This report contains the collective views of an international group of experts and does not necessarily represent the decisions or the stated policy of the United Nations Environment Programme, the International Labour Organization, or the World Health Organization.

Concise International Chemical Assessment Document 30

1,3-BUTADIENE: HUMAN HEALTH ASPECTS

Please note that the layout and pagination of this pdf file are not identical to those of the printed CICAD

First draft prepared by K. Hughes, M.E. Meek, M. Walker, and R. Beauchamp, Health Canada, Ottawa, Canada

Published under the joint sponsorship of the United Nations Environment Programme, the International Labour Organization, and the World Health Organization, and produced within the framework of the Inter-Organization Programme for the Sound Management of Chemicals.

World Health Organization Geneva, 2001 The International Programme on Chemical Safety (IPCS), established in 1980, is a joint venture of the United Nations Environment Programme (UNEP), the International Labour Organization (ILO), and the World Health Organization (WHO). The overall objectives of the IPCS are to establish the scientific basis for assessment of the risk to human health and the environment from exposure to chemicals, through international peer review processes, as a prerequisite for the promotion of chemical safety, and to provide technical assistance in strengthening national capacities for the sound management of chemicals. The Inter-Organization Programme for the Sound Management of Chemicals (IOMC) was established in 1995 by UNEP, ILO, the Food and Agriculture Organization of the United Nations, WHO, the United Nations Industrial Development Organization, the United Nations Institute for Training and Research, and the Organisation for Economic Co-operation and Development (Participating Organizations), following recommendations made by the 1992 UN Conference on Environment and Development to strengthen cooperation and increase coordination in the field of chemical safety. The purpose of the IOMC is to promote coordination of the policies and activities pursued by the Participating Organizations, jointly or separately, to achieve the sound management of chemicals in relation to human health and the environment.

WHO Library Cataloguing-in-Publication Data

1,3-Butadiene : human health aspects.

(Concise international chemical assessment document ; 30)

1.Butadienes - toxicity 2.Risk assessment 3.Environmental exposure 4.Occupational exposure I.International Programme on Chemical Safety II.Series

ISBN 92 4 153030 8 (NLM Classification: QD 305.H7) ISSN 1020-6167

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Printed by Wissenschaftliche Verlagsgesellschaft mbH, D-70009 Stuttgart 10 TABLE OF CONTENTS

FOREWORD ...... 1

1. EXECUTIVE SUMMARY ...... 4

2. IDENTITY AND PHYSICAL/CHEMICAL PROPERTIES ...... 5

3. ANALYTICAL METHODS ...... 5

4. SOURCES OF HUMAN EXPOSURE ...... 6

4.1 Natural sources ...... 6 4.2 Anthropogenic sources ...... 6 4.3 Production and uses ...... 7

5. ENVIRONMENTAL TRANSPORT, DISTRIBUTION, AND TRANSFORMATION ...... 7

5.1 Air ...... 7 5.2 Water ...... 8 5.3 Sediment and soil ...... 8 5.4 Biota ...... 8 5.5 Environmental modelling ...... 8

6. ENVIRONMENTAL LEVELS AND HUMAN EXPOSURE ...... 9

6.1 Environmental levels ...... 9 6.1.1 Ambient air ...... 9 6.1.2 Surface water ...... 9 6.1.3 Groundwater ...... 9 6.2 Human exposure ...... 9 6.2.1 Indoor air ...... 9 6.2.2 Drinking-water ...... 10 6.2.3 Food ...... 10 6.2.4 Consumer products ...... 10 6.2.5 Occupational exposure ...... 10

7. COMPARATIVE KINETICS AND METABOLISM IN LABORATORY ANIMALS AND HUMANS ...... 11

8. EFFECTS ON LABORATORY MAMMALS AND IN VITRO TEST SYSTEMS ...... 13

8.1 Single exposure ...... 13 8.2 Irritation and sensitization ...... 13 8.3 Repeated exposure ...... 13 8.4 Carcinogenicity ...... 13 8.5 Genotoxicity and related end-points ...... 18 8.6 Reproductive toxicity ...... 23 8.6.1 Effects on fertility ...... 23 8.6.2 Developmental toxicity ...... 24 8.7 Immunotoxicity ...... 24

iii Concise International Chemical Assessment Document 30

9. EFFECTS ON HUMANS ...... 24

9.1 Clinical studies ...... 24 9.2 Epidemiological studies ...... 25 9.2.1 Cancer ...... 25 9.2.2 Non-neoplastic effects ...... 28 9.2.3 Genotoxicity ...... 28

10. EVALUATION OF HEALTH EFFECTS ...... 29

10.1 Hazard identification ...... 29 10.1.1 Carcinogenicity and genotoxicity ...... 29 10.1.2 Non-neoplastic effects ...... 32 10.2 Exposure–response assessment and criteria for setting tolerable concentrations or guidance values ...... 33 10.2.1 Carcinogenicity ...... 33 10.2.1.1 Epidemiological data ...... 33 10.2.1.2 Data from studies in experimental animals ...... 34 10.2.2 Non-neoplastic effects ...... 35 10.3 Sample exposure and risk characterization ...... 36 10.3.1 Sample exposure characterization ...... 36 10.3.2 Sample risk characterization ...... 36 10.4 Uncertainties and degree of confidence in human health hazard characterization and sample risk characterization ...... 37

11. PREVIOUS EVALUATIONS BY INTERNATIONAL BODIES ...... 40

REFERENCES ...... 41

APPENDIX 1 — SOURCE DOCUMENT ...... 49

APPENDIX 2 — CICAD PEER REVIEW ...... 50

APPENDIX 3 — CICAD FINAL REVIEW BOARD ...... 51

APPENDIX 4 — QUANTITATION OF EXPOSURE–RESPONSE FOR CRITICAL EFFECTS ASSOCIATED WITH EXPOSURE TO 1,3-BUTADIENE ...... 52

INTERNATIONAL CHEMICAL SAFETY CARD ...... 68

RÉSUMÉ D’ORIENTATION ...... 70

RESUMEN DE ORIENTACIÓN ...... 72

iv 1,3-Butadiene: Human health aspects

FOREWORD 1701 for advice on the derivation of health-based tolerable intakes and guidance values. Concise International Chemical Assessment Documents (CICADs) are the latest in a family of While every effort is made to ensure that CICADs publications from the International Programme on represent the current status of knowledge, new informa- Chemical Safety (IPCS) — a cooperative programme of tion is being developed constantly. Unless otherwise the World Health Organization (WHO), the International stated, CICADs are based on a search of the scientific Labour Organization (ILO), and the United Nations literature to the date shown in the executive summary. In Environment Programme (UNEP). CICADs join the the event that a reader becomes aware of new informa- Environmental Health Criteria documents (EHCs) as tion that would change the conclusions drawn in a authoritative documents on the risk assessment of CICAD, the reader is requested to contact IPCS to inform chemicals. it of the new information.

International Chemical Safety Cards on the Procedures relevant chemical(s) are attached at the end of the CICAD, to provide the reader with concise information The flow chart shows the procedures followed to on the protection of human health and on emergency produce a CICAD. These procedures are designed to action. They are produced in a separate peer-reviewed take advantage of the expertise that exists around the procedure at IPCS. They may be complemented by world — expertise that is required to produce the high- information from IPCS Poison Information Monographs quality evaluations of toxicological, exposure, and other (PIM), similarly produced separately from the CICAD data that are necessary for assessing risks to human process. health and/or the environment. The IPCS Risk Assess- ment Steering Group advises the Co-ordinator, IPCS, on CICADs are concise documents that provide sum- the selection of chemicals for an IPCS risk assessment, maries of the relevant scientific information concerning whether a CICAD or an EHC is produced, and which the potential effects of chemicals upon human health institution bears the responsibility of the document and/or the environment. They are based on selected production, as well as on the type and extent of the national or regional evaluation documents or on existing international peer review. EHCs. Before acceptance for publication as CICADs by IPCS, these documents undergo extensive peer review The first draft is based on an existing national, by internationally selected experts to ensure their regional, or international review. Authors of the first completeness, accuracy in the way in which the original draft are usually, but not necessarily, from the institution data are represented, and the validity of the conclusions that developed the original review. A standard outline drawn. has been developed to encourage consistency in form. The first draft undergoes primary review by IPCS and The primary objective of CICADs is characteri- one or more experienced authors of criteria documents in zation of hazard and dose–response from exposure to a order to ensure that it meets the specified criteria for chemical. CICADs are not a summary of all available data CICADs. on a particular chemical; rather, they include only that information considered critical for characterization of the The draft is then sent to an international peer risk posed by the chemical. The critical studies are, review by scientists known for their particular expertise however, presented in sufficient detail to support the and by scientists selected from an international roster conclusions drawn. For additional information, the compiled by IPCS through recommendations from IPCS reader should consult the identified source documents national Contact Points and from IPCS Participating upon which the CICAD has been based. Institutions. Adequate time is allowed for the selected experts to undertake a thorough review. Authors are Risks to human health and the environment will required to take reviewers’ comments into account and vary considerably depending upon the type and extent revise their draft, if necessary. The resulting second draft of exposure. Responsible authorities are strongly encouraged to characterize risk on the basis of locally measured or predicted exposure scenarios. To assist the reader, examples of exposure estimation and risk 1 International Programme on Chemical Safety (1994) characterization are provided in CICADs, whenever Assessing human health risks of chemicals: derivation possible. These examples cannot be considered as of guidance values for health-based exposure limits. representing all possible exposure situations, but are Geneva, World Health Organization (Environmental provided as guidance only. The reader is referred to EHC Health Criteria 170).

1 Concise International Chemical Assessment Document 30

CICAD PREPARATION FLOW CHART

SELECTION OF PRIORITY CHEMICAL

SELECTION OF HIGH QUALITY NATIONAL/REGIONAL ASSESSMENT DOCUMENT(S)

FIRST DRAFT PREPARED

PRIMARY REVIEW BY IPCS ( REVISIONS AS NECESSARY)

REVIEW BY IPCS CONTACT POINTS/ SPECIALIZED EXPERTS

REVIEW OF COMMENTS (PRODUCER/RESPONSIBLE OFFICER), PREPARATION OF SECOND DRAFT 1

FINAL REVIEW BOARD 2

FINAL DRAFT 3

EDITING

APPROVAL BY DIRECTOR, IPCS

PUBLICATION

1 Taking into account the comments from reviewers. 2 The second draft of documents is submitted to the Final Review Board together with the reviewers’ comments. 3 Includes any revisions requested by the Final Review Board.

2 1,3-Butadiene: Human health aspects is submitted to a Final Review Board together with the reviewers’ comments.

A consultative group may be necessary to advise on specific issues in the risk assessment document.

The CICAD Final Review Board has several important functions:

– to ensure that each CICAD has been subjected to an appropriate and thorough peer review; – to verify that the peer reviewers’ comments have been addressed appropriately; – to provide guidance to those responsible for the preparation of CICADs on how to resolve any remaining issues if, in the opinion of the Board, the author has not adequately addressed all comments of the reviewers; and – to approve CICADs as international assessments.

Board members serve in their personal capacity, not as representatives of any organization, government, or industry. They are selected because of their expertise in human and environmental toxicology or because of their experience in the regulation of chemicals. Boards are chosen according to the range of expertise required for a meeting and the need for balanced geographic representation.

Board members, authors, reviewers, consultants, and advisers who participate in the preparation of a CICAD are required to declare any real or potential conflict of interest in relation to the subjects under discussion at any stage of the process. Representatives of nongovernmental organizations may be invited to observe the proceedings of the Final Review Board. Observers may participate in Board discussions only at the invitation of the Chairperson, and they may not participate in the final decision-making process.

3 Concise International Chemical Assessment Document 30

1. EXECUTIVE SUMMARY metabolic capability for 1,3-butadiene in humans, related to genetic polymorphism for relevant enzymes.

This CICAD on 1,3-butadiene was prepared by the 1,3-Butadiene is of low acute toxicity in experimen- Environmental Health Directorate of Health Canada tal animals. However, long-term exposure to 1,3- based on documentation prepared concurrently as part butadiene was associated with the development of of the Priority Substances Program under the Canadian ovarian atrophy at all concentrations tested in mice. Environmental Protection Act (CEPA). The objective Other effects in the ovaries have also been observed in of health assessments on Priority Substances under shorter-term studies. Atrophy of the testes was also CEPA is to assess the potential effects of indirect observed in male mice at concentrations greater than exposure in the general environment on human health. those associated with effects in females. Based on Data identified as of the end of April 1998 were limited available data, there is no conclusive evidence considered in this review. Information on the nature of that 1,3-butadiene is teratogenic in experimental animals the peer review and availability of the source document following maternal or paternal exposure or that it induces is presented in Appendix 1. Information on the peer significant fetal toxicity at concentrations below those review of this CICAD is presented in Appendix 2. This that are maternally toxic. CICAD was approved as an international assessment at a meeting of the Final Review Board, held in Helsinki, 1,3-Butadiene also induced a variety of effects on Finland, on 26–29 June 2000. Participants at the Final the blood and bone marrow of mice; although data are Review Board meeting are listed in Appendix 3. The limited, similar effects have not been observed in rats. International Chemical Safety Card (ICSC 0017) for 1,3- butadiene, produced by the International Programme on Inhaled 1,3-butadiene is a potent carcinogen in Chemical Safety (IPCS, 1993), has also been reproduced mice, inducing tumours at multiple sites at all concentra- in this document. tions tested in all identified studies. 1,3-Butadiene was also carcinogenic in rats at all exposure levels in the only 1,3-Butadiene (CAS No. 106-99-0) is a product of relevant study available; although only much higher incomplete combustion resulting from natural processes concentrations were tested in rats than in mice, rats and human activity. It is also an industrial chemical used appear to be the less sensitive species, based on primarily in the production of polymers, including poly- comparison of tumour incidence data. The greater butadiene, styrene-butadiene rubbers and lattices, and sensitivity in mice than in rats to induction of these nitrile-butadiene rubbers. 1,3-Butadiene enters the effects by 1,3-butadiene is likely related to species environment from exhaust emissions from gasoline- and differences in metabolism to the active epoxide diesel-powered vehicles, from non-transportation fuel metabolites. combustion, from biomass combustion, and from indus- trial on-site uses. 1,3-Butadiene is mutagenic in somatic cells of both mice and rats, although the mutagenic potency was While 1,3-butadiene is not persistent, it is ubiqui- greater in mice than in rats. Similarly, 1,3-butadiene tous in the urban environment because of its widespread induced other genetic damage in somatic cells of mice, combustion sources. The highest atmospheric but not in those of rats. 1,3-Butadiene was also consis- concentrations have been measured in air in cities and tently genotoxic in germ cells of mice, but not in the close to industrial sources. single assay in rats identified. However, there were no apparent differences in species sensitivity to genetic The general population is exposed to 1,3-butadiene effects induced by epoxide metabolites of 1,3-butadiene. primarily through ambient and indoor air. In comparison, There is also limited evidence from occupationally other media, including food and drinking-water, contrib- exposed populations that 1,3-butadiene is genotoxic in ute negligibly to exposure to 1,3-butadiene. Tobacco humans, inducing mutagenic and clastogenic damage in smoke may contribute significant amounts of 1,3-buta- somatic cells. diene. An association between exposure to 1,3-butadiene Metabolism of 1,3-butadiene appears to be qualita- in the occupational environment and leukaemia fulfils tively similar across species, although there are quantita- several of the traditional criteria for causality. In the tive differences in the amounts of putatively toxic metab- largest and most comprehensive study conducted to olites formed; mice appear to oxidize 1,3-butadiene to the date, involving a cohort of workers from multiple plants, monoepoxide, and subsequently the diepoxide, mortality due to leukaemia increased with estimated metabolite to a greater extent than do rats or humans. cumulative exposure to 1,3-butadiene in the styrene- However, there may also be interindividual variation in butadiene rubber industry; this association remained

4 1,3-Butadiene: Human health aspects after controlling for exposure to styrene and benzene 2. IDENTITY AND PHYSICAL/CHEMICAL and was strongest in those subgroups with highest PROPERTIES potential exposure. Similarly, an association between exposure to 1,3-butadiene and leukaemia was observed in an independently conducted case–control study of 1,3-Butadiene (H2C=CH@CH=CH2) is also known as largely the same population of workers. However, there butadiene, ",(-butadiene, buta-1,3-diene, bivinyl, was no increase in mortality due to leukaemia in divinyl, erythrene, vinylethylene, biethylene, and butadiene monomer production workers who were not pyrrolylene. Its Chemical Abstracts Service (CAS) concomitantly exposed to some of the other substances registry number is 106-99-0, and its Registry of Toxic present in the styrene-butadiene rubber industry, Effects of Chemical Substances (RTECS) number is although there was some limited evidence of an EI9275000. association with mortality due to lymphosarcoma and reticulosarcoma in some subgroups. At room temperature, butadiene is a colourless, flammable gas with a mild aromatic odour. The molecular The available epidemiological and toxicological weight of butadiene is 54.09 g/mol. It has a high vapour data provide evidence that 1,3-butadiene is carcinogenic pressure (281 kPa at 25 °C), a vapour density of 1.9, a in humans and may also be genotoxic in humans. The moderately low water solubility (735 mg/litre at 25 °C), a carcinogenic potency (the concentration associated with low boiling point (!4.4 °C), a low octanol/water partition a 1% increase in mortality due to leukaemia) was deter- coefficient (K 1.99) (Mackay et al., 1993), and a Henry’s 3 ow mined to be 1.7 mg/m , based on the results of the largest law constant of 7460 Pa@m3/mol (equivalent to an well conducted epidemiological investigation in exposed air/water partition coefficient, or dimensionless Henry’s workers. This value is similar to the lower end of the law constant, of 165.9). range of tumorigenic concentrations determined on the basis of studies in rodents. 1,3-Butadiene also induced Further chemical and physical characteristics of reproductive toxicity in experimental animals. As a butadiene are given in the International Chemical Safety measure of its potency to induce reproductive effects, a Card reproduced in this document. benchmark concentration of 0.57 mg/m3 was derived for ovarian toxicity in mice. The conversion factor for butadiene in air is as follows: 1 ppm = 2.21 mg/m3. Although the health effects associated with expo- sure to 1,3-butadiene and the mode of action for induc- tion of these effects have been extensively investigated, there continues to be considerable research on this sub- 3. ANALYTICAL METHODS stance in an effort to address some of the uncertainties associated with the database. Selected methods for the analysis of butadiene in various matrices are listed in Table 1 (IARC, 1999). Gas detection tubes can also be used to detect butadiene.

Table 1: Methods for analysis of butadiene (modified from IARC, 1999).

Assay Limit of Sample matrix Sample preparation procedurea detection Reference

Air Adsorb (charcoal); extract (carbon disulfide) GC/FID 200 µg/m3 US OSHA, 1990 Adsorb (coconut, charcoal); extract GC/FID 0.2 mg/sample NIOSH, 1994 (dichloromethane) (5–25 litres) Adsorb on Perkin-Elmer ATD 400 packed GC/FID 200 µg/m3 UK HSE, 1992 with polymeric or synthetic adsorbent material; thermal desorption

Foods and plastic Dissolve (dimethylacetamide) or melt; inject GC/MS-SIM ~1 µg/kg Startin & Gilbert, 1984 food-packing material headspace sample

Plastics, liquid foods Dissolve in o-dichlorobenzene; inject GC/FID 2–20 µg/kg US FDA, 1987 headspace sample Solid foods Cut or mash sample; inject headspace GC/FID 2–20 µg/kg US FDA, 1987 sample a Abbreviations: GC/FID: gas chromatography/flame ionization detection; GC/MS-SIM: gas chromatography/mass spectrometry with single-ion monitoring.

5 Concise International Chemical Assessment Document 30

4. SOURCES OF HUMAN EXPOSURE collected for industrial processes, plastic products industries, refined petroleum and coal products industries, and chemical and chemical products Data on sources and emissions from Canada, the industries as part of the National Pollutant Release source country of the national assessment on which this Inventory (NPRI) (Environment Canada, 1996a, 1997). CICAD is based, are presented here as an example. Emissions other than those reported to the NPRI may Sources and patterns of emissions in other countries are occur, including from combustion of other fuels (e.g., expected to be similar, although quantitative values may natural gas, oil, and wood space heating), prescribed vary. forest burning, cigarettes, waste incineration, releases from polymer products, releases from the use and Estimates of emissions of butadiene are highly disposal of products containing butadiene, and spillage variable, depending on the method of estimation and the (Ligocki et al., 1994; Environment Canada, 1996b; OECD, quality of the data upon which they are based. Total 1996). Canadian emissions for 1994 were estimated to range between 12 917 and 41 622 tonnes (Environment Canada, The following amounts of butadiene were 1998). Major uncertainties are associated with estimates estimated to have been released into the Canadian for combustion sources, notably forest fires. environment in 1994 from key transportation and related sources (Environment Canada, 1996a; CPPI, 1997): 4.1 Natural sources 3376–7401 tonnes from on-road gasoline- and diesel- powered motor vehicles (with about 45–89% of those Butadiene is released from biomass combustion, releases from gasoline engines and 11–55% from diesel especially forest fires. Total global emissions of buta- engines); 150–258 tonnes from aircraft; 84–1689 tonnes diene from biomass combustion were estimated to be 770 from off-road motor vehicles; 84 tonnes from lawn- 000 tonnes per year (Ward & Hao, 1992). Releases from mowers; 40 tonnes from the marine sector; and 17 tonnes forest fires in Canada were estimated to range between from the rail sector. 3607 and 26 966 tonnes, which constituted 49.3% (range of estimates is 28–65%) of the total annual emissions of In addition, data from NPRI for 1994 (Environment butadiene in Canada (CPPI, 1997). Although Altshuller et Canada, 1996a) listed a total of 270.4 tonnes released al. (1971) suggested that butadiene can be released from from the chemical and chemical products industries. Of natural gas losses and diffusion through soil from this, 270.3 tonnes were released into air, 0.058 tonnes petroleum deposits, no data were identified on this into water (St. Clair River, Ontario), and 0.002 tonnes possible source. onto land. There were releases of 17.5 tonnes into air from the plastic products industries. A total of 22.3 4.2 Anthropogenic sources tonnes was released from the refined petroleum and coal products industries, of which 22.2 tonnes were released All internal combustion engines may produce into air. Off-site transfer of wastes (material sent for final butadiene as a result of incomplete combustion. The disposal or treatment prior to final disposal) from amount generated and released depends primarily on the industrial sites in Canada in 1994 was estimated to composition of fuel, the type of engine, the emission include a total of 131.3 tonnes of butadiene, with 128.7 control used (i.e., presence and efficiency of catalytic tonnes being sent to incineration, 2.1 tonnes to landfill, converter), the operating temperature, and the age and and 0.5 tonnes to municipal sewage treatment plants state of repair of the vehicle.1 Cyclohexane, 1-hexene, (Environment Canada, 1996a). Based on 1995 NPRI data 1-pentene, and cyclohexene have been identified as (Environment Canada, 1997), the amount of butadiene primary fuel precursors for butadiene (Schuetzle et al., estimated to have been released into the Canadian 1994). As well, very low levels of butadiene itself may be environment was 225.8 tonnes from industrial on-site present in gasoline and in liquefied petroleum gas. uses, with 0.058 tonnes released into water, 0.002 tonnes into land, and 225.4 tonnes into air. Releases into air Butadiene can also enter the environment from any included air fugitive releases (172.8 tonnes), air stack stage in the production, storage, use, transport, or dis- releases (36.3 tonnes), air storage releases (4.8 tonnes), posal of products with residual, free, or unreacted buta- air spill releases (1.1 tonnes), and other air releases (10.4 diene. Data on Canadian industrial emissions have been tonnes).

Based on data in NPRI, it was estimated that the 1 Personal communication from L.A. Graham, River Road total release of butadiene from fuel distribution in 1994 Environmental Technology Centre, Environment Canada, was 24 tonnes (Environment Canada, 1996a), although Ottawa, Ontario, to Commercial Chemicals Evaluation gasoline and diesel fuel contain little or no butadiene Branch, Environment Canada, Hull, Quebec, 1996. (US EPA, 1989).

6 1,3-Butadiene: Human health aspects

CPPI (1997) estimated that releases into the Canadian environment in 1994 were 1191 tonnes from 5. ENVIRONMENTAL TRANSPORT, prescribed forest burning, 3706 tonnes from wood space DISTRIBUTION, AND TRANSFORMATION heating, 11 tonnes from natural gas/oil space heating, and 1–9 tonnes from cigarettes. 5.1 Air 4.3 Production and uses Since butadiene is released primarily to air, its fate Butadiene is produced during the combustion of in that medium is of primary importance. Butadiene is not organic matter in both natural processes and human expected to persist in air, since it oxidizes rapidly with activities. In addition, it is produced commercially for use several oxidant species. Destruction of atmospheric in the chemical polymer industry. butadiene by the gas-phase reaction with photochemi- cally produced hydroxyl radicals is expected to be the Butadiene is purified by extraction from a crude dominant photo-initiated pathway. Products that can be petroleum butadiene stream. In 1994, there was one formed include formaldehyde, acrolein, and furan. Canadian commercial producer of butadiene (located in Destruction by nitrate radicals is expected to be a sig- Sarnia, Ontario), with a domestic production of nificant nighttime process in urban areas. Acrolein, 103.7 kilotonnes. Importation into Canada from the USA trans-4-nitroxy-2-butenal, and 1-nitroxy-3-buten-2-one was 1.7 kilotonnes in 1994. The Canadian domestic use have been identified as products of this reaction. Reac- of butadiene in 1994 amounted to 105.4 kilotonnes tion with ozone is also rapid but less important than (98.3 kilotonnes for total domestic demand and 7.1 for reaction with hydroxyl radicals. The products of the export sales) (Camford Information Services, 1995). In the reaction of butadiene with ozone are acrolein, formalde- 1 USA, total production in 1993 was 1.4 billion kilograms. hyde, acetylene, ethylene, formic acid, formic anhydride, According to data summarized in IARC (1999), in 1996, carbon monoxide, carbon dioxide, hydrogen gas, hydro- production of butadiene in China (Taiwan), France, peroxyl radical, hydroxyl radical, and 3,4-epoxy-1-butene Germany, Japan, the Republic of Korea, and the USA (Atkinson et al., 1990; Howard et al., 1991; McKone et was 129, 344, 673, 1025, 601, and 1744 kilotonnes, al., 1993; US EPA, 1993). respectively. Average atmospheric half-lives for photo-oxidation The largest end use of butadiene in Canada is the of butadiene, based on measured as well as calculated production of polybutadiene rubber (51.4 kilotonnes; data, range from 0.24 to 1.9 days (Darnell et al., 1976; 52.3% of total Canadian consumption for 1994) (Camford Lyman et al., 1982; Atkinson et al., 1984; Becker et al., Information Services, 1995). Other derivatives produced 1984; Klöpffer et al., 1988; Howard et al., 1991; Mackay et include styrene-butadiene lattices (31.0 kilotonnes; al., 1993). However, half-lives for butadiene in air can 31.5% of total Canadian consumption for 1994), nitrile- vary considerably under different conditions. butadiene rubbers (10.0 kilotonnes; 10.2% for 1994), Estimations for atmospheric residence time in several US acrylonitrile-butadiene-styrene terpolymer (3.4 kilo- cities ranged from 0.4 h under clear skies at night in the tonnes; 3.5% for 1994), and specialty styrene-butadiene summer to 2000 h (83 days) under cloudy skies at night rubbers (2.5 kilotonnes; 2.5% of total Canadian in the winter. Daytime residence times for different cities consumption for 1994). within a given season varied by factors of 2–3. Nighttime residence times varied by larger factors. The differences Butadiene has a long history of use, notably between summer and winter conditions were large at all related to production of polymers. Several industrial and sites, with winter residence times 10–30 times greater commercial products are manufactured with it or may than summer residence times (US EPA, 1993). Because of contain it as a component. Examples include tires, car the long residence times under some conditions, sealants, plastic bottles and food wrap, epoxy resins, especially in winter under cloudy conditions, there is a lubricating oils, hoses, drive belts, moulded rubber possibility of day-to-day carryover. Nonetheless, given goods, adhesives, paint, latex foams for carpet backing the generally short daytime residence times, the net or underpad, shoe soles, moulded toys/household atmospheric lifetime of butadiene is short, and there is goods, medical devices, and chewing gum (CEH-SRI generally limited potential for long-range transport of International, 1994; OECD, 1996). this compound.

It is predicted from its physical/chemical properties that when butadiene is released into air, almost all of it 1 Hazardous Substances Databank, National Library of will exist in the vapour phase in the atmosphere Medicine’s TOXNET system, searched 10 December (Eisenreich et al., 1981; Environment Canada, 1998). Wet 1999. and dry deposition are not expected to be important as

7 Concise International Chemical Assessment Document 30 transfer processes. Evaporation from rain may be rapid, fresh mass) to concentration in soil solids (mg/kg) was and the compound is returned to the atmosphere estimated to range from 0.32 to 15 (dimensionless). relatively quickly unless it is leached into the soil. The partition coefficient of butadiene concentra- 5.2 Water tion in whole plants (mg/kg, fresh mass) to its concentra- tion in soil solids (mg/kg) was estimated to range from Volatilization, biodegradation, and oxidation by 0.1 to 2.9 (dimensionless). The steady-state plant/air singlet oxygen are the most prominent processes partition coefficient for foliar uptake of butadiene in involved in determining the fate of butadiene in water. plant leaves was estimated to be 0.63 m3/kg. There are no The estimated half-lives of butadiene by reaction in reported bioaccumulation data for any terrestrial water range from 4.2 to 28 days (Howard et al., 1991; invertebrates. Mackay et al., 1993). 5.5 Environmental modelling 5.3 Sediment and soil Fugacity modelling was conducted to provide an The processes that are most prominent in deter- overview of key reaction, intercompartment, and advec- mining the environmental fate of butadiene in sediment tion (movement out of a system) pathways for butadiene are biotic and abiotic degradation. The modelled half- and of its overall distribution in the environment. A lives of butadiene by reaction in sediment range from steady-state, non-equilibrium model (Level III fugacity 41.7 to 125 days (Mackay et al., 1993). modelling) was run using the methods developed by Mackay (1991) and Mackay and Paterson (1991). Based on its vapour pressure and its solubility, Assumptions, input parameters, and results are volatilization of butadiene from soil and other surfaces is presented in Environment Canada (1998). Based on expected to be significant. Butadiene’s organic carbon/ butadiene’s physical/chemical properties, Level III water partition coefficient indicates that it should not fugacity modelling predicts that: adsorb to soil particles to a great degree and would be considered moderately mobile (Kenaga, 1980; Swann et • when butadiene is released into air, the distribution al., 1983). However, the rapid rate of volatilization and of mass is almost 100% in air, with very small the potential for degradation in soil suggest that it is amounts in soil and water; unlikely that butadiene will leach into groundwater. • when butadiene is released into water, the distribu- Based on modelling predictions, the half-life of butadi- tion of mass is 99.0% in water, with small amounts ene by reaction, given by Howard et al. (1991) and in air; Mackay et al. (1993), ranges from 7 to 41.7 days. • when butadiene is released into soil, the distribu- tion of mass is 38.6% in soil, 59.3% in air, and 2.1% 5.4 Biota in water.

There are no measured bioconcentration factors. Modelling predictions do not purport to reflect Butadiene is metabolized by the mixed-function oxidase actual expected measurements in the environment but system in higher organisms, which contributes to the rather indicate the broad characteristics of the fate of the expected lack of accumulation by many organisms. substance in the environment and its general Estimated bioconcentration factors for butadiene in fish distribution between media. Thus, when butadiene is have been reported to range from 4.6 to 19 (Lyman et al., discharged into air or water, most of it is expected to be 1982; OECD, 1996). Even though estimation methods found in the medium receiving the discharge directly. For likely overestimate the true bioconcentration potential example, if butadiene is discharged into air, almost all of for a readily metabolized substance, they indicate that it will exist in the atmosphere, where it will react rapidly butadiene is not expected to bioconcentrate in aquatic and will also be transported away. If butadiene is organisms or to biomagnify in the aquatic food chain. discharged to water, it will react in water, and some will also evaporate into air. If butadiene is discharged to soil, There are no reported measurements of plant root most will be present in air or soil, where it will react bioconcentration in soils. However, McKone et al. (1993) (Mackay et al., 1993; Environment Canada, 1998). estimated the uptake of butadiene by roots from soil solution to be 1.84 litres/kg, which is the ratio of butadiene concentration in root (mg/kg, fresh mass) to the concentration in soil solution (mg/litre). The partition coefficient of butadiene concentration in roots (mg/kg,

8 1,3-Butadiene: Human health aspects

6. ENVIRONMENTAL LEVELS AND vehicle exhaust. Similarly, butadiene was frequently HUMAN EXPOSURE detected in samples from 10 parking structures in Cali- fornia, with the maximum concentration being 28 µg/m3 (Wilson et al., 1991). Butadiene has also been detected in Data on environmental levels and human exposure urban road tunnels during rush hours in Australia (mean 3 from Canada, the source country of the national assess- concentration 28 µg/m ; Duffy & Nelson, 1996) and 3 3 ment on which this CICAD is based, are presented here Sweden (mean concentrations 17 µg/m and 25 µg/m in as a basis for the sample risk characterization. Patterns of two tunnels; Barrefors, 1996). Butadiene was measured at 3 exposure in other countries are expected to be similar, concentrations ranging from 0.2 to 28 µg/m in 96 of 97 5- although quantitative values may vary. min air samples collected from a pumping island at randomly identified self-service filling stations in 6.1 Environmental levels California (Wilson et al., 1991).

6.1.1 Ambient air 6.1.2 Surface water

Butadiene was detected (detection limit 0.05 µg/m3) No data on concentrations of butadiene in Cana- in 7314 (or 80%) of 9168 24-h samples collected between dian lake, river, estuarine, or marine waters were identi- 1989 and 1996 from 47 sites across Canada.1 The mean fied in the literature. Butadiene is being monitored in concentration in all samples was 0.3 µg/m3 (in the effluents discharged into the St. Clair River from the calculation of the mean, a value of one-half the detection butadiene production plant in Sarnia, Ontario. It was limit was assumed for samples in which levels were detected only twice, at 2 and 5 µg/litre, in 2103 composite below the detection limit), and the maximum samples of aqueous effluent taken every 4 h in 1996 concentration measured was 14.1 µg/m3. Concentrations (detection limit 1 µg/litre). In daily sampling of effluents of butadiene in ambient air corresponding to the 50th from the four individual outfalls (detection limit 1 µg/litre and 95th percentiles were 0.21 and 1.0 µg/m3, in 736 samples and 50 µg/litre in 789 samples), butadiene respectively. Concentrations were generally higher in was detected in only three samples, at concentrations of 3 urban areas, with a mean exposure to 0.4 mg/m3 (95th 21, 80, and 130 µg/litre. percentile 1.3 mg/m3) estimated as a “reasonable worst- case scenario,” based on data from four sites. Similar 6.1.3 Groundwater levels were measured in smaller surveys in Canada (Bell et al., 1991, 1993; Hamilton-Wentworth, 1997; Conor Butadiene was detected but not quantified in a Pacific Environmental, 1998).2 In areas influenced by groundwater plume near a waste site in Quebec where industrial point sources of butadiene, concentrations in refinery oil residues and a variety of organic chemicals air were greater, with maximum and mean levels of 28 and had been dumped (Pakdel et al., 1992). 0.62 mg/m3, respectively (95th percentile 6.4 mg/m3) Human exposure being measured between 1 and 3 km from the source 6.2 (MOEE, 1995). 6.2.1 Indoor air Butadiene has also been detected in air in enclosed structures. Concentrations of butadiene between 4 and In available surveys in Canada, 1,3-butadiene was 49 µg/m3 were measured during the winter months of detected up to 6 times more frequently in indoor air in 1994–1995 in Canadian underground parking garages homes than in corresponding samples of outdoor air, (Environment Canada, 1994) because of its presence in with concentrations being up to 10-fold higher indoors than outdoors (Bell et al., 1993; Hamilton-Wentworth, 1997; Conor Pacific Environmental, 1998).4 Concentra- tions in air of indoor environments are highly variable 1 Unpublished data on butadiene levels in Canada from and depend largely on individual activities and circum- National Air Pollution Surveillance program, provided by T. Dann, River Road Environmental Technology Centre, Environment Canada, Ottawa, Ontario, to Commercial Chemicals Evaluation Branch, Environment Canada, Hull, 3 Personal communication from H. Michelin, Bayer Inc., Quebec, April 1997. Sarnia, Ontario, to Commercial Chemicals Evaluation Branch, Environment Canada, Hull, Quebec, 1997. 2 Also letter dated 28 August 1996 from P. Steer, Science and Technology Branch, Ontario Ministry of 4 Also personal communication dated 24 December 1997 Environment and Energy, to J. Sealy, Health Canada, re. from X.-L. Cao to Health Canada, re. method detection 1,3-butadiene and chloroform data (File No. limits for 24-h air samples from multimedia exposure pilot 1E080149.MEM). study (File No. MDL.XLS).

9 Concise International Chemical Assessment Document 30 stances, including the use of consumer products (e.g., 6.2.4 Consumer products cigarettes), the infiltration of vehicle exhaust from nearby traffic and possibly from attached garages, and cooking Data on emissions of butadiene from potential activities involving heated fats and oils (see section indoor sources such as styrene-butadiene rubber were 6.2.3). While data are inadequate to determine the not identified. relative contributions of each of these potential indoor sources, the highest concentrations of butadiene in Butadiene has been detected in both mainstream indoor air in Canada have generally been detected in smoke and sidestream smoke from cigarettes in Canada indoor environments contaminated with environmental and the USA. For 18 brands of Canadian cigarettes, the tobacco smoke (ETS). In a survey of 94 homes across mean butadiene content ranged from 14.3 to 59.5 µg/cig- Canada, the mean level in “non-smoking” homes was arette (overall mean concentration 30.0 µg/cigarette) in <1 µg/m3 (data censored by considering levels to be one- the mainstream smoke and from 281 to 656 µg/cigarette half the detection limit in samples in which butadiene (overall mean concentration 375 µg/cigarette) in the side- was not detected), compared with a mean of 2.5 µg/m3 stream smoke, according to “preliminary” data (Labstat, (data censored) in homes where smoking was present Inc., 1995). The US DHHS (1989) reported that the (Conor Pacific Environmental, 1998). Similarly, mean vapour phase of mainstream smoke of non-filter concentrations in indoor air from “non-smoking” loca- cigarettes contained butadiene at levels of 25–40 µg/cig- tions in Windsor, Ontario, ranged from 0.3 to 1.6 µg/m3, arette. Brunnemann et al. (1989) measured butadiene while mean levels in “smoking” locations ranged from 1.3 levels ranging from 16 to 75 µg/cigarette in mainstream to 18.9 µg/m3. At non-residential indoor sampling sites in smoke from seven brands of cigarettes and levels Windsor, the frequency of detection of butadiene was ranging from 205 to 361 µg/cigarette in the sidestream 75–100% where ETS was present (Bell et al., 1993). smoke from six types of cigarettes. As discussed in section 6.2.1, the presence of ETS contributes to 6.2.2 Drinking-water elevated levels of butadiene in indoor air.

There are no data available concerning the pres- 6.2.5 Occupational exposure ence of butadiene in drinking-water. In an investigation on whether the use of polybutylene pipe in water distri- Potential occupational exposure to butadiene can bution systems is likely to result in the contamination of occur in petroleum refining and related operations, pro- drinking-water with butadiene, Cooper1 did not detect duction of butadiene monomer, production of butadiene- the substance in water from these types of pipes (no based polymers, or the manufacture of rubber and plas- further information was presented in the secondary tics products (IARC, 1999). Arithmetic mean concentra- account [CARB, 1992] of this study). tions in petroleum and petrochemical operations in several European countries ranged from 0.1 to 6.4 mg/m3 6.2.3 Food during 1984–1987 (IARC, 1999; European Chemicals Bureau, 2001). Based on occupational hygiene surveys There are no data available concerning the of butadiene production facilities in the United Kingdom, presence or concentrations of butadiene in food in personal airborne exposures are generally below a mean Canada. In the USA, the migration of butadiene from concentration of 5 ppm (11 mg/m3), with most below 1 rubber-modified plastic containers to food was inves- ppm (2.2 mg/m3). In polymer manufacture in the United tigated by McNeal & Breder (1987). Butadiene was Kingdom, most time-weighted average exposures are detected in some of the containers, but was generally below 2–3 ppm (4.4–6.6 mg/m3). Similar concentrations not detected in the foods (detection limits 1–5 ng/g). were reported in other facilities in the European Union Similarly, in the United Kingdom, butadiene was not (IARC, 1999). In monomer production facilities in the detected (detection limit 0.2 ng/g) in five brands of soft USA surveyed in 1985, arithmetic mean concentrations margarine, although its presence was demonstrated (at ranged from 1 to 277 mg/m3, while those in polymer concentrations ranging from <5 to 310 ng/g) in the production industries ranged from 0.04 to 32 mg/m3 plastic containers (Startin & Gilbert, 1984). Butadiene has (IARC, 1999). been detected in the emissions from heated cooking oils, including Chinese rapeseed, peanut, soybean, and canola oils, at levels ranging from 23 to 504 µg/m3 (Pellizzari et al., 1995; Shields et al., 1995).

1 Personal communication from R. Cooper, Department of Biomedical and Environmental Health, School of Public Health, University of California, Berkeley, California, 1989 (cited in CARB, 1992).

10 1,3-Butadiene: Human health aspects

RS H H C C C C OH O 2 H H H H2 H C C C C RS OHOH 2 H OHOH 1,3,4-trihydroxy-2-(N-acetylcysteinyl)butane S-(2-hydroxy-3,4-epoxybut-1-yl)glutathione GSH GT GSH OH OH H O H GT H 2 EH H 2C C C CH2 O RS O O H EH H2 C C C C OH OH H C C C CH H2C C C CH2 H 2 2 H H OHOH H erythritol OHOH 1,2,3,4-diepoxybutane (DEB) 1,2-dihydroxy-3,4-epoxybutane (EBdiol) S-(1-hydroxy-3,4-epoxybut-2-yl)glutathione RS

H 2C C C C OH CO H H 2 O GSH H2 1-hydroxy-2-(N-acetylcysteinly)-3-butene (MII) H2C C C CH 2 GT H C C C CH P450 H H via 1-hydroxy-2-(glutathionyl)-3-butene 2 H H 2 1,2-epoxy-3-butene (EB) OH 1,3-butadiene H C C C C RS 2 H H EH H2 1-(N-acetylcysteinyl)-2-hydroxy-3-butene O via 1-(glutathionyl)-2-hydroxy-3-butene H C C C C 2 H H 2 H 3-butenal H H 2 GSH H H C C C C H 2 2 H RS C C C C OH OH GT H2 H 2 O O OH OH H C C C C 1,3-dihydroxy-3-butene H2C C C CH3 3 1,2-dihydroxy-4-(N-acetylcysteinyl)butane (MI) H H H H crotonaldehyde acrolein P + 450 CO2 OH OH RS O H H H GSH H 2 EH H C C C CH H C C C C OH 2 2 CO 2 H 2C C C C H 2 H H2 GT H OH OH OH OH OHOH erythritol 1,3,4-trihydroxy-2-(N-acetylcysteinyl)butane 1,2-dihydroxy-3,4-epoxybutane (EBdiol)

Fi gure 1: Proposed metabolism of 1,3-butadiene.

7. COMPARATIVE KINETICS AND in vitro than in those of mice, available data are METABOLISM IN LABORATORY ANIMALS insufficient to characterize interindividual variability in AND HUMANS humans. Although there are known genetic polymorphisms for a number of the enzymes involved in the metabolism of butadiene, information on genotype The database on the toxicokinetics and metabolism was not included in most investigations in humans. of butadiene is relatively extensive. The proposed metabolism is outlined in Figure 1, based on the path- Based on the metabolic pathways described in ways described by Henderson et al. (1993, 1996) and Figure 1, butadiene is first oxidized via cytochrome P-450 Himmelstein et al. (1997). Available data for the path- enzymes (primarily P-450 2E1 in humans, although other ways most extensively investigated indicate that isoforms may also be involved, the relative contributions metabolism is qualitatively similar among the various of which vary between tissues and species) to the species studied, although there may be quantitative monoepoxide 1,2-epoxy-3-butene, or EB, which is subse- differences in the amount of butadiene absorbed as well quently further oxidized via P-450 enzymes to the as in metabolic rates and the proportion of metabolites diepoxide 1,2,3,4-diepoxybutane, or DEB, or hydrolysed generated. These differences appear to be in via epoxide hydrolase (EH) to butenediol (1,2-dihydroxy- concordance with the observed variation in sensitivity 3-butene). The monoepoxide, the diepoxide, and the to butadiene-induced toxic effects of the few strains of butenediol may all be conjugated with glutathione (GSH) rodent species tested to date, in that mice appear to to form mercapturic acids, which are eventually metabolize a greater proportion of butadiene to active eliminated in the urine. Hydrolysis of the diepoxide via epoxide metabolites than do rats. While less of these epoxide hydrolase or oxidation of the butenediol via metabolites are also formed in samples of human tissues cytochrome P-450 will result in the formation of the

11 Concise International Chemical Assessment Document 30 monoepoxide diol (EBdiol). A small amount of butadiene DEB may not play a significant role in the induction of may be converted to 3-butenal, which is subsequently mammary tumours in rats. Available in vitro data in transformed to crotonaldehyde (about 2–5% of the human liver and lung samples suggest that humans also amount that is oxidized to the monoepoxide in human form less of the active metabolites of butadiene than do liver microsomes [Duescher & Elfarra, 1994] or mice (although somewhat varying results have been microsomes of kidney, lung, or liver of B6C3F1 mice reported with respect to the magnitude of the differences [Sharer et al., 1992]). However, this pathway has not between species) (Csanády et al., 1992; Duescher & been extensively investigated, nor was crotonaldehyde Elfarra, 1994; Krause & Elfarra, 1997). detected in a sensitive analysis (using nuclear magnetic resonance spectroscopy) of urinary metabolites of rats Although epoxide metabolites of butadiene are and mice exposed to 13C-butadiene (Nauhaus et al., formed to a greater extent in mice than in rats or humans, 1996). they are also cleared via glutathione conjugation more rapidly in mice (Kreuzer et al., 1991; Sharer et al., 1992; Metabolism of butadiene and subsequent conver- Boogaard et al., 1996a, 1996b). Conversely, hydrolysis of sion of EB to DEB may also take place to a more limited EB and DEB is greater in humans than in rats (based on degree in the bone marrow (e.g., Maniglier-Poulet et al., in vitro data, as DEB has not been detected in tissues of 1995) by means other than P-450 oxidation (possibly via exposed humans), and hydrolysis of EB and DEB in rats myeloperoxidase; Elfarra et al., 1996), based on in vitro is in turn greater than that in mice (Csanády et al., 1992; observations and the detection of the epoxides in the Krause et al., 1997). In both humans and monkeys, bone marrow of rodents (Thornton-Manning et al., removal of EB via hydrolysis appears to predominate 1995a, 1995b), although this potential pathway has not over conjugation with glutathione, based on analysis of yet been extensively investigated. EB may also react urinary metabolites (Sabourin et al., 1992; Bechtold et al., with both myeloperoxidase and chloride to form a 1994). Although hydrolysis of the epoxide metabolites is chlorohydrin (1-chloro-2-hydroxy-3-butene) (Duescher & generally considered to be a detoxifying mechanism, it Elfarra, 1992). Metabolites arising from other possible may also lead to the formation of the diolepoxide, EBdiol, pathways have been identified in the urine of mice which is biologically reactive. However, no data were exposed to butadiene (including metabolites known to identified on species differences in the formation of be derived from metabolism of acrolein or acrylic acid) EBdiol via metabolism of both epoxide metabolites. (Nauhaus et al., 1996), but no further research has yet been generated. The formation of stable adducts of both the mono- epoxide and monoepoxide diol metabolites of butadiene There is a substantial amount of evidence from in with the N-terminal valine of haemoglobin has been vitro and in vivo investigations that B6C3F1 mice oxidize observed in experimental animals and humans exposed butadiene to the monoepoxide via P-450 in the liver to a to butadiene (Albrecht et al., 1993; Osterman-Golkar et greater extent than do Sprague-Dawley rats and humans. al., 1993, 1996; Neumann et al., 1995; Sorsa et al., 1996b; Levels of EB in the blood and other tissues of mice were Tretyakova et al., 1996; Pérez et al., 1997).1 Consistent two- to eightfold higher than those in rats exposed to with the greater formation of epoxide metabolites, greater similar levels of butadiene (Bond et al., 1986; Himmel- concentrations of haemoglobin–EB adducts were stein et al., 1994, 1995; Bechtold et al., 1995; Thornton- measured in mice than in rats exposed to the same Manning et al., 1997). concentration of butadiene. However, levels of haemoglobin–EB adducts in butadiene-exposed workers, Available data also suggest that there are similar although significantly elevated compared with levels in species differences in the amount of the diepoxide non-exposed workers, were considerably less than formed from oxidation of the monoepoxide. Levels of would be expected on the basis of results of studies in DEB were 40- to 160-fold higher in blood and other mice and rats (Osterman-Golkar et al., 1993). Based on tissues of B6C3F1 mice than in Sprague-Dawley rats observations in rats and humans exposed to butadiene, exposed to the same concentration of butadiene (Thorn- levels of haemoglobin–EBdiol adducts are substantially ton-Manning et al., 1995a, 1995b). While concentrations greater than levels of haemoglobin–EB adducts of EB at various sites were similar in male and female (although it is noted that the same adduct can result rats, levels of DEB were at least fivefold higher in from binding with DEB). Metabolites of butadiene may females than in males, which correlates with the greater also form adducts with DNA (see sections 8.5 and 9.2.3). incidence of tumours in female rats. Although the mam- mary gland is a target tissue in rats, extended exposure to butadiene at 8000 ppm (17 696 mg/m3) for 10 days did not result in any accumulation of DEB at this site (Thornton-Manning et al., 1998), which suggests that 1 Also personal communication (correspondence dated 25 March 1998) from J.A. Swenberg, University of North Carolina, Chapel Hill, NC, to Health Canada.

12 1,3-Butadiene: Human health aspects

In addition to quantitative interspecies differences (Deutschmann & Laib, 1989). Depletion of non-protein in the metabolism of butadiene, there is also evidence sulfhydryl content may inhibit detoxification of epoxide that there is significant variation within the human popu- metabolites vie glutathione conjugation. lation. Indeed, although available data are inadequate to assess interindividual variation in metabolism, which has 8.2 Irritation and sensitization been observed in in vitro investigations in microsomes from a small number of subjects (Boogaard & Bond, No investigations in experimental animals on the 1996; Krause et al., 1997), there has been significant potential for irritation or sensitization of butadiene have interindividual variability in the extent of formation of been identified. haemoglobin adducts with butadiene metabolites in human populations (Neumann et al., 1995; Osterman- 8.3 Repeated exposure Golkar et al., 1996). Such variability is not unexpected, in view of the complexity of the metabolic pathways The majority of short-term and subchronic studies involved in the biotransformation of butadiene: i.e., the were designed as either range-finding studies three principal enzymatic processes that determine the preliminary to chronic bioassays or investigations of extent of exposure to the putatively toxic epoxide potential mechanisms of action for butadiene-induced metabolites, namely formation via cytochrome P-450 2E1 cancer and are not adequate for determination of critical and removal via epoxide hydrolase and glutathione effect levels. Effects on body weight were observed in 3 conjugation. For example, the inducibility of cytochrome B6C3F1 mice exposed to 625 ppm (1383 mg/m ) butadiene P-450 2E1 by low molecular weight compounds such as or more for 2 weeks; no histopathological changes were ethanol is likely to contribute to interindividual variabil- noted at any concentration at or below 8000 ppm (17 696 ity in sensitivity. Moreover, genetic polymorphisms for mg/m3) (NTP, 1984). glutathione-S-transferases and epoxide hydrolase might also contribute to considerable variation in sensitivity. Haematological effects consistent with megalo- While the influence of genotype for epoxide hydrolase blastic anaemia and effects on bone marrow, including has not been well investigated (although data indicate alterations in stem cell development, have been that hydrolysis of EB predominates over oxidation and observed in two strains of mice (B6C3F1 and NIH Swiss) glutathione conjugation in humans), interindividual exposed to 1000 or 1250 ppm (2212 or 2765 mg/m3) sensitivity to the genetic effects of the epoxide metabo- butadiene for up to 31 weeks (Irons et al., 1986a, 1986b; lites in in vitro studies has been clearly related to geno- Leiderman et al., 1986; Bevan et al., 1996). Other effects, type for the glutathione-S-transferases (see section including decreased survival and body weight gain (with 9.2.3). males being more sensitive than females), altered organ weights, and ovarian or testicular atrophy, have also

been observed in B6C3F1 mice exposed subchronically to similar or higher levels of butadiene (NTP, 1984; 8. EFFECTS ON LABORATORY Bevan et al., 1996). In addition, an increased incidence of

MAMMALS AND IN VITRO TEST SYSTEMS a variety of tumours has been observed in B6C3F1 mice exposed to 625 ppm (1383 mg/m3) butadiene for as little as 13 weeks (NTP, 1993) (see section 8.4). Although 8.1 Single exposure histopathological changes and haematological effects were reported in early studies in rats exposed to low 3 Although few data are available, butadiene concentrations (3 or 10 mg/m ) (Batinka, 1966; Ripp, appears to be of low acute toxicity in experimental 1967; Nikiforova et al., 1969), these results were not confirmed in more recent investigations of rats exposed animals, with reported LC50 values for rats and mice of 3 for up to 13 weeks to much higher concentrations (e.g., >100 000 ppm (>221 000 mg/m ). Lowest LC50 values for 3 butadiene are reported for mice, at 117 000 ppm (256 000 17 600 mg/m ) (e.g., Crouch et al., 1979; Bevan et al., mg/m3) (duration not specified) (Batinka, 1966) and 1996). In view of the limitations of the studies in rats, it is 121 000 ppm (268 000 mg/m3) (2 h) (Shugaev, 1969). The not possible to draw any conclusions regarding species nervous system and the blood appear to be the principal differences in response to subchronic exposure to targets; however, in only one study were data sufficient butadiene. to determine a lowest-observed-effect level (LOEL) of 200 ppm (442 mg/m3) for haematological effects (Leavens 8.4 Carcinogenicity et al., 1997). Exposure to butadiene for 7 h caused a concentration-dependent depletion (by as much as 80%) The carcinogenic potential of inhaled butadiene of cellular non-protein sulfhydryl content of liver, lung, has been studied in two strains of mice and one strain or heart in mice, with a LOEL of 100 ppm (221 mg/m3) of rats. Butadiene was a multi-site carcinogen in all

13 Concise International Chemical Assessment Document 30 identified long-term experiments, inducing common and The NTP also conducted a “stop-exposure” experi- rare tumours in mice and rats, although there appear to ment in male B6C3F1 mice designed to investigate be marked species and strain differences in sensitivity. whether tumour induction was associated with the In an early bioassay by the National Toxicology Program exposure concentration or the duration of exposure. 3 (NTP, 1984) in which male and female B6C3F1 mice were Animals were exposed to 200 ppm (442 mg/m ) for exposed to 0, 625, or 1250 ppm (0, 1383, or 2765 mg/m3) 40 weeks or 625 ppm (1383 mg/m3) for 13 weeks (both butadiene for up to 61 weeks, there were exposure- equivalent to 8000 ppm-weeks) or to 312 ppm (690 mg/m3) related increases in the incidences of malignant for 52 weeks or 625 ppm (1383 mg/m3) for 26 weeks (both lymphomas, cardiac haemangiosarcomas (an extremely equivalent to 16 000 ppm-weeks); all groups were rare tumour in B6C3F1 mice), and lung tumours in both observed for the remainder of the 2-year study. Again, sexes. There were also increased incidences of papillo- survival was reduced in all exposed mice, largely due to mas or carcinomas of the forestomach, hepatocellular malignant neoplasms, with significant increases in the adenomas or carcinomas, ovarian granulosa cell incidences of lymphocytic lymphomas, histiocytic tumours, acinar cell carcinomas of the mammary gland, sarcomas, cardiac haemangiosarcomas, Harderian gland brain gliomas, and Zymbal gland carcinomas (the latter adenomas or carcinomas, hepatocellular adenomas or two tumour types have only rarely been observed in this carcinomas, alveolar/bronchiolar adenomas or strain of mice at the NTP) in one or both sexes. carcinomas, and squamous cell papillomas or carcinomas of the forestomach (even in mice exposed for only 13 Because of the poor survival of mice in the earlier weeks) (incidence data presented in Table 2). In addition, bioassay and to better characterize the exposure– low incidences of several uncommon tumour types response relationship, the NTP (1993) subsequently (preputial gland carcinomas, Zymbal gland carcinomas, exposed B6C3F1 mice to lower concentrations (0, 6.25, 20, malignant gliomas and neuroblastomas of the brain, 62.5, 200, or 625 ppm [0, 13.8, 44.2, 138, 442, or 1383 Harderian gland carcinomas, and renal tubule adenomas) mg/m3]) of butadiene for up to 2 years. Survival was were again observed in one or more of the exposed again decreased in most groups ($20 ppm groups. Concentration may be more important than the [$44.2 mg/m3]); at the highest concentration, deaths duration of exposure in tumour development, as the were principally due to lymphatic lymphomas, which incidence of malignant lymphomas and squamous cell appeared to arise from the thymus and occurred as early carcinomas of the forestomach was greater in the groups as week 23. Non-neoplastic effects observed in exposed that had been exposed to 625 ppm (1383 mg/m3) for a mice included a variety of haematological effects, alter- shorter period than in those exposed to 200 ppm (442 ations in organ weights, bone marrow atrophy and mg/m3) for a longer period (i.e., similar total cumulative hyperplasia, atrophy of the thymus, atrophy and exposure) (NTP, 1993). angiectasis of the ovaries, uterine atrophy, mineralization of the cardiac endothelium, liver necrosis, and olfactory Acute exposure of B6C3F1 mice for 2 h to up to epithelial atrophy. There were significant increases in the 10 000 ppm (22 120 mg/m3) butadiene, followed by incidence of tumours at a variety of sites (incidence data observation for 2 years, did not induce an increased presented in Table 2), including malignant lymphomas incidence of tumours at any site (Bucher et al., 1993). (particularly lymphocytic lymphomas), histiocytic sarco- mas, cardiac haemangiosarcomas, Harderian gland Sensitivity to butadiene-induced thymic lympho- adenomas and carcinomas, hepatocellular adenomas and ma/leukaemia appears to be enhanced by the presence of carcinomas, alveolar/bronchiolar adenomas and carcino- an endogenous ecotropic retrovirus in B6C3F1 mice, as mas, mammary gland adenoacanthomas, carcinomas, the incidence of this tumour was greater in male B6C3F1 and malignant mixed tumours, ovarian granulosa cell mice exposed to 1250 ppm (2765 mg/m3) butadiene for 52 tumours, and forestomach squamous cell papillomas and weeks than in male Swiss mice, which do not express an carcinomas, particularly when the incidences were endogenous retrovirus (57% versus 14%). Exposed mice adjusted for survival. The incidence of alveolar/bronchi- of both strains had elevated incidences of thymic olar adenomas or carcinomas was significantly increased lymphoma/leukaemia compared with controls, as did 3 in females at all concentrations (i.e., $6.25 ppm B6C3F1 mice exposed to 1250 ppm (2765 mg/m ) for [$13.8 mg/m3]). Low incidences of uncommon tumours, 12 weeks and then observed for an additional 40 weeks, such as preputial gland carcinomas, Zymbal gland car- although the MuLV env sequence for the retrovirus was cinomas in males, and renal tubule adenomas in both detected only in tumours of the B6C3F1 mice. Other sexes, were also suspected of being related to exposure. tumours reported in the mice exposed for 52 weeks In addition, exposure to butadiene induced malignant included haemangiosarcomas of the heart (mainly in tumours at several sites, whereas, in general, tumours at B6C3F1 mice) and lung tumours. Neoplasms of the the same sites in control animals were benign. glandular and non-glandular stomach were observed in

14 Table 2: Incidences of neoplastic lesions in critical carcinogenicity bioassays for butadiene in B6C3F1 mice (NTP, 1993).

Protocol Results Comments Mice (70 males and 70 Numbers of animals surviving until study termination were 35, 39, 24, 22, 4, and 0 (males) and 37, 33, 24, 11, 0, and 0 (females) Haemangiosarcomas had not previously females per group; 90 males at 0, 6.25, 20, 62.5, 200, and 625 ppm, respectively. been observed in 573 male and 558 and 90 females per group at female NTP 2-year historical controls. the highest concentration) were exposed to 0, 6.25, 20, Renal tubule adenomas are rare in this 62.5, 200, or 625 ppm (0, strain, with a range of 0–1% occurrence 13.8, 44.2, 138, 442, or 1383 in NTP 2-year historical controls. mg/m3) butadiene for 6 h/day, Lymphohaematopoietic system 5 days/week, for 103 weeks. Exposure was associated with the development of malignant lymphomas (particularly lymphocytic lymphomas, which occurred as Carcinomas of the preputial gland are Up to 10 mice of each sex early as week 23). The incidences were significantly increased in males at 625 ppm (p < 0.001) and females at 200 and 625 ppm rare in B6C3F1 mice, with none having from each group were killed (p < 0.001) (although all incidences in the females were within the range of historical control values [8–44%]). been observed in NTP historical after 9 and 15 months of Incidences for the 0, 6.25, 20, 62.5, 200, and 625 ppm groups were: controls; similarly, sarcomas of the exposure. males: 4/50, 2/50, 4/50, 6/50, 2/50, and 51/73, or 8, 4, 8, 12, 4, and 70% subcutaneous tissue are uncommon, females: 6/50, 12/50, 11/50, 7/50, 9/50, and 32/80, or 12, 24, 22, 14, 18, and 40% with the incidence in NTP historical Histopathological After poly-3 adjustment for survival, the incidences were: control females being 2/561. examination of a males: 9.0, 4.4, 10.0, 15.3, 7.4, and 97.3% comprehensive range of females: 13.1, 27.2, 27.5, 20.2, 40.1, and 85.5% Zymbal gland neoplasms have not been tissues was carried out on Histiocytic sarcomas were significantly increased in both males (p < 0.001) and females (p = 0.002) at 200 ppm, and the observed in NTP historical controls, nor mice in the control and 200 incidence of these tumours was marginally higher than that in controls in males at 20, 62.5, and 625 ppm (p = 0.021–0.051) and have carcinomas of the small intestine and 625 ppm exposure groups females at 625 ppm (p = 0.038). been observed in recent NTP controls. killed after 9 months, on all Incidences: mice killed at 15 months males: 0/50, 0/50, 4/50, 5/50, 7/50, and 4/73, or 0, 0, 8, 10, 14, and 5% Exposure to butadiene tended to be except females exposed to females: 3/50, 2/50, 7/50, 4/50, 7/50, and 4/80, or 6, 4, 14, 8, 14, and 5% associated with malignant neoplasms in 6.25 or 20 ppm, and on all Adjusted incidences: several organs, whereas tumours at the mice exposed for 2 years. males: 0, 0, 10.0, 12.9, 24.3, and 50.7% same sites in controls were generally females: 6.5, 4.4, 17.2, 11.8, 34.0, and 35.5% benign. Heart The incidences of cardiac haemangiosarcomas were significantly increased compared with controls in males at 62.5 ppm and above and in females at 200 ppm and above. Incidences: males: 0/50, 0/49, 1/50, 5/48, 20/48, and 4/73, or 0, 0, 2, 10, 42, and 5% females: 0/50, 0/50, 0/50, 1/49, 21/50, and 23/80, or 0, 0, 0, 2, 42, and 29% Adjusted incidences: males: 0, 0, 2.6, 13.5, 64.1, and 52.9% females: 0, 0, 0, 3.1, 71.9, and 83.4% Lungs There was evidence of increased incidences of alveolar/bronchiolar adenomas or carcinomas compared with controls in males at 62.5 ppm and above (p < 0.001) and in females at all concentrations (p < 0.001–0.004). Incidences: males: 21/50, 23/50, 19/50, 31/49, 35/50, and 3/73, or 42, 46, 38, 63, 70, and 4% females: 4/50, 15/50, 19/50, 24/50, 25/50, and 22/78, or 8, 30, 38, 48, 50, and 28% Adjusted incidences: males: 47.5, 49.0, 44.9, 74.2, 87.8, and 45.1% females: 8.8, 33.0, 46.5, 61.1, 81.5, and 82.4% Forestomach An increased incidence of forestomach tumours (squamous cell papillomas or carcinomas) was observed in males at 200 and 625 ppm (p < 0.001) and females at 62.5 ppm and above (p < 0.001–0.044). Incidences: males: 1/50, 0/50, 0/50, 1/50, 8/50, and 4/73, or 2, 0, 0, 2, 16, and 5% females: 0/50, 0/50, 3/50, 2/50, 4/50, and 22/80, or 0, 0, 6, 4, 8, and 28% Adjusted incidences: males: 2.3, 0, 0, 2.7, 28.7, and 53.5% females: 0, 0, 7.8, 6.1, 22.5, and 82.6% Table 2 (contd). Protocol Results Comments Ovary Increased incidences of malignant and benign granulosa cell tumours were reported in females exposed to 62.5 ppm and above (p < 0.001). Incidences (benign and malignant): 1/49, 0/49, 1/48, 9/50, 8/50, and 6/79, or 2, 0, 2, 18, 16, and 8% Adjusted incidences: 2.3, 0, 2.6, 26.3, 41.1, and 46.5% Harderian gland The incidence of Harderian gland adenomas and carcinomas was increased in both sexes at 62.5 and 200 ppm (p < 0.001–0.016). Incidences: males: 6/50, 7/50, 9/50, 20/50, 31/50 and 6/73, or 12, 14, 18, 40, 62 and 8% females: 8/50, 10/50, 7/50, 15/50, 20/50, and 9/80, or 16, 20, 14, 30, 40, and 11% Adjusted incidences: males: 13.5, 15.2, 22.4, 50.8, 80.6, and 64.2% females: 17.5, 22.7, 17.4, 41.2, 70.9, and 58.0% Mammary gland The incidence of mammary gland tumours (adenoacanthomas, carcinomas, and malignant mixed tumours) was increased in females at 62.5 ppm and above (p < 0.001–0.004). Most of the neoplasms were carcinomas. Incidences: 0/50, 2/50, 4/50, 12/50, 15/50, and 16/80, or 0, 4, 8, 24, 30, and 20% Adjusted incidences: 0, 4.5, 10.2, 32.6, 56.4, and 66.8% Liver In males, the incidence of hepatocellular neoplasms (adenomas and carcinomas) at 200 ppm was significantly greater than that in the controls (p = 0.03). The authors also reported increases in hepatocellular neoplasms at 62.5 ppm in females (p = 0.027). Incidences (adenomas and carcinomas): males: 21/50, 23/50, 30/50, 25/48, 33/48, and 5/72, or 42, 46, 60, 52, 69, and 7% females: 15/49, 14/49, 15/50, 19/50, 16/50, and 2/80, or 31, 29, 30, 38, 32, and 3% Adjusted incidences (adenomas and carcinomas): males: 44.6, 48.2, 65.2, 61.6, 85.9, and 61.2% females: 33.3, 30.3, 36.4, 51.4, 64.9, and 21.7% Other tumours Low incidences of certain uncommon neoplasms also occurred in exposed mice and were considered to be probably related to treatment. These included preputial gland carcinomas (in 5/50 males at 200 ppm, p < 0.001) and renal tubule adenomas in both sexes (in 1/50 males at 6.25, 3/48 at 62.5 ppm, and 1/49 at 200 ppm, and in 2/50 females at 200 ppm, compared with none in controls). Tumours at other sites that “may be related to exposure” included neurofibrosarcomas or sarcomas of the subcutaneous tissue in females (1/50, 2/50, 3/50, 5/50, 3/50, and 3/80) and Zymbal gland neoplasia (one adenoma in control males, one adenoma and one carcinoma in females at 625 ppm). The investigators were uncertain whether the low incidence of carcinomas of the small intestine in the treated animals (females: 2 at 6.25 ppm, 1 at 625 ppm, males: 1 each at 6.25, 20, and 62.5 ppm, 2 at 200 ppm, compared with none in controls) was exposure related. Table 2 (contd). Protocol Results Comments Mice (males; 50 per group) Lymphohaematopoietic system Renal tubule adenomas have only rarely were exposed to butadiene for The incidence of malignant lymphomas (the majority of which were lymphocytic lymphomas) was markedly increased in both been observed in NTP historical controls 6 h/day, 5 days/week, at groups exposed to 625 ppm (p < 0.001) and occurred as early as 23 weeks in the 625 ppm (26 weeks) group. (1/571). concentrations of 200 ppm Incidences: 4/50, 8/50, 22/50, 8/50, and 33/50, or 8, 16, 44, 16, and 66%, in the control, 200 ppm (40 weeks), 625 ppm (13 (442 mg/m3) for 40 weeks weeks), 312 ppm (52 weeks), and 625 ppm (26 weeks) groups, respectively. Malignant gliomas and neuroblastomas (equivalent to a total Poly-3 adjusted incidences (adjusted for survival): 9.0, 24.1, 56.1, 35.0, and 87.2% of the brain rarely develop exposure of 8000 ppm-weeks), The incidence of histiocytic sarcomas was increased in exposed groups (p < 0.001– 0.036). spontaneously in this strain of mice, with 312 ppm (690 mg/m3) for 52 Incidences: 0/50, 5/50, 2/50, 7/50, and 2/50, or 0, 10, 4, 14, and 4% none having been observed in 574 NTP weeks (16 000 ppm-weeks), or Adjusted incidences: 0, 16.3, 9.4, 30.6, and 20.4% historical controls. 625 ppm (1383 mg/m3) for 13 or 26 weeks (8000 and 16 000 Heart ppm-weeks, respectively). The incidence of cardiac haemangiosarcomas was significantly (p < 0.001) increased in all groups, but particularly in mice After exposure ceased, mice exposed to 200 or 312 ppm. were kept in control chambers Incidences: 0/50, 15/50, 7/50, 33/50, and 13/50, or 0, 30, 14, 66, and 26% until 103 weeks and Adjusted incidences: 0, 47.1, 30.9, 85.2, and 74.5% evaluated. Histopathological Lungs examination of a There was a significant (p < 0.001) increase in the incidence of pulmonary neoplasms (alveolar/bronchiolar adenoma or comprehensive range of carcinoma) in all exposed groups, particularly when the figures were adjusted to take account of mortality. tissues was conducted on all Incidences: 21/50, 36/50, 28/50, 32/50, and 17/50, or 42, 72, 56, 64, and 34% mice. Adjusted incidences: 47.5, 88.6, 89.5, 88.0, and 87.2% Liver The incidence of adenomas or carcinomas in the liver was significantly greater in the 200 ppm group (p = 0.004) than in the controls and in all exposed groups when adjusted for survival (p < 0.01–0.05). Incidences: 21/50, 33/49, 24/49, 24/50, and 13/50, or 42, 67, 49, 48, and 26% Adjusted incidences: 44.6, 82.4, 80.3, 75.9, and 77.3% Forestomach There was a significant (p < 0.001) increase in the incidence of squamous cell papillomas or carcinomas of the forestomach in mice exposed to 312 or 625 ppm (both 13 and 26 weeks). Incidences: 1/50, 3/50, 7/50, 9/50, and 10/50, or 2, 6, 14, 18, and 20% Adjusted incidences: 2.3, 10.2, 28.7, 39.2, and 60.7% Harderian gland The incidence of Harderian gland adenomas or carcinomas was significantly (p < 0.001) increased compared with controls in all exposed groups. Incidences: 6/50, 27/50, 23/50, 30/50, and 13/50, or 12, 54, 46, 60, and 26% Adjusted incidences: 13.5, 72.1, 82.0, 88.6, and 76.5% Other tumours The incidences of kidney adenomas were 0/50, 4/48, 1/50, 3/49, and 1/50 in the control, 200 ppm (40 weeks), 625 ppm (13 weeks), 312 ppm (52 weeks), and 625 ppm (26 weeks) groups, respectively. The incidence of adenomas or carcinomas of the preputial gland was significantly (p < 0.001–0.003) increased in the 312 ppm and 625 ppm (13 or 26 weeks) groups, with incidences of 0/50, 1/50, 5/50, 4/50, and 3/50. Malignant gliomas, which were considered to be exposure related, occurred in two mice exposed to 625 ppm for 13 weeks and one exposed to 625 ppm for 26 weeks. Malignant neuroblastomas were observed in two mice exposed to 625 ppm for 13 weeks. The incidence of adenomas or carcinomas of the Zymbal gland was significantly (p = 0.009) increased in mice exposed to 625 ppm for 26 weeks (1/50, 1/50, 0/50, 2/50, and 2/50). Concise International Chemical Assessment Document 30

the B6C3F1 mice, whereas adenocarcinomas of the 8.5 Genotoxicity and related end-points Harderian gland and the thyroglossal duct were observed in the Swiss mice (Irons et al., 1989). The genotoxicity of butadiene has been investi- gated in a limited range of in vitro assays and a more In the only identified long-term bioassay in rats extensive range of in vivo tests. Butadiene was muta- (Hazleton Laboratories Europe Ltd., 1981a; Owen et al., genic in Salmonella typhimurium strains TA1530 and 1987; Owen & Glaister, 1990), male and female Sprague- TA1535 in the presence of metabolic activation with Dawley rats were exposed to 0, 1000, or 8000 ppm (0, rodent or human S9 preparations (de Meester et al., 1978, 2212, or 17 696 mg/m3) butadiene for up to 111 weeks. At 1980; Arce et al., 1990; NTP, 1993; Araki et al., 1994), 8000 ppm (17 696 mg/m3), survival was reduced in both although it was generally inactive in strains TA97, TA98, sexes; there were also changes in the relative weights of and TA100 with or without exogenous activation under a number of organs in males at this concentration, along similar experimental conditions (Victorin & Ståhlberg, with an increase in the severity of nephrosis of the 1988; Arce et al., 1990; NTP, 1993). Results of mouse kidney relative to controls. Relative liver weights were lymphoma assays have been conflicting, with an increased in all exposed groups, although there were no increased frequency of mutations at the tk locus in one exposure-related histopathological effects on the liver. study at very high concentrations (i.e., 200 000–800 000 At 8000 ppm (17 696 mg/m3), there were increased ppm [442 400–1 796 600 mg/m3]) in the presence of incidences of follicular cell adenomas and carcinomas of metabolic activation (Sernau et al., 1986), while there was the thyroid gland in females and exocrine adenomas of no convincing activity at concentrations of up to 300 000 the pancreas in males (with a carcinoma occurring in a rat ppm (663 600 mg/m3) in another study (although the of either sex) (incidence data presented in Table 3). In authors noted that the lack of a positive response may females, the incidence of benign or malignant mammary have been due to the low solubility of butadiene in the gland tumours, along with the incidence of animals with culture medium; NTP, 1993). Butadiene dissolved in multiple mammary gland tumours, was increased at both ethanol induced sister chromatid exchanges in cultured 1000 and 8000 ppm (2212 and 17 696 mg/m3). The mammalian cells (hamsters and humans) (Sasiadek et al., incidence of sarcomas of the uterus and carcinomas of 1991a, 1991b), while in vitro exposure to gaseous the Zymbal gland increased significantly with level of butadiene did not induce this effect in preparations from exposure in females; in addition, a Zymbal gland rats, mice, and humans (Arce et al., 1990; Walles et al., carcinoma occurred in one male rat at each exposure 1995). level. The incidence of Leydig cell tumours of the testes was increased in both groups of exposed males. The An overview of the results of available in vivo investigators suggested that the occurrence of tumours assays for genotoxicity in germ and somatic cells in mice of the testes and Zymbal gland may have been unrelated and rats is presented in Table 4; in general, the data are to exposure, as the incidences observed were reportedly consistent with species-specific differences in sensitivity similar to those in other control rats of the same strain in to butadiene-induced genetic damage, likely related to the study laboratory, although it is noted that Zymbal the quantitative differences in the formation of active gland tumours were noted in the chronic bioassays in metabolites, although fewer studies have been mice discussed above. conducted in rats. Butadiene induced dominant lethal mutations in two strains of mice (CD-1 and (102/E1 ×

Both the mono- and diepoxide metabolites (EB and C3H/E1)F1) following short-term or subchronic exposure DEB) have induced local tumours at the site of applica- of males to concentrations as low as 500 ppm (1106 tion in Swiss mice or Sprague-Dawley rats (Van Duuren mg/m3) for 5 days or 65 ppm (144 mg/m3) for 4 weeks; et al., 1963, 1965, 1966), although available studies are however, exposure to 6250 ppm (13 825 mg/m3) for 6 h inadequate to evaluate species differences in sensitivity. did not induce dominant lethal mutations in CD-1 mice. The results of these studies, which depended upon the It has been hypothesized that the observed greater timing of mating relative to exposure, suggested that the sensitivity of B6C3F1 mice compared with Sprague- induction of dominant lethal mutations in mice was likely Dawley rats to the induction of thymic lymphoma by caused by effects on mature germ cells. In the only simi- butadiene may be related to differences in the potential lar study in rats identified, there was no evidence of of EB to affect haematopoietic stem cell differentiation dominant lethal mutations in Sprague-Dawley rats observed in in vitro investigations, as suppression of exposed to up to 1250 ppm (2765 mg/m3) butadiene for 10 clonogenic response was greater in bone marrow cells weeks. from C56BL/6 mice than in those from Sprague-Dawley rats or humans; it was also hypothesized that the sub- Short-term exposure to 500 or 1300 ppm (1106 or population of progenitor cells affected in mice is not 2876 mg/m3) butadiene also induced an exposure-related present in humans (Irons et al., 1995). increase in the incidence of heritable chromosomal

18 Table 3: Incidence of neoplastic lesions in critical carcinogenicity bioassays for butadiene in Sprague-Dawley CD rats (Hazleton Laboratories Europe Ltd., 1981a; Owen et al., 1987; Owen & Glaister, 1990).

Protocol Results Comments Rats (110 males and 110 females Survival at study termination was 45, 51, and 32% (males) and 48, 34, and 25% (females) at 0, 1000, The authors indicated that the incidence of per group) were exposed to and 8000 ppm, respectively (based on interpretation of survival curves in published accounts). pancreatic adenomas may be overestimated, due to concentrations of 0, 1000, or 8000 difficulties in distinguishing between adenomas and 3 ppm (0, 2212, or 17 696 mg/m ) Mammary gland hyperplastic foci or nodules in this organ. butadiene for 6 h/day, 5 The incidence of total mammary gland tumours (adenomas or carcinomas) was significantly increased days/week. Ten rats per sex per in treated females in both groups (p < 0.01; incidences 50/100, 79/100, and 81/100 in the control, The authors noted that the incidence of testicular group were killed at 52 weeks. The 1000, and 8000 ppm groups, respectively). The positive trend was significant (p < 0.001). The tumours was similar to that observed in historical study was terminated at 105 weeks incidence of multiple mammary gland tumours was also increased in exposed females (8/100, 42/100, controls at Hazleton Laboratories (i.e., 0–6%). in females and 111 weeks in and 38/100, or 1.38, 3.70, and 3.33 adenomas per adenoma-bearing rat; latter values from Melnick & males. A comprehensive range of Huff, 1992). It was stated that the incidences of both uterine tissues from rats at the high sarcomas and Zymbal gland carcinomas were similar concentration and control rats and Pancreas to those reported in untreated Sprague-Dawley rats at a more limited range from rats at The incidence of pancreatic exocrine adenomas was increased at 8000 ppm in males (p < 0.001) the study laboratory and may not have been the lower concentration were (incidences 3/100, 1/100, and 10/100); exocrine carcinomas also occurred in one male and one female treatment related. The authors also indicated that examined microscopically in exposed to 8000 ppm, compared with none in other groups. additional support for the observed increases in animals killed after 52 weeks and tumour incidences not being associated with exposure at the end of the study. Testes was provided by the fact that the majority of the There was a significant (p < 0.01) exposure-related increase in the incidence of Leydig cell tumours in Zymbal gland tumours were present in animals killed the testis (incidences 0/100, 3/100, and 8/100). within 76–90 weeks, while none was observed at the end of the study. Thyroid gland In females, there were significant exposure-related trends for the occurrence of thyroid follicular cell adenomas or carcinomas (0/100, 4/100, and 11/100; p < 0.001) and uterine sarcomas (1/100, 4/100, and 5/100; p < 0.05). Other tumours The incidence of Zymbal gland carcinomas was significantly increased (p < 0.01) at the highest concentration in females (0/100, 0/100, and 4/100); in males, the incidences were 0/100, 1/100, and 1/100. Concise International Chemical Assessment Document 30

Table 4: Overview of genotoxicity of butadiene and its metabolites in rodents.

End-point Mice (strain) Rats (strain) Comments References BUTADIENE Germ cells

Dominant lethal + (CD-1) ! (Sprague-Dawley) results in mice depended upon Morrissey et al., 1990; mutations duration of exposure and timing Anderson et al., 1993; of exposure relative to mating; Adler et al., 1994, 1998; rats were exposed to BIBRA International, + ((102/E1 × C3H/E1)F1) concentrations similar to those 1996a, 1996b; Brinkworth that induced effects in mice et al., 1998

Heritable + (C3H/E1) NT Adler et al., 1995a, 1998 translocations

Other genetic effects + ((102/E1 × C3H/E1)F1) NT Morrissey et al., 1990; Xiao on male germ cells & Tates, 1995; Brinkworth (chromosomal + (CD-1) et al., 1998; Pacchierotti et aberrations in al., 1998a; Tommasi et al., embryos, DNA 1998 + (B6C3F1) damage, sperm head morphology, + (102 × C3H) micronuclei)

Somatic cells

Chromosomal + (B6C3F1) NT Irons et al., 1987; Tice et aberrations (bone al., 1987; Shelby, 1990; marrow) + (Swiss) NTP, 1993

Sister chromatid + (B6C3F1) ! (Sprague-Dawley) rats were exposed to much higher Choy et al., 1986; exchanges (bone concentrations than those that Cunningham et al., 1986; marrow) induced effects in mice Tice et al., 1987; Arce et al., 1990; Shelby, 1990; NTP, 1993

Micronuclei (bone + (NMRI) ! (Sprague-Dawley) effects in mice were observed at Choy et al., 1986; marrow, blood, the lowest concentration tested Cunningham et al., 1986; spleen) (i.e., 6.25 ppm); male mice Irons et al., 1986a, 1986b; + (B6C3F1) ! (Wistar) appeared to be more sensitive Tice et al., 1987; Jauhar et than female mice; rats were al., 1988; Arce et al., 1990;

+ (CB6F1) exposed to concentrations similar Shelby, 1990; Victorin et to those that induced effects in al., 1990; NTP, 1993; mice Przygoda et al., 1993; +((102/E1 × C3H/E1)F1) Adler et al., 1994; Autio et al., 1994; Leavens et al., + (NIH Swiss) 1997; Stephanou et al., 1998

– hprt mutations + ((102/E1 × C3H/E1)F1) + (F344) mice appeared to be more Cochrane & Skopek, 1993, (spleen, thymus) sensitive than rats 1994b; Tates et al., 1994, + (B6C3F1) 1998; Meng et al., 1998, ! (CD-1) 2000

Specific locus + ((102/E1 × C3H/E1)F1) NT Adler et al., 1994 mutations (mouse spot test)

Transgenic systems + (CD2F1 derived) NT Recio et al., 1992, 1993, (lacZ, lacI) 1996; Sisk et al., 1994; + (B6C3F1 derived) Recio & Meyer, 1995

Unscheduled DNA ! (B6C3F1) ! (Sprague-Dawley) Vincent et al., 1986; Arce synthesis (liver) et al., 1990

DNA–DNA or +/! (B6C3F1) ! (Sprague-Dawley) Jelitto et al., 1989; Ristau DNA–protein cross- et al., 1990; Vangala et al., links (liver) 1993

DNA binding (liver, + (B6C3F1) + (Wistar) levels of adducts were slightly Kreiling et al., 1986; Sorsa lung) higher in mice than in rats et al., 1996b; Koivisto et + (CB6F1 ) + (Sprague-Dawley) al., 1997, 1998; Tretyakova + (F344) et al., 1998a, 1998b

20 1,3-Butadiene: Human health aspects

Table 4 (contd). End-point Mice (strain) Rats (strain) Comments References

DNA strand breaks + (B6C3F1) + (Sprague-Dawley) results were dependent on Vangala et al., 1993; and other damage analytical method used; there was Walles et al., 1995; (liver, lung, testes) + (NMRI) little quantitative species Anderson et al., 1997 difference in the degree of strand ! (CD-1) breakage

1,2-EPOXY-3-BUTENE (EB)

Germ cells

Dominant lethal ! ((102/E1 × NT Adler et al., 1997 mutations C3H/E1)F1)

Other genetic effects + (F1(102 × C3H)) + (Lewis) Lewis rats appeared to be slightly Xiao & Tates, 1995; on male germ cells more sensitive than mice Lähdetie et al., 1997; (micronuclei) + (BALB/c) + (Sprague-Dawley) Russo et al., 1997

Somatic cells

Chromosomal + (C57Bl/6) NT Sharief et al., 1986 aberrations (bone marrow) Sister chromatid + (BALB/c) NT Stephanou et al., 1997 exchanges (spleen)

Micronuclei (spleen, + (F1(102 × C3H)) + (Lewis) (F1(102 × C3H) mice appeared to Xiao & Tates, 1995; Adler blood, bone marrow) be more sensitive than Lewis rats; et al., 1997; Anderson et + (BALB/c) !/+ (Sprague- CD-1 mice appeared to be more al., 1997; Lähdetie & Dawley) sensitive than Sprague-Dawley Grawé, 1997; Russo et al., + ((102/E1 × C3H/E1)F1) rats 1997; Stephanou et al., + (CD-1) 1997

– hprt mutations + (B6C3F1) ! (Lewis) Cochrane & Skopek, 1994b; (spleen) Tates et al., 1998; Meng et + ((102/E1 × C3H/E1)F1) ! (F344) al., 1999

Transgenic systems ! (B6C3F1 derived) + (F344 derived) rats appeared to be more Saranko et al., 1998 (lacI) sensitive than mice

DNA strand breaks + (CD-1) +/! (Sprague- damage was observed only in Anderson et al., 1997 and other damage Dawley) bone marrow cells of rats (bone marrow, testes)

Unscheduled DNA ! (CD-1) NT Anderson et al., 1997 synthesis (testes)

1,2,3,4-DIEPOXYBUTANE (DEB) Germ cells

Dominant lethal + ((102/E1 × C3H/E1)F1) NT Adler et al., 1995b mutations

Other genetic effects + ((C57Bl/Cne × + (Lewis) Lewis rats appeared to be more Adler et al., 1995b; Xiao & on male germ cells C3H/Cne)F1) sensitive to induction of Tates, 1995; Lähdetie et

(chromosomal micronuclei than F1(102 × C3H) al., 1997; Russo et al., + (F1(102 × C3H)) + (Sprague-Dawley) aberrations in mice 1997 zygotes, micronuclei) + (BALB/c)

Effects on female + (B6C3F1) NT Tiveron et al., 1997 germ cells (chromosomal aberrations in embryos)

Somatic cells

Chromosomal + (NMRI) NT positive results were also obtained Walk et al., 1987 aberrations (bone in Chinese hamsters, with NMRI marrow) mice being more sensitive than hamsters

Sister chromatid + (NMRI) NT positive results were also obtained Conner et al., 1983; Walk exchanges (bone in Chinese hamsters, with NMRI et al., 1987 marrow, lung, liver) + (Swiss Webster) mice being more sensitive than

+ (BDF1) hamsters

21 Concise International Chemical Assessment Document 30

Table 4 (contd). End-point Mice (strain) Rats (strain) Comments References

Micronuclei (spleen, + (F1(102 × C3H)) + (Lewis) there was little difference in Adler et al., 1995b; Xiao &

blood, bone marrow) sensitivity between F1(102 × C3H) Tates, 1995; Anderson et + (BALB/c) + (Sprague-Dawley) mice and Lewis rats or between al., 1997; Lähdetie & CD-1 mice and Sprague-Dawley Grawé, 1997; Russo et al., rats 1997; Stephanou et al., + ((102/E1 × C3H/E1)F1) 1997

+ (CD-1)

– hprt mutations + (B6C3F1) ! (Lewis) F344 rats appeared to be more Cochrane & Skopek, 1994b;

(spleen) sensitive than B6C3F1 mice Tates et al., 1998; Meng et ! ((102/E1 × + (F344) al., 1999 C3H/E1)F1)

Transgenic systems ! (B6C3F1 derived) ! (F344 derived) Recio et al., 1998 (lacI)

DNA binding + (ICR) NT Mabon et al., 1996 DNA strand breaks +/! (CD-1) +/! (Sprague- damage was noted in bone Anderson et al., 1997 and other damage Dawley) marrow cells only (bone marrow, testes)

Unscheduled DNA + (CD-1) NT Anderson et al., 1997 synthesis (testes)

1,2-DIHYDROXY-3,4-EPOXYBUTANE (EBdiol)

Germ cells

Dominant lethal ! ((102/E1 × NT Adler et al., 1997

mutations C3H/E1)F1)

Other genetic effects NT + (Sprague-Dawley) Lähdetie et al., 1997 on male germ cells (micronuclei)

Somatic cells

Micronuclei (bone + ((102/E1 × C3H/E1)F1) + (Sprague-Dawley) Adler et al., 1997; Lähdetie marrow) & Grawé, 1997

translocations in mice; an increased incidence of the monoepoxide diol metabolites (EB and EBdiol, chromosomal aberrations was also noted in zygotes of respectively) have been observed. The degree of adduct male mice exposed to $500 ppm ($1106 mg/m3) for formation was generally similar in the two species or, in 5 days. Other butadiene-induced effects observed in some studies, up to twofold greater in mice than in rats. male germ cells of mice include sperm head abnormal- Similarly, there was little quantitative difference in the ities, micronuclei in spermatids, and DNA damage amount of butadiene-induced single strand breaks in (strand breaks and alkaline-labile sites). Investigations DNA of mice and rats. DNA–DNA and DNA–protein of these end-points in rats have not been identified. cross-links were noted in one of two studies in mice, but not in rats exposed to higher concentrations of Butadiene was consistently genotoxic in somatic butadiene. cells of several strains of mice, inducing chromosomal aberrations, sister chromatid exchanges, and micronuclei Metabolites of butadiene have also been in numerous assays; micronuclei have been observed mutagenic and clastogenic in numerous in vitro and in following exposure to concentrations as low as 6.25 ppm vivo assays (see Table 4 for overview of results of in (13.8 mg/m3) butadiene for 13 weeks or 62.5 ppm vivo assays). EB, DEB, and EBdiol all induced mutations (138 mg/m3) for 8 h. Although only few studies were in bacteria and yeast in the absence of exogenous identified, these effects were not observed in rats metabolic activation (IARC, 1992; NTP, 1993; Thier et al., exposed to much higher concentrations. However, gene 1994; Adler et al., 1997); mutagenic activity was also mutations at the hprt locus have been induced in both observed for all three metabolites at two foci in human mice and rats, with a four- to sevenfold greater muta- TK6 lymphoblastoid cells, with DEB being much more genic potency being determined for mice than for rats. potent (Cochrane & Skopek, 1993, 1994a). Conversely, Mutagenic activity was also observed in two transgenic the monoepoxide was much more potent than the mouse systems and in the mouse spot test. Binding to diepoxide in the induction of mutations at the lacI DNA has been observed in all strains of mice and rats transgene of fibroblasts obtained from a transgenic rat tested; following exposure to butadiene, adducts of both strain (Saranko & Recio, 1998; Saranko et al., 1998). Both guanine and adenine with the monoepoxide as well as EB and DEB also induced sister chromatid exchanges,

22 1,3-Butadiene: Human health aspects chromosomal aberrations, and micronuclei in cultured 8.6 Reproductive toxicity mammalian (including human) cells (IARC, 1992; Xi et al., 1997). Aneuploidy in chromosomes 12 and X was also 8.6.1 Effects on fertility induced in human lymphocytes, which is notable in view of the fact that aneuploidy in these chromosomes is Few data on the effects of butadiene on reproduc- commonly observed in lymphocytic leukaemias (Xi et al., tive ability were identified. Exposure to up to 1300 ppm 1997). In vitro exposure to DEB, but not EB or EBdiol, (2876 mg/m3) for 5 days did not affect the reproductive induced micronuclei in spermatids isolated from rats abilities of male (102/E1 × C3H/E1)F1 mice, based on (Sj`blom & L@hdetie, 1996). percentages of successful pairings with unexposed females and unfertilized metaphase I oocytes (Pacchi- The monoepoxide, diepoxide, and monoepoxide erotti et al., 1998a). Similarly, there were no decreases diol metabolites all induced micronuclei in germ cells of in mating frequency or pregnancy rate in the dominant male mice and rats; in one of these studies, the magni- lethal studies in mice and rats (Anderson et al., 1993, tude of the effect was greater in Lewis rats than in F1(102 1998; BIBRA International, 1996a, 1996b; Brinkworth et × C3H) mice. There were no consistent patterns in the al., 1998). Documentation of an earlier study in rats, relative potency of the three metabolites. Chromosomal guinea-pigs, and rabbits (Carpenter et al., 1944) is too aberrations in zygotes produced by exposed males and limited for evaluation. dominant lethal mutations were induced by DEB in mice

(strains (C57Bl/Cne × C3H/Cne)F1 and (102/E1 × C3H/ The reproductive organs have consistently been E1)F1), respectively), whereas EB and EBdiol did not targets of non-neoplastic effects induced by butadiene induce dominant lethal mutations. In the only identified in subchronic and long-term bioassays in B6C3F1 mice investigation of the potential effects on female germ but not in Sprague-Dawley rats, although butadiene- cells, pre-mating exposure of female B6C3F1 mice to DEB induced tumours of the reproductive organs have been resulted in an increased frequency of chromosomal observed in both species. Ovarian atrophy and aberrations in embryos in the absence of ovarian decreased weight were observed in mice exposed to 1000 toxicity. ppm (2212 mg/m3, the only concentration tested) for 13 weeks (Bevan et al., 1996). In the 2-year bioassay EB, DEB, and EBdiol were also genotoxic in conducted by the NTP, there was a significant increase somatic cells (bone marrow, peripheral blood, lung, and in the incidence of ovarian atrophy in females exposed spleen), inducing sister chromatid exchanges, chromo- for up to 2 years to all concentrations tested (i.e., $6.25 somal aberrations, or micronuclei in several strains of ppm [$13.8 mg/m3]); both the incidence and the severity mice, rats, and hamsters, with little consistent evidence of this lesion increased with exposure. Ovarian atrophy of interspecies differences in sensitivity; in general, the was also observed at the interim sacrifices at 9 and 15 diepoxide was more potent than the monoepoxide or the months at higher concentrations ($200 and $62.5 ppm monoepoxide diol. Although negative results were [$442 and $138 mg/m3], respectively). Atrophied ovaries obtained in Lewis rats, both EB and DEB induced an characteristically had no evidence of oocytes, follicles, – increased frequency of hprt mutations in B6C3F1 mice or corpora lutea. Angiectasis and germinal epithelial and F344 rats, with rats being more sensitive than mice, hyperplasia of the ovaries were reported at $62.5 and which may be related to slower clearance in rats. EB $200 ppm ($138 and $442 mg/m3), respectively, after induced mutations in the bone marrow of lacI transgenic exposure for 2 years. Uterine atrophy was also noted at rats, but not in lacI transgenic mice; DEB did not induce concentrations of 200 ppm (442 mg/m3) or greater. lacI mutations in either species. Meng et al. (1999) Survival was decreased at $20 ppm ($44.2 mg/m3), suggested that the hprt assay is more sensitive to the principally due to neoplastic lesions at several sites, detection of large deletions induced by DEB than the including the ovaries (Melnick et al., 1990; NTP, 1993). lacI transgene assay. DNA damage (strand breaks or alkaline-labile sites) was caused by EB and DEB in the Effects on the testes, including reduced weight, bone marrow of rats and mice, with DEB being less degeneration, or atrophy, were observed in B6C3F1 potent than EB; the only damage observed in haploid mice exposed to concentrations at or above 200 ppm testicular cells was in mice exposed to EB. It was (442 mg/m3) for 2 years or to higher levels for shorter suggested that the apparent greater potency of EB durations (NTP, 1993; Bevan et al., 1996). Cytotoxic compared with DEB may be due to the bifunctional effects on differentiating spermatogonia were noted in alkylating ability of DEB, subsequent induction of DNA (102/E1 × C3H/E1)F1 mice 21 days after exposure to $130 repair, and the inability of the alkaline Comet assay ppm ($288 mg/m3) for 5 days; a decrease in elongated employed to measure cross-links (Anderson et al., 1997). spermatids was noted in mice exposed to 1300 ppm (2876 mg/m3) (Pacchierotti et al., 1998a).

23 Concise International Chemical Assessment Document 30

No non-neoplastic effects were noted in the repro- Although evidence of male-mediated ductive organs of male or female Sprague-Dawley rats teratogenicity was observed when male CD-1 mice exposed to up to 8000 ppm (17 696 mg/m3) butadiene for exposed to 12.5 ppm (27.7 mg/m3) butadiene for 10 weeks 2 years (Hazleton Laboratories Europe Ltd., 1981a; Owen were mated with unexposed females (Anderson et al., et al., 1987). 1993), there was no increase in malformations when the study was repeated at 12.5 and 125 ppm (27.7 and 277 Both the mono- and diepoxide metabolites of mg/m3) (Brinkworth et al., 1998). The authors suggested butadiene induced ovarian toxicity (depletion of small that the discrepant results may be a function of the and growing follicles) and alkylation with macromole- statistical significance in the first study being due to the cules in the ovary in B6C3F1 mice repeatedly exposed via lack of abnormalities in controls (compared with 2.5% in intraperitoneal injection. In contrast, effects in the ovary exposed), whereas a low incidence was noted in exposed in Sprague-Dawley rats were observed only in rats and control mice in the follow-up study. Similarly, there exposed to the diepoxide at doses higher than those that were no significant increases in fetal abnormalities in were active in the mice (Doerr et al., 1996). Since the CD-1 mice following paternal exposure to up to 6250 ppm results of structure–activity studies with 4-vinylcyclo- (13 825 mg/m3) butadiene for 6 h (Anderson et al., 1993) hexene and several of its analogues, butadiene mono- or concentrations up to 130 ppm (288 mg/m3) for 4 weeks epoxide and diepoxide, epoxybutane, and isoprene (BIBRA International, 1996a), although it was noted in indicated that compounds that form only monoepoxides the latter study that some females may have been do not induce ovarian toxicity (Doerr et al., 1995), it sacrificed too early for detection of abnormalities. There appears that conversion to the bifunctional diepoxide was no evidence of male-mediated teratogenicity in may be required for the induction of these effects. A offspring of male Sprague-Dawley rats exposed for 10 single intraperitoneal injection of DEB reduced various weeks to concentrations as high as 1250 ppm (2765 testicular cell populations and induced morphological mg/m3) butadiene and then mated with unexposed changes in the epithelium of the seminiferous tubules in females (BIBRA International, 1996b). male B6C3F1 mice (Spano et al., 1996). 8.7 Immunotoxicity 8.6.2 Developmental toxicity Although the haematopoietic system is a target of Few studies on the potential for butadiene to butadiene-induced toxicity, no effects on immune system induce developmental effects have been identified. function of biological significance were observed in the

There was no evidence of teratogenicity following only relevant study identified in which B6C3F1 mice were exposure of pregnant CD-1 mice to up to 1000 ppm (2212 exposed to 1250 ppm (2765 mg/m3) butadiene for up to 24 mg/m3) butadiene on days 6 through 15 of gestation, weeks, although there were depressions in cellularity although maternal toxicity (decreased body weight gain) and plaque-forming cells as well as histopathological and fetal toxicity (reduced fetal weight and skeletal changes in the spleen (Thurmond et al., 1986). abnormalities) occurred at 200 ppm (442 mg/m3) and above, and there was a slight reduction in male fetal body weight at 40 ppm (88 mg/m3) of questionable biological significance (Hackett et al., 1987b; Morrissey 9. EFFECTS ON HUMANS et al., 1990). In Sprague-Dawley rats exposed to 8000 ppm (17 696 mg/m3) butadiene on days 6 through 15 of gestation, there was an increased incidence of “major” 9.1 Clinical studies abnormalities of the skull, spine, sternum, long bones, and ribs. Abnormalities believed to be associated with Slight irritation of the eyes or respiratory tract was retarded embryonic growth were observed at 200 and observed in volunteers exposed to very high concentra- 3 1000 ppm (442 and 2212 mg/m ). Maternal toxicity tions of butadiene (i.e., >2000 ppm [>4400 mg/m3]) in the (decreased body weight gain or loss of body weight) few early clinical investigations identified (Larionov et was observed in all exposed groups (Hazleton al., 1934; Carpenter et al., 1944); however, these studies Laboratories Europe Ltd., 1981b, 1982). However, there are inadequate for evaluation of the potential effects of was no evidence of developmental toxicity in Sprague- butadiene in humans, as only subjective symptoms 3 Dawley rats exposed to up to 1000 ppm (2212 mg/m ) appear to have been evaluated. butadiene, also on days 6 through 15 of gestation, although maternal toxicity (decreased body weight gain) was noted at the highest concentration (Hackett et al., 1987a; Morrissey et al., 1990).

24 1,3-Butadiene: Human health aspects

Table 5: Summary of measures of risk for cancers of the lymphohaematopoietic system in populations occupationally exposed to butadiene.a

Cohort Number Risk measureb Cohort description size of cases Exposure (95% CI) Comments References Leukaemia

Styrene-butadiene 15 649 48 SMR = 131 (97–174) SMR was significant Delzell et al., rubber workers 7 0 ppm-years RR = 1.0 in some subgroups; 1995 14 >0–19 ppm-years RR = 1.4 (0.4–4.8) increasing trend in 18 20–99 ppm-years RR = 2.3 (0.7–7.9) RRs remained when 7 100–199 ppm-years RR = 2.6 (0.7–10.0) adjusted for styrene 5 $200 ppm-years RR = 4.2 (1.0–17.4)

Butadiene 2795 13 SMR = 113 (60–193) no association Divine & production workers 996 3 low SMR = 67 (13–195) between qualitative Hartman, 1874 11 varied SMR = 154 (77–275) measure of cumula- 1996 tive exposure and leukaemia risk

Butadiene 364 2 SMR = 123 (15–444) E.M. Ward et production workers al.,1995, 1996

Lymphosarcoma

Styrene-butadiene 15 649 11 SMR = 80 (40–144) SMRs were Delzell et al., rubber workers increased for 1995 maintenance workers (O = 8; SMR = 192; 95% CI = 83–379) and labourers (O = 3; SMR = 123; 95% CI = 25–359), but not in production or laboratory workers Butadiene 2795 9 SMR = 191 (87–364) no association with Divine & production workers 0 background SMR = 0 (0–591) duration of employ- Hartman, 2 low SMR = 109 (12–395) ment, based on only 1996 7 varied SMR = 249 (100–513) two and one cases in the two higher cate- gories Butadiene 364 4 SMR = 577 trend with duration E.M. Ward et production workers 1 <2 years (157–1480) of employment when al., 1995, 3 $2 years SMR = 303 dichotomized at 2 1996 SMR = 827 (p < 0.05) years a SMR = standardized mortality ratio; CI = confidence interval; RR = relative risk; O = observed cases. b SMR = observed/expected × 100.

9.2 Epidemiological studies for lymphohaematopoietic cancers is presented in Table 5. 9.2.1 Cancer In the most recent update of the largest of the The carcinogenicity of butadiene has been cohorts of male monomer workers (n = 2795) at the Port investigated in several populations of workers Neches butadiene production facility in Texas, USA occupationally exposed during its manufacture or use. (Divine & Hartman, 1996), mortality due to lympho- Although most of these studies are limited by the haematopoietic cancer was significantly elevated paucity of historical monitoring data, there is evidence (standardized mortality ratio [SMR]2 = 147; 95% that occupational exposure to butadiene in the styrene- confidence interval [CI] = 106–198), due largely to a non- butadiene rubber industry is associated with excess significant increase in the number of deaths due to mortality due to leukaemia and weaker evidence of an lymphosarcoma and reticulosarcoma (SMR = 191, 95% CI association with lymphosarcoma1 in butadiene monomer = 87–364), based on nine cases. However, there was no production workers. A summary of the measures of risk association with duration of employment (SMRs of 261,

1 The terminology for cancers of the 2 SMRs are presented here in the format used by the lymphohaematopoietic system is that used by authors of authors; i.e., SMR = observed/expected or SMR = the individual study accounts. observed/expected × 100.

25 Concise International Chemical Assessment Document 30

182, and 79, for <5, 5–19, and $20 years of employment, to supplant those of earlier investigations, as there is respectively, based on six, two, and one cases). The considerable overlap in the cohort population with the greatest excess was observed in men first employed earlier studies (i.e., 14 869 of these subjects had been during the Second World War (observed cases [O] = 7; employed at one of the two plants studied previously by SMR = 241; 95% CI = 97–497), during which the greatest Meinhardt et al. [1982] or at seven of the eight plants concentrations of butadiene likely occurred, although no investigated by Matanoski et al. [1990, 1993] and Santos- data were presented (Divine et al., 1993). When the entire Burgoa et al. [1992], although they had been employed cohort was subdivided on the basis of a qualitative for different time periods [Delzell et al. (1995) included measure of exposure (based on job history information, several more years of follow-up] and selected using discussions and reviews of classifications with long- different inclusion criteria).1 Estimates of cumulative term employees, and recent industrial hygiene surveys), exposure and peak exposure frequency were derived for the SMR for lymphosarcoma and reticulosarcoma was workers from six of the eight plants based on complete greatest (SMR = 249; 95% CI = 100–513) in those whose work histories for 97% of these employees, information jobs involved “varied” exposure to butadiene (i.e., the on processes and plant conditions based on available group with potentially the greatest exposure), although records, walk-through surveys, and interviews with the number of cases in each subgroup was small, and long-term employees, plant engineers, and managers and only one of the seven cases in this group had been were compared with monitoring data from surveys exposed for more than 10 years. There was also a non- conducted from the late 1970s onward. significant increase in mortality due to leukaemia in the varied exposure group (SMR = 154; 95% CI = 77–275; There was an increase in mortality due to leukae- based on 11 cases, 10 of which had been employed less mia, which was of borderline statistical significance, in than 10 years), although there was no such increase in the overall cohort, based on 48 cases (SMR = 131; 95% the total cohort (SMR = 113; 95% CI = 60–193). However, CI = 97–174); this excess was concentrated in workers there was no association between estimates of who had had 10 or more years of employment and 20 or cumulative exposure and any form of more years since date of hire (O = 29; SMR = 201; 95% CI lymphohaematopoietic cancer. = 134–288). Similarly, there was a significant increase in mortality due to leukaemia in “ever hourly” workers (i.e., Mortality due to lymphosarcoma and reticulo- workers who had ever been paid on an hourly basis) sarcoma was significantly increased in a smaller cohort whose jobs were most likely to have involved exposure study of 364 workers who had ever been employed in to butadiene (O = 45; SMR = 143; 95% CI = 104–191), butadiene production at two Union Carbide plants in which was again concentrated in workers with longer West Virginia, USA, based on four cases (SMR = 5.77; duration of employment and time since hire and was 95% CI = 1.57–14.8). The risk was greater in those greater in black workers than in white workers in this employed for 2 years or more than in those employed for subgroup. The SMRs for leukaemia also increased with less than 2 years (SMRs of 8.27 and 3.03, respectively), duration of employment for ever hourly workers. When although the number of cases in each category was very examined by type of employment, the number of deaths small. No significant increase in mortality due to due to leukaemia was significantly increased in leukaemia or aleukaemia was observed (observed/ production workers (O = 22; SMR = 159; 95% CI = expected = 2/1.62). However, no monitoring data were 100–241), labourers (O = 16; SMR = 195; 95% CI = available to assess exposure of individuals in the cohort 112–317; concentrated among black workers), laboratory (E.M. Ward et al., 1995, 1996). In the only other study of workers (O = 12; SMR = 462; 95% CI = 238–806), and monomer production workers, there were no deaths due black workers in other operations (O = 3; SMR = 680; to cancer of the lymphohaematopoietic system; 95% CI = 137–1986); no significant increases were however, the size of the cohort (n = 614) and duration of observed in maintenance workers (O = 13; SMR = 107; follow-up were insufficient to detect excess risks of 95% CI = 57–184). (Although analyses were not lymphohaematopoietic cancer of less than fivefold presented and no further information is given, Delzell et (Cowles et al., 1994). al. [1995], in the Discussion section of their report, reported that there was no increase in mortality due to Several studies have been conducted in workers leukaemia in 851 workers in butadiene production areas employed in the manufacture of synthetic rubber in North America who were exposed to butadiene as well as to styrene and other substances. The largest and most comprehensive study to date involved 15 649 workers employed at eight styrene-butadiene rubber manufactur- 1 ing facilities in North America (Delzell et al., 1995). The It is not possible to determine, with any certainty, the results of this study are emphasized here and considered size of the population in these earlier studies that was not subsumed in the later investigation by Delzell et al. (1995). One of the small plants of approximately 600 workers included in the Matanoski et al. (1990, 1993) cohort was not examined by Delzell et al. (1995).

26 1,3-Butadiene: Human health aspects

[which would not involve exposure to styrene], with had little influence on the exposure–response relation- very low numbers of observed and expected deaths [1 ship. Risk of leukaemia also increased with duration of versus 2.1, respectively].) As well, the SMRs for employment in areas in which there was “definite” expo- lymphosarcoma were non-significantly increased for sure to peaks (RRs of 1.0, 2.3, and 2.7 for 0, >0–4, and >5 maintenance workers (O = 8; SMR = 192; 95% CI = years) and in areas for which elevated SMRs had been 83–379) and labourers (O = 3; SMR = 123; 95% CI = noted in the previous analyses (RRs of 1.0, 1.9, and 3.1 25–359), while there was no increase in mortality due to for 0, >0–4, and >5 “high SMR-years”). The authors lymphosarcoma in production workers or laboratory noted that it was not possible to distinguish between the workers. When individual plants were considered roles of estimated peak or cumulative exposure. separately, there were non-statistically significant increases in mortality due to leukaemia at most (but not The estimates of exposure were further refined for all) plants (SMRs ranged from 72 to 780, excluding workers at one of the plants included in the investigation groups in which zero cases were observed and less than by Delzell et al. (1995) through more extensive research one case was expected); numbers of observed cases of of historical conditions (Macaluso et al., 1997). Although lymphosarcoma were too low to permit meaningful there was little change in classification of various conclusions with respect to mortality at individual workers as exposed or non-exposed, the revised plants. estimates of cumulative exposure to butadiene for many job groups were generally greater (two- to threefold) In regression analyses, mortality due to leukaemia than the original; the most substantial increase (by an was observed to increase with cumulative exposure to order of magnitude) was determined for tasks among butadiene, as relative risk (RR) values for exposure unskilled labourers during the 1950s and 1960s. It was categories of 0, >0–19, 20–99, 100–199, and $200 ppm- not indicated in the report if the rank order of the cumu- years were 1.0, 1.4, 2.3, 2.6, and 4.2, respectively (for lative exposure estimates differed (although it is likely cases in which leukaemia was considered the underlying that it did not; Gerin & Siemiatycki, 1998). There was cause of death). There was only limited evidence of an little change in estimated exposure to peak levels of association with cumulative exposure to peak levels of butadiene or in cumulative exposure to styrene. These butadiene. The authors also investigated the potential revised exposure estimates have not yet been incorpor- influence of exposure to styrene or benzene on mortality ated into cancer mortality analyses. and determined that the trend for increased risk with increased cumulative exposure to styrene was less Sathiakumar et al. (1998) re-examined the mortality pronounced, while exposure to benzene was considered of this cohort based on currently accepted terminology to be too infrequent (few subjects were exposed) and too for lymphohaematopoietic cancers (other than low to be a confounding factor. There was no leukaemia). There were no significant increases in deaths association between cumulative exposure to butadiene in the overall cohort due to non-Hodgkin’s lymphoma, and non-Hodgkin’s lymphoma in regression analyses. Hodgkin’s disease, multiple myeloma, or cancers of other lymphatic tissue, nor were there any associations Based on the results of this study, Delzell et al. between mortality due to these causes and duration of (1995) concluded that there was a relationship between exposure and year of hire. Similarly, mortality due to employment in the styrene-butadiene industry and these causes was not associated with any process leukaemia, with the increased risk of leukaemia being group; however, the authors noted that an association most strongly associated with exposure to butadiene or for non-Hodgkin’s lymphoma may be obscured by the to butadiene and styrene in combination (although the possibility that some cases of non-Hodgkin’s lymphoma association with butadiene remained after controlling for had transformed to leukaemia, with the latter form of exposure to styrene). Data were insufficient to draw any cancer being recorded on the death certificate. firm conclusions with respect to an association with any specific form of leukaemia. An association between exposure to butadiene and leukaemia, as well as Hodgkin’s disease, was also In a subsequent study in which Delzell et al. (1996) observed in a recent independently conducted nested attempted to better define exposure of this cohort to case–control study of 58 cases of lymphohaematopoietic peak levels of butadiene, the RR for leukaemia increased cancers from a cohort of styrene-butadiene rubber with increasing average annual number of peaks to workers (from many of the same plants investigated by which workers were exposed (RRs of 1.0, 2.3, and 3.1 for Delzell et al. [1995]), in which exposure was estimated 0, >0–3288, and >3288 peaks), as well as again with based on analyses of monitoring data obtained in the cumulative exposure to butadiene (RRs of 1.0, 1.1, 2.0, last 15–20 years of operation (Matanoski et al., 1997). 2.4, and 4.6 for 0, >0–19, 20–99, 100–199, and $200 ppm- years). Although the analyses were not presented, adjusting for cumulative exposure to styrene apparently

27 Concise International Chemical Assessment Document 30

Irons & Pyatt (1998) noted the concordance 9.2.3 Genotoxicity between the potential for exposure to dimethyldithiocar- bamate (a “stopper” used in the polymerization process The potential genotoxicity of butadiene has in the styrene-butadiene rubber industry) and mortality recently been investigated in several studies of groups due to leukaemia in various processes. However, of workers exposed in the production of butadiene, although the biological plausibility of a potential styrene-butadiene rubber, or polybutadiene rubber. association is recognized (since dimethyldithiocarbamate Although the data available to date are not completely is a potent inhibitor of clonogenic response in human consistent, they indicate that there is some evidence that CD34+ bone marrow cells), it is not possible to draw any exposure to butadiene induces genetic effects in conclusion concerning its potential role in the observed occupationally exposed populations and that sensitivity increases in leukaemia mortality at this time in view of to the induction of these effects is related to genetic the current lack of quantification of exposure levels of polymorphism for enzymes involved in the metabolism of this substance in the plants examined and the absence of butadiene, most notably those within the glutathione-S- data on its leukaemogenic potential. transferase class. The results of several in vitro studies in human lymphocytes have demonstrated that Other epidemiological studies in populations sensitivity to DEB-induced sister chromatid exchanges occupationally exposed to butadiene have been and micronuclei is associated with the presence or identified in the literature (e.g., McMichael et al., 1974, absence of homozygous deletion of the GSTT1 gene, 1976; Andjelkovich et al., 1976, 1977; Linet et al., 1987; which codes for GST2 (Kelsey et al., 1995; Norppa et al., Siemiatycki, 1991; Bond et al., 1992; Downs et al., 1993). 1995; Wiencke et al., 1995; Landi et al., 1996; Pelin et al., However, owing to limitations of these studies 1996; Vlachodimitropoulos et al., 1997), for which the (including small numbers of observed and expected prevalence of the null genotype is reported to be cases of lymphohaematopoietic cancer or lack of expo- between 15 and 30% (Nelson et al., 1995; Abdel-Rahman sure characterization), they contribute little to evaluation et al., 1996; Bailey et al., 1998). Similarly, sensitivity to of the association between exposure to butadiene and sister chromatid exchanges induced by EB appears to be these forms of cancer. related to genotype for GSTM1, which codes for GSTµ (Wiencke & Kelsey, 1993; Uuskula et al., 1995), and Significantly increased mortality due to cancers possibly also GSTT1 genotype in GSTM1-null other than those of the lymphohaematopoietic system individuals (Bernardini et al., 1998). However, there were has not been consistently observed in these studies. no differences in sensitivity to sister chromatid exchanges induced by EBdiol in individuals with and 9.2.2 Non-neoplastic effects without deletions for GSTT1 or GSTM1 (Bernardini et al., 1996). Mortality due to all causes was similar to or significantly lower than that expected in all of the major Although no increased frequencies of sister chro- cohorts of workers potentially exposed to butadiene. matid exchanges, chromosomal aberrations, or micro- Although increased mortality due to arteriosclerotic or nuclei were observed in earlier studies in butadiene ischaemic heart disease or circulatory disease in general production workers in Portugal and the Czech Republic has been observed in some subgroups of workers in compared with controls (Sorsa et al., 1994, 1996b), some of these cohorts (McMichael et al., 1974, 1976; positive results for chromosomal aberrations and sister Matanoski et al., 1990; Delzell et al., 1995), the potential chromatid exchanges were obtained in the most recent association with exposure to butadiene has not been study of the Czech workers (Tates et al., 1996; Šrám et al., extensively investigated. 1998). When genotype was considered, there was a significant increase in the frequency of chromosomal There were no differences in morbidity or various aberrations in both exposed and control subjects from haematological parameters between 438 workers exposed both plants who were deficient for the GSTT1 gene to mean concentrations of butadiene ranging up to 10 (Sorsa et al., 1996a). ppm (22 mg/m3) (with a maximum time-weighted average concentration of 143 ppm [316 mg/m3]) and 2600 An increased frequency of hprt– mutants in periph- unexposed workers at a butadiene production facility in eral blood lymphocytes has been observed in two Texas, USA (Cowles et al., 1994). However, Checkoway studies of exposed workers at a butadiene production & Williams (1982) observed changes in haematological facility in Texas, USA (Legator et al., 1993; Ward et al., parameters consistent with bone marrow depression in 1994; Au et al., 1995) and in preliminary results of a eight workers exposed to high concentrations of study of styrene-butadiene rubber workers from the butadiene (up to about 53 ppm [117 mg/m3]) when compared with values for 145 workers exposed to much lower levels (i.e., <1 ppm [<2.2 mg/m3]).

28 1,3-Butadiene: Human health aspects same region (J.B. Ward et al., 1996).1 Although analyses 10. EVALUATION OF HEALTH EFFECTS by genotype are not yet available, it was noted that the highest frequency of hprt– variants occurred in an individual who was GSTT1 null. In contrast to the 10.1 Hazard identification observations in the Texan plants, however, no increase in hprt– mutant frequency was observed in workers Although the metabolism of butadiene appears to exposed to similar levels of butadiene at the monomer be qualitatively similar across species, there are plant in the Czech Republic (Tates et al., 1996) or in a extensive data that indicate that the putatively active population of polybutadiene rubber workers in China epoxide metabolites are formed to a greater degree in (Hayes et al., 1996) (no information on genotype was mice than in rats. Similarly, although in vivo data are presented). These investigations involved different limited, humans appear to metabolize butadiene to the analytical methodologies (autoradiographic versus mono- and diepoxide metabolites to a much lesser extent clonal assays), which may account for the discordance than mice. However, based on the observed variability in in the results; in addition, differences in occupational the formation of adducts of haemoglobin with butadiene scenarios, exposure levels, age, smoking habits, or other metabolites in occupationally exposed human popula- lifestyle factors may have contributed to the tions, there appears to be interindividual variation in discrepancy. Current ongoing research (including humans, which is likely related to polymorphism for genotyping) may explain the differences in the results. genes that code for enzymes involved in the metabolism of butadiene. The weight of evidence for the carcino- Decreased DNA repair ability was also observed in genicity, genotoxicity, and non-neoplastic effects of peripheral blood lymphocytes of exposed workers at the butadiene needs to be considered, therefore, in the monomer production and styrene-butadiene rubber context of these interspecies and interindividual facilities in Texas in both a (-radiation challenge assay variations. and a CAT-Host Cell Reactivation assay (Hallberg et al., 1997).2 However, the difference between exposed and 10.1.1 Carcinogenicity and genotoxicity “unexposed” monomer workers in the response to the challenge assay was no longer significant after ambient Data supporting the interspecies differences in levels in the plant were reduced. Similarly, the effect on production of active epoxide metabolites are in DNA repair ability in styrene-butadiene rubber workers concordance with the observed difference in sensitivity was less when only non-smokers were considered. The between mice and rats (at least for the few strains detection of alkylated DNA (the same adduct as detected investigated) to butadiene-induced carcinogenicity, in in the liver of mice and rats exposed to butadiene; Jelitto that the substance appears to be much more potent in et al., 1989; Koivisto et al., 1997) in the urine of an mice than in rats. Although butadiene was a multi-site exposed worker (Peltonen et al., 1993) also provides carcinogen in both mice and rats at all exposure levels some evidence of the interaction of butadiene or its tested (Hazleton Laboratories Europe Ltd., 1981a; NTP, metabolites with genetic material in humans. 1984, 1993; Irons et al., 1989), the concentrations that induced tumours in the only study available in rats were much greater than those that were tumorigenic in mice (i.e., $1000 ppm [$2212 mg/m3] versus $6.25 ppm [$13.8 mg/m3]).

Species differences in sensitivity to genetic effects induced by butadiene have also been observed. Although butadiene was mutagenic in somatic cells of both mice and rats, its mutagenic potency was greater in mice. Other genotoxic end-points (chromosomal aberrations, sister chromatid exchanges, and micronuclei) were noted in somatic cells of mice but not in those of rats exposed to much higher concentrations. 1 Also personal communication (letter dated 17 October Butadiene was genotoxic in germ cells of male mice in 1997) from J.B. Ward, Jr., Division of Environmental multiple assays, while negative results were obtained in Toxicology, Department of Preventative Medicine and the single dominant lethal study in rats. Unlike the Community Health, University of Texas Medical Branch, observations with the parent compound, however, there Galveston, TX, to Health Canada. is little evidence that there are species differences in the sensitivity to genotoxic effects induced by the epoxide 2 Personal communication (electronic correspondence metabolites of butadiene (EB, DEB, and EBdiol), dated 15 November 1997) from J.B. Ward, Jr., Division of although there was some indication of interstrain Environmental Toxicology, Department of Preventative variability. These data suggest that interspecies Medicine and Community Health, University of Texas differences in sensitivity to butadiene-induced Medical Branch, Galveston, TX, to Health Canada.

29 Concise International Chemical Assessment Document 30 genotoxicity are related to quantitative differences in the therefore, these results are not inconsistent with the formation of active metabolites. association observed by Delzell et al. (1995).

There is also limited evidence of the genotoxicity However, no increase in mortality due to leukaemia of butadiene in exposed workers; although data are not was observed in studies of workers involved in the completely consistent, increased frequencies of chromo- production of butadiene monomer who were not con- somal aberrations, sister chromatid exchanges, and hprt– comitantly exposed to the other substances present in mutations and decreased DNA repair capability have the styrene-butadiene rubber industry (E.M. Ward et al., been reported in some studies of workers in the mono- 1995, 1996; Divine & Hartman, 1996). Although there was mer and/or styrene-butadiene rubber manufacturing some evidence of increased mortality due to industries (Legator et al., 1993; J.B. Ward et al., 1994, lymphosarcoma and reticulosarcoma in the subgroup of 1996; Au et al., 1995; Tates et al., 1996; Hallberg et al., workers potentially exposed to the highest 1997; Šrám et al., 1998).1 The discrepancy in the results concentrations of butadiene in the largest of these may be due to the use of different methods for the investigations, there was no association with duration of detection of mutations or differences in exposure levels. employment or estimated cumulative exposure (based on In addition, since sensitivity to induction of genetic qualitative ranking of potential for exposure). Although effects by butadiene and its metabolites has been linked mortality due to lymphosarcoma was non-significantly to genotype for glutathione-S-transferase enzymes in elevated in some process groups in the styrene- several in vitro and a few in vivo studies, interpretation butadiene rubber cohort (Delzell et al., 1995), there were of the inconsistent observations in the available no consistent patterns (other than for leukaemia), even database is complicated by the lack of information on when currently accepted terminology for genotype for most of the small populations examined. lymphohaematopoietic cancers was used (Sathiakumar et al., 1998). There have been several epidemiological investi- gations of the carcinogenicity of butadiene that serve as The traditional criterion of consistency for the a basis for assessment of the weight of evidence for observed association between exposure to butadiene causality based on traditional criteria. In the most recent and leukaemia is fulfilled, at least in part, in that similar cohort study (Delzell et al., 1995), which is also the excesses were observed among plants in the large cohort largest and most comprehensive investigation study of styrene-butadiene rubber workers (Delzell et al., conducted to date and that in which exposure was most 1995); i.e., there is internal consistency. A similar extensively characterized, an association between exposure–response was also noted in an independent exposure to butadiene in the styrene-butadiene rubber nested case–control study of mostly the same industry and leukaemia was observed (i.e., there was a population in which different exposure assessment quantifiable exposure–response relationship). SMRs for methodology was employed (Matanoski et al., 1997). leukaemia were elevated for the overall cohort of workers Observation of external consistency with results of other from eight plants; the strength of this association was cohort studies of styrene-butadiene rubber workers is generally greater when specific subgroups with greater largely precluded, in view of the scope of the large potential for exposure were considered. In addition, there epidemiological cohort study that included a large was an increase in the RR for leukaemia with increased proportion of all of the styrene-butadiene rubber workers cumulative exposure to butadiene in workers from the six in North America. Indeed, it is difficult to envisage plants for which exposure was best characterized. The additional studies in this occupational group that would association between leukaemia and exposure to buta- contribute meaningfully to weight of evidence for diene remained when the potential role of two other consistency of the observed association. substances present in the work environment (i.e., styrene and benzene) was considered. Although further One criterion for causality of observed refinement of the estimates of exposure at one of these associations in epidemiological studies, namely plants resulted in increases for several job categories coherence, may not have been adequately fulfilled, in (Macaluso et al., 1997), it is unlikely that these changes view of the difference in the specific form of would affect the relative ranking of the categories and lymphohaematopoietic cancer in excess in available analyses in which exposed workers were compared with investigations for the two principal types of populations “non-exposed” workers (Gerin & Siemiatycki, 1998); of workers studied. Indeed, increases in lymphosarcoma and reticulosarcoma have been observed in monomer production workers, whereas increases in leukaemia have been observed in styrene-butadiene rubber workers. 1 Also personal communications (letter dated 17 October Although it is plausible that this difference may be 1997 and electronic correspondence dated 15 November related to variation in the extent of information available 1997) from J.B. Ward, Jr., Division of Environmental for characterization of exposure or to the nature of Toxicology, Department of Preventative Medicine and exposures in the two industries, this has not been Community Health, University of Texas Medical Branch, systematically investigated. There is also the possibility Galveston, TX, to Health Canada.

30 1,3-Butadiene: Human health aspects of misclassification of cause of death on death Therefore, although not completely convincing in certificates (although Sathiakumar et al. [1998] did not their own right, the available epidemiological studies of observe an association with forms of lymphohaemato- the association between leukaemia and exposure to poietic cancer other than leukaemia in the large cohort of butadiene in occupationally exposed human populations styrene-butadiene rubber workers when causes of death fulfil several of the traditional criteria for causality, were examined using current terminology). The potential including strength of association (RR of 4.2 in the for transformation of one form of lymphohaematopoietic highest exposure group [based on five cases], which cancer to another (e.g., non-Hodgkin’s lymphoma to would be considered moderately strong), quantifiable leukaemia) has also been noted (Sathiakumar et al., 1998). exposure–response relationship, temporal relationship In addition, available data for the large study of styrene- (the critical investigation [i.e., Delzell et al., 1995] is a butadiene rubber workers were insufficient to determine historical cohort study), biological plausibility, and, to if butadiene was causally associated with a specific form some degree, consistency, although the criterion for of leukaemia. Moreover, it is noteworthy that these coherence is not fully satisfied. different tumours observed in styrene-butadiene rubber workers and monomer production workers are of the Assessment of the weight of evidence for same organ system, and perhaps even share the same carcinogenicity in human populations should not, pluripotential stem cell. however, be considered in isolation from the extensive supporting data on carcinogenicity, genotoxicity, and An association between exposure to butadiene inter- and intraspecies variations in metabolism and and the induction of leukaemia is also biologically response. The association between exposure to plausible. The haematopoietic system is a target for butadiene and development of cancer is supported by butadiene-induced effects in rodents (i.e., lymphocytic limited evidence of genetic damage in exposed workers, lymphomas [NTP, 1993], cytogenetic effects in bone as well as the wealth of evidence that butadiene is marrow [Cunningham et al., 1986; Irons et al., 1986a, carcinogenic and/or genotoxic in all species of 1987; Tice et al., 1987; NTP, 1993; Leavens et al., 1997], experimental animals tested (mice, rats, and hamsters), and suppression of stem cell differentiation [Irons et al., inducing a wide range of tumours and genetic damage at 1996]). Aneuploidy, which is believed to be associated relatively low concentrations in mice (i.e., within the with leukaemia in humans, has been induced in human same order of magnitude as current occupational health lymphocytes exposed in vitro to the mono- and diepox- limits). Moreover, while there are quantitative differences ide metabolites of butadiene (Vlachodimitropoulos et al., in the potency of the substance to induce tumours in 1997; Xi et al., 1997). Moreover, the presence of relevant various species, likely related to observed quantitative metabolizing enzymes in progenitor cells believed to be differences in metabolism, there are indications of important targets for the induction of leukaemia in considerable interindividual variations in the metabolism humans (i.e., CD34+ cells) has been demonstrated in of butadiene in the human population, consistent with studies of the metabolism of benzene (a documented expectations for a complex metabolic pathway. human leukaemogen) (Schattenberg et al., 1994; Ross et al., 1996) (although exposure of human CD34+ cells to EB The observation of an association between at “physiologically relevant concentrations” did not alter exposure in the occupational environment and leukaemia cytokine-induced clonogenic response, an early change that fulfils several of the traditional criteria for causality frequently observed in the development of leukaemia; of associations observed in epidemiological studies, as Irons et al., 1996).1 Therefore, available data also support well as supporting limited data on genotoxicity in human the biological plausibility of an association between populations and the well documented carcinogenicity exposure to butadiene and leukaemia observed in and genotoxicity at relatively low concentrations in some humans, although the active metabolite has not been species of experimental animals, provides weight of identified. evidence that butadiene is carcinogenic in humans.

Although relevant data in humans are limited, the results of in vivo studies in experimental animals indicate that butadiene induces mutations in somatic cells and male germ cells as well as male-mediated heritable clastogenic damage. While most of the studies have been conducted in mice, rats appear to be less sensitive to these effects, which is consistent with species differences in metabolism. However, in view of the likely considerable heterogeneity in the metabolism of butadiene in human populations, butadiene may be a human somatic and germ cell genotoxicant. 1 Also personal communication (correspondence dated 30 March 1998) from R.D. Irons, University of Colorado Health Sciences Center, Denver, CO, to Health Canada.

31 Concise International Chemical Assessment Document 30

10.1.2 Non-neoplastic effects changes,1 it may be that butadiene is exacerbating these changes. It should be noted, though, that the incidence The available data on effects of butadiene other of these lesions was increased as early as 9 months than carcinogenicity or genotoxicity are limited. Based (although the slides from these interim sacrifices have on the limited data available, species differences in the not been re-examined). That butadiene is causally ability of butadiene to induce other non-neoplastic associated with these lesions is also difficult to dismiss effects again appear to be consistent with variations in on the basis of currently available data, in view of the metabolism of butadiene to active metabolites. However, consistency with the results of other studies, including butadiene is of low acute toxicity in both rats and mice, the earlier NTP (1984) bioassay and a subchronic study in contrast to its ability to induce cancer and genetic at higher concentrations (Bevan et al., 1996) in which damage at relatively low concentrations in mice. such lesions were also observed, the presence of a clear dose–response relationship, and biological plausibility. Haematological effects suggestive of macrocytic Based on the observation of depletion of ovarian anaemia have been consistently observed in mice (two follicles and alkylation with ovarian macromolecules in strains) following short-term, subchronic, or chronic mice following intraperitoneal administration of the exposure to butadiene at concentrations similar to or monoepoxide or diepoxide metabolite and in rats lower than those that induced general toxicity (as administered the diepoxide (Doerr et al., 1995), it is indicated by decreased body weight gain and increased possible that the ovarian toxicity is mediated through organ weights) (Irons et al., 1986a, 1986b; NTP, 1993; generation of the active epoxide metabolites. Bevan et al., 1996). For example, changes in haemato- logical parameters were noted in mice exposed to Testicular atrophy was noted only in male mice $62.5 ppm ($138 mg/m3) butadiene for 9 months or exposed to concentrations greater than those that longer in the NTP bioassay. Butadiene also induced induced effects in females (NTP, 1993). Consistent with effects on bone marrow (including atrophy, decreased metabolic differences, butadiene did not induce ovarian cellularity, regeneration, and alterations in stem cell or testicular toxicity in the limited number of available development) in mice (Irons et al., 1986a, 1986b; studies in rats, although, as noted above, the diepoxide Leiderman et al., 1986; NTP, 1993), although available metabolite was ovotoxic in both species (Doerr et al., data are inadequate to assess the potential effects on 1995, 1996). immune system function. While effects on the blood and bone marrow have not been reported in rats in recent Although available data are limited, there is no investigations (including the only identified chronic conclusive evidence that butadiene is teratogenic in bioassay; Hazleton Laboratories Europe Ltd., 1981a), the mice or rats following maternal or paternal exposure or database is considerably more limited. In addition, the that it induces significant fetal toxicity at concentrations lack of observation of haematotoxicity in rats may again below those that are maternally toxic. reflect the species differences in metabolism. Although the available epidemiological studies are too limited to Available epidemiological data are inadequate for assess the haematotoxicity in humans, available data evaluation of potential reproductive or developmental support the haematopoietic system being a critical target toxicity; in fact, none of the identified analytical studies for butadiene-induced toxicity, since the was conducted in women. However, in view of the quali- lymphohaematopoietic system is a target for butadiene- tative similarities in the metabolism of butadiene in mice, induced leukaemia in humans. However, it has not been rats, and humans and the likely variation across the gen- established if the non-neoplastic effects observed in eral population associated with genetic polymorphism animals may be preliminary to, or associated with, the for the relevant enzymes, and on the basis of the development of lymphohaematopoietic cancers. observed ovarian toxicity in butadiene-exposed mice, butadiene may be a reproductive toxicant in humans, The reproductive organs are also critical targets of although additional work to clarify the relevance of these butadiene-induced non-neoplastic effects in mice. observed effects is clearly desirable. Ovarian atrophy, the severity and incidence of which increased with concentration or duration of exposure, Available data on other systemic or organ-specific was observed at all concentrations (i.e., $6.25 ppm effects are inadequate to determine if such effects might [$13.8 mg/m3]) in the chronic bioassay conducted by the be considered critical. NTP (1993); in all exposure groups, the level of degen- eration at 2 years, characterized by lack of oocytes, follicles, or corpora lutea, was incompatible with reproductive capacity. Although recent re-examination of some of the tissue samples indicated that the atrophy observed in the ovaries may be related to senile 1 Personal communication (electronic correspondence dated 26 June 1998) from B. Davis, National Institute for Environmental Health and Safety, National Toxicology Program, Research Triangle Park, NC, to Health Canada.

32 1,3-Butadiene: Human health aspects

10.2 Exposure–response assessment and exposure–response for non-cancer effects, where con- criteria for setting tolerable sidered appropriate, benchmark concentrations2 have concentrations or guidance values been calculated on the basis of data from long-term studies in mice. As per the approach adopted for several genotoxic carcinogens, measures of the potency of a substance to Several physiologically based pharmacokinetic induce effects for which there is believed to be no (PBPK) models have been developed as a basis for threshold may be used to establish guideline values for reducing uncertainty in interspecies extrapolations for environmental media. Since air is the principal route of butadiene by various groups of investigators. However, exposure to butadiene in the general environment none of the models currently available has adequately (available data indicate that other routes contribute accounted for the distribution of metabolites in the negligibly), quantitation of exposure–response for compartments included; the principal researchers in this cancer as well as non-cancer effects is limited to field have concluded that there are likely more factors exposure by inhalation. involved in butadiene metabolism than have been included in the models developed to date (Csanády et In order to eliminate the uncertainty associated al., 1996; Sweeney et al., 1997). In addition, none of the with extrapolation from animal species, quantitative models has included the formation of EBdiol, a puta- measures of carcinogenic potency (i.e., tumorigenic tively active metabolite that is believed to be important concentrations, or TCs)1 have been developed on the in humans, since it has been observed to bind to haemo- basis of available epidemiological data. This is based on globin to a greater degree than EB in workers exposed to the conclusion that the weight of evidence for an butadiene. Nor has bone marrow been incorporated as a association between butadiene and leukaemia satisfies compartment, although it appears to be a target site of several of the traditional criteria for causality in butadiene-induced toxic effects. Moreover, none of the epidemiological studies. However, uncertainties in the PBPK models has been validated in humans. For these exposure estimates for the critical cohort of workers as reasons, therefore, such models have not been used to well as confounding or effect-modifying aspects that quantitatively account for interspecies variations in could impact on quantitative estimates of risk are metabolism in the quantitation of exposure–response for recognized. In view of these factors and to serve as a critical end-points based on studies in experimental basis for comparison, quantitative measures of cancer animals presented here. potency have also been developed on the basis of results of long-term bioassays in rats and mice, with In addition, owing to its relatively slow metabo- those in mice being considered justifiably conservative, lism, butadiene achieves a steady state during prolonged considering the likely heterogeneity in metabolic inhalation exposure. Exposures of the same concentra- transformation of butadiene in humans. (See discussion tion and duration would be expected to result in equiva- of relevance of specific tumour types in animals to lent toxicity across species, and interspecies scaling to humans in section 10.2.1.2.) account for variations in inhalation rate to body weight ratios or body surface areas between humans and In addition to inducing tumours at multiple sites in animals is not considered necessary. experimental animals, butadiene is also genotoxic in somatic and germ cells and induces reproductive and 10.2.1 Carcinogenicity haematological effects in animals. As a measure of 10.2.1.1 Epidemiological data

In only one epidemiological investigation of the 1 The potency estimate for carcinogenicity is determined association between butadiene and leukaemia have data by calculating the dose or concentration associated with on exposure of the study population been sufficiently an increase in cancer incidence or mortality of an characterized to permit quantitation of exposure– appropriate percentage. When based on toxicological response (Delzell et al., 1995). The Delzell et al. (1995) data from studies in experimental animals, a 5% increase study also presents results for the largest cohort studied is generally chosen, as these values usually lie within or close to the observable range (i.e., a TC05 is calculated). When epidemiological data form the basis for derivation of a tumorigenic concentration, the percent increase selected is that which falls within the area of the 2 exposure–response curve that represents the majority of Similar to tumorigenic concentrations (TC05s), the observable data; this is often less than 5%. In the benchmark concentrations for non-cancer effects (or case of butadiene, the carcinogenic potency calculated BMC05s), when based on data in experimental animals, on the basis of modelling of epidemiological data (as represent the dose or concentration associated with a described herein) was considered to be best defined as a 5% increase in the incidence of an effect compared with

1% increase in mortality due to leukaemia (i.e., a TC01). controls.

33 Concise International Chemical Assessment Document 30 to date (including subjects from eight plants, six of formed in mice compared with rats. In addition, the which were included in the exposure–response different profiles of tumours observed in the two species analyses); it is also considered to subsume the may be related to differential roles of the epoxide observations of mortality in workers at these plants metabolites in the induction of the various tumours; i.e., reported previously by other researchers (i.e., Meinhardt the diepoxide may be more critical to tumour induction in et al., 1982; Matanoski et al., 1990, 1993; Santos-Burgoa mice than is EB (since it was reported recently that et al., 1992), because of the considerable overlap in the formation of DEB increased with level of exposure to cohort definition. The exposure assessment of study butadiene in mice but not in rats; Thornton-Manning et subjects was of extremely high quality, being very al., 1998), while the monoepoxide or monoepoxide diol thorough and based on research of plant records may be more important in rats. concerning work histories, processes and local emissions, and consultation with staff from each plant, The relevance for extrapolation to humans of and is, therefore, considered appropriate for exposure–response for some of the types of tumours quantification of exposure–response (limited industrial observed in rodents has been questioned. For example, hygiene monitoring data were also available, although Irons et al. (1989) hypothesized that the thymic lympho- used primarily for comparison with estimated ma/leukaemia induced in B6C3F1 mice may be related to concentrations). For comparison with estimates based the presence of an endogenous ecotropic retrovirus, as a on the data from the cohort study, carcinogenic potency much lower incidence was observed in Swiss mice that was also calculated on the basis of the results of the do not possess this retrovirus (although the incidence case–control study nested within essentially the same was significantly elevated compared with controls). population of workers (Matanoski et al., 1997), although Therefore, although the haematopoietic system is a data available in the published report were too limited to target for the induction of cancer by butadiene in permit detailed analysis here. humans, the observed exposure–response relationship for this end-point is not considered appropriate for A detailed description of the methods employed quantitative extrapolation to humans — on the basis that and the assumptions made in the derivation of tumori- this retrovirus is not present in humans and its presence genic potency estimates (i.e., TC01, or the concentration in B6C3F1 mice renders this strain quite susceptible to associated with a 1% increase in mortality due to induction of lymphoma — although the relevant infor- leukaemia based on the observations in Delzell et al. mation is included for comparative purposes. [1995]) is presented in Appendix 4. Although several mathematical models were applied, the choice of model It has also been suggested that the tumours had little impact on the resulting TC01, as there was only observed in the study in rats (i.e., mammary gland, a threefold variation in the range of values. However, the thyroid gland, pancreas, uterus, and testes) and some of estimated TC01 in which confidence is highest (i.e., that the tumours induced in mice (i.e., ovaries and mammary for which the model provided the best fit) is 1.7 mg/m3. gland) may be mediated through effects on the

The TC01 calculated on the basis of the data of Matan- endocrine system. Indeed, tumours at these sites are oski et al. (1997) was only slightly lower than this value. often associated with disruption of hormonally mediated functions. In addition, non-neoplastic or pre-neoplastic 10.2.1.2 Data from studies in experimental animals effects, including atrophy, degeneration, and hyperplasia, have also been observed in mice exposed As described in section 10.1.1, butadiene induced subchronically to butadiene. However, the mechanism an increase in the incidence of tumours at multiple sites by which butadiene induces tumours at these sites has in both B6C3F1 mice (liver, lung, Harderian gland, mam- not yet been adequately investigated; i.e., it has not mary gland, ovaries, forestomach, Zymbal gland, and been established whether these tumours are induced via kidney, along with malignant lymphomas, histiocytic a mechanism for which there may be a threshold of sarcomas, and cardiac haemangiosarcomas) and exposure (e.g., through induction of hormonally Sprague-Dawley rats (mammary gland, thyroid gland, mediated effects), although the possibility is recognized. uterus, Zymbal gland, pancreas, and testes). As In addition, the results of in vivo genotoxicity assays discussed above, consistent with the species differences indicate that butadiene or its metabolites induce genetic in metabolism, mice were much more sensitive to effects in the reproductive organs of multiple strains of butadiene-induced cancer than were rats for the strains mice. investigated. Based on data available (i.e., evidence from genotoxicity studies that butadiene and its metabolites Based on these considerations, estimates of car- are active in both species), this difference in sensitivity cinogenic potency were calculated on the basis of the is quantitative rather than qualitative and is related to malignant lymphomas, histiocytic sarcomas, cardiac the greater amounts of putatively active metabolites haemangiosarcomas, alveolar/bronchiolar adenomas or

34 1,3-Butadiene: Human health aspects carcinomas, hepatocellular adenomas or carcinomas, chronic exposure of mice to concentrations of butadiene squamous cell papillomas or carcinomas of the fore- considerably lower than those associated with adverse stomach, adenomas or carcinomas of the Harderian effects on the testes, investigation of the response of gland, granulosa cell tumours of the ovaries, and female germ cells in mice to butadiene is desirable, since adenoacanthomas, carcinomas, or malignant mixed this may well be the most sensitive end-point for devel- tumours of the mammary gland observed in B6C3F1 mice opment of quantitative estimates of heritable damage. in the chronic bioassay conducted by the NTP (1993) (Determination of putatively toxic metabolites in the and the mammary gland tumours, pancreatic exocrine ovaries of butadiene-exposed female mice would also be adenomas, Leydig cell tumours, Zymbal gland informative.) For this reason, quantitation of exposure– carcinomas, thyroid follicular cell adenomas or carci- response for heritable genetic damage is not presented nomas, and uterine sarcomas in Sprague-Dawley rats here. However, in view of the apparent greater reported by Hazleton Laboratories Europe Ltd. (1981a). It sensitivity of the reproductive organs in female mice, a is noted that the characterization of exposure–response benchmark concentration was derived for non-neoplastic is much better in the study in mice (which involved five effects in the ovary, which is considerably more closely spaced exposure levels) than in the bioassay in protective than that for male-mediated heritable damage rats (in which only two more widely spaced exposure developed by Pacchierotti et al. (1998b). (Although the levels were used, the higher of which was likely above relative role of butadiene in the induction of the the level of metabolic saturation). (Although there were observed atrophy in mice in the NTP study is unclear, as also increased incidences of tumours at several sites in discussed in section 10.1.2, information currently

B6C3F1 mice in the “stop-exposure” study conducted by available is not considered a sufficient basis upon which the NTP [1993], only TC05s determined on the basis of to dismiss this end-point as being inappropriate for the 2-year study were included, as the latter study quantification of exposure–response. However, this provides better information for characterization of uncertainty should be kept in mind in the interpretation exposure–response in mice following long-term exposure or application of the BMC05s derived below.) [i.e., more exposure levels for up to 2 years].) Ovarian atrophy was observed in both long-term The methods employed for development of esti- NTP (1984, 1993) bioassays in mice and a subchronic mates of tumorigenic potency (i.e., the concentrations study (Bevan et al., 1996). Although limited, available associated with a 5% increased incidence of tumours, or data indicate that rats are less sensitive to induction of

TC05s) based on these data are described in Appendix 4. this effect, which may, again, be a consequence of inter- TC05s based on observations in mice ranged from species variations in metabolism. Therefore, although 2.3 mg/m3 (95% lower confidence limit [LCL] = 1.7 mg/m3) additional research into the etiology of the observed for Harderian gland tumours in males to 99 mg/m3 (95% ovarian atrophy in mice would be desirable, the data LCL = 23 mg/m3) for malignant lymphomas in males. For from the later NTP study are considered most appropri- 3 rats, calculated TC05s ranged from 6.7 mg/m (95% LCL = ate for characterization of exposure–response (i.e., 3 3 3 4.7 mg/m ) to 4872 mg/m (95% LCL = 766 mg/m ) for development of a BMC05). In this investigation, the tumours of the mammary gland and Zymbal gland in incidence of atrophy of the ovaries was significantly females, respectively. increased in an exposure-related manner at all concen- trations tested (i.e., $6.25 ppm [$13.8 mg/m3]). The Although not presented here, modelling of the severity of this effect also increased with exposure. incidence of micronucleated polychromatic erythrocytes in B6C3F1 mice exposed to butadiene for up to 15 months The derivation of the BMC05 for ovarian atrophy is in the NTP bioassay resulted in a benchmark presented in detail in Appendix 4. Because the expo- concentration (BMC05) that was very similar to the lower sure–response curve plateaus at the higher exposure end of the range of estimates of tumorigenic potency. levels, the two highest exposure groups were omitted

from the calculations. The resulting BMC05 for ovarian 10.2.2 Non-neoplastic effects atrophy in mice was determined to be 0.57 mg/m3 (95% LCL = 0.44 mg/m3), when all degrees of severity were There have been recent attempts to quantitatively considered. If only lesions of moderate or marked estimate risk of heritable genetic damage in humans severity were considered, the resulting BMC05 would be based on a parallelogram approach and data on male- about fivefold higher. mediated heritable translocations and bone marrow micronuclei in mice and chromosomal aberrations in Haematotoxicity is considered to be a critical effect lymphocytes of exposed workers (Pacchierotti et al., associated with exposure to butadiene. Although the 1998b). In view, however, of the reported ovarian haematopoietic system appears to be a target for atrophy due to reduction of primordial follicles (to a butadiene-induced cancer in humans, available data on degree that would preclude reproduction) following the potential non-neoplastic effects on this system are

35 Concise International Chemical Assessment Document 30 inadequate for quantitation of exposure–response. characterize the distributions of concentrations of However, since statistically significant changes were butadiene in various indoor environments. In general, observed in mice only at concentrations greater than butadiene is detected more frequently and at higher those that induced other toxic effects, and since concentrations in indoor environments contaminated by benchmark concentrations derived for effects on the ETS than in areas where smoking does not occur. Non- blood are greater than those for these other effects, smokers who spend a considerable proportion of their quantitation of the exposure–response for time in indoor environments where ETS is present can be haematological effects has not been presented here. exposed to concentrations of butadiene that are an order of magnitude higher than the average levels in the 10.3 Sample exposure and risk outdoor air. Tobacco use (e.g., 20 cigarettes per day) can characterization increase the daily intake of butadiene by smokers by five times over the daily intake by non-smokers in ETS- 10.3.1 Sample exposure characterization contaminated indoor locations. The daily intake of butadiene by smokers can be 100 times greater than the The principal source of environmental exposure to daily intake of non-smokers who are not exposed to ETS. butadiene is air. Although few data were identified regarding levels in drinking-water and food, due to its 10.3.2 Sample risk characterization physical/chemical properties (e.g., vapour pressure and partition coefficients) and environmental release patterns Butadiene is released to air from both industrial (i.e., principally atmospheric emissions), intake of point sources and more dispersive, non-point sources, butadiene in these media is expected to be negligible in the latter due to its production primarily during comparison with that in air. incomplete combustion. Based on estimates derived using monitoring data from Canada, intake for the As an example of population exposure characteri- general population is primarily from air, with intake from zation, estimates are presented on the basis of data other media likely being negligible in comparison. The available for Canada. Based on concentrations measured focus of the human health risk characterization is, in outdoor air in several rural, suburban, and urban therefore, the general population exposed in outdoor and locations across Canada1 (see section 6.1.1), 95% of the indoor air in the general environment and those exposed general population can be expected to be exposed to through air in the vicinity of industrial point sources. average concentrations of up to 1.0 µg/m3. However, since levels are generally greater in highly urbanized For compounds such as butadiene, where data are areas, estimated “reasonable worst-case exposure” is sufficient to support a plausible mode of action for expected to be up to 1.3 µg/m3 (95th percentile). In areas induction of tumours by direct interaction with genetic influenced by industrial point sources, exposure could material, estimates of exposure are compared with be as high as 6.4 µg/m3, based on the 95th percentile of quantitative estimates of cancer potency to characterize concentrations measured near a source in Ontario risk. (MOEE, 1995). Tumorigenic concentrations were calculated on the Individuals may also be exposed to butadiene for basis of data from both epidemiological studies and short durations while at self-service gasoline filling investigations in experimental animals. For the critical stations or in parking garages; however, these intakes epidemiological investigation (Delzell et al., 1995), a TC01 are still much less than average daily intakes for the (i.e., the concentration associated with a 1% increase in general population from inhalation of background mortality due to leukaemia) was considered the concentrations in outdoor and indoor air. appropriate measure of carcinogenic potency, since the majority of the observable data fell within this range. Although available Canadian data indicate that Although four different mathematical models were butadiene is detected with greater frequency in indoor considered, the TC01 generated by the model with the air than in outdoor air, there are insufficient data to best fit was 1.7 mg/m3.

Quantitative estimates of carcinogenic potency derived on the basis of data in experimental animals were

calculated as TC05s (i.e., the concentration associated with a 5% increase in tumour incidence). Based on the 2- 1 Unpublished data on butadiene levels in Canada from year bioassay in mice (NTP, 1993), TC s ranged from 2.3 National Air Pollution Surveillance program, provided by 05 mg/m3 (95% LCL = 1.7 mg/m3) to 99 mg/m3 (95% LCL = 23 T. Dann, River Road Environmental Technology Centre, mg/m3). The TC s derived on the basis of the more Environment Canada, Ottawa, Ontario, to Commercial 05 limited study in rats (Hazleton Laboratories Europe Ltd., Chemicals Evaluation Branch, Environment Canada, Hull, Quebec, April 1997.

36 1,3-Butadiene: Human health aspects

3 3 1981a) ranged from 6.7 mg/m (95% LCL = 4.7 mg/m ) to the margin between the BMC05 for ovarian toxicity and a 4872 mg/m3 (95% LCL = 766 mg/m3). sample exposure characterization are presented in Table 7. It should be noted, though, that even if the mode of The values derived on the basis of studies in induction of ovarian atrophy does not involve direct humans are preferred as the basis for comparison with interaction with genetic material, the margin between estimates of exposure to characterize risk. While there are exposure and effect level (i.e., for which a tolerable a number of uncertainties in the use of the epidemi- concentration is normally developed) is still small — i.e., ological data for both hazard evaluation and exposure– exposure levels in Canada are 90–570 times lower than response analyses (section 10.4), these are likely far less the benchmark concentration, as presented in Table 7. than uncertainties associated with interspecies extrapo- lation. Moreover, estimated potency for humans is 10.4 Uncertainties and degree of similar to that developed on the basis of the cancer confidence in human health hazard bioassays in experimental animals. (Indeed, although in characterization and sample risk an area of the exposure–response curve where data were characterization more sparse, it is noteworthy that TC05s calculated on the basis of epidemiological data [as opposed to the There is a high degree of certainty that butadiene

TC01s presented above] are within the range of values is being released to ambient air in Canada in significant derived from the studies in rodents.) amounts in vehicular exhaust. There is a moderate degree of certainty that exhaust emissions of butadiene Based on the sample exposure scenarios presented are lower in well maintained vehicles equipped with above (section 10.3.1), 95% of the population in Canada catalytic converters than in older non-equipped vehicles, is exposed to concentrations of butadiene in outdoor air and that evaporative emissions during refuelling and of 1.0 µg/m3 or less. For the proportion of the general vehicle operation contribute less to concentrations of population that is regularly exposed to higher concentra- butadiene in ambient air than do emissions in vehicular tions of butadiene in urban areas (i.e., the “reasonable exhaust. worst-case scenario”), the 95th percentile of the distrib- ution of concentrations is 1.3 µg/m3. In the only area of There is a moderate degree of certainty that buta- Canada identified as having an industrial point source, diene is not being released to the Canadian environment the 95th percentile of the distribution of concentrations in significant amounts from industrial activities in is 6.4 µg/m3. Canada, as only a single major point source (i.e., in Sarnia, Ontario) of discharge to the atmosphere has been The margins between carcinogenic potency and identified. Although there is some uncertainty that the estimated exposure for the general population (including available measurements of butadiene in samples taken ambient and reasonable worst case) and those in the over a few days in the vicinity of this source are repre- vicinity of a point source are presented in Table 6. sentative of population exposure over the long term, Equivalent low-dose risk estimates are also presented in since the samples were taken at distances of up to a few this table. kilometres from the source, there is a moderate degree of certainty that a segment of the population would be In view of the relative potency of butadiene to exposed to the measured concentrations. There is a high induce some non-cancer effects, these end-points are degree of certainty that populations in rural areas are also important in risk characterization. As presented exposed to lower concentrations of butadiene in ambient 3 above, a benchmark concentration (BMC05) of 0.57 mg/m air than are communities in more densely populated (95% LCL = 0.44 mg/m3) was derived on the basis of data areas. for the incidence of ovarian atrophy of all severities (i.e., female reproductive toxicity) in mice exposed to Available data on concentrations of butadiene in butadiene for up to 2 years (NTP, 1993). And while there ambient air in Canada are quite extensive. A large pro- is uncertainty about the relevance of the ovarian atrophy portion of the numerous samples from several sampling observed in mice for humans (section 10.4), the BMC05 is sites across the country contained concentrations of slightly less than the lower end of the range of estimates butadiene above the level of detection. Therefore, there of cancer potency based on the incidence of tumours in is a high degree of certainty in the estimations of expo- the same study in mice, as well as the TC05 for cancer sure to butadiene via ambient air. based on the epidemiological data. The mode of induction of ovarian atrophy is unknown. However, if it is (reasonably) assumed that the mode of action is related to that by which tumours are induced (i.e., direct interaction with genetic material), risk to human health for reproductive effects may be characterized in the same manner as presented for cancer. Therefore, estimates of

37 Concise International Chemical Assessment Document 30

Table 6: Comparison of estimates of carcinogenic potency with exposure levels.

Margin between potency and Equivalent low-dose risk

Exposure Potency (TC 01 or TC 05) exposure estimate

3 3 –6 1.0 µg/m (95th percentile 1.7 mg/m (TC01 for leukaemia in humans) 1700 5.9 × 10 for all sites in Canada) 3 –6 2.3 mg/m (TC05 for most sensitive tumour 2300 22 × 10 site in mice [Harderian gland])

3 –6 1.7 mg/m (95% LCL of TC05 for most 1700 29 × 10 sensitive tumour site in mice)

3 –6 6.7 mg/m (TC05 for most sensitive tumour 6700 7.5 × 10 site in rats [mammary gland])

3 –6 4.7 mg/m (95% LCL of TC05 for most 4700 11 × 10 sensitive tumour site in rats)

3 3 –6 1.3 µg/m (95th percentile 1.7 mg/m (TC01 for leukaemia in humans) 1300 7.7 × 10 for reasonable worst-case 3 –6 scenario) 2.3 mg/m (TC05 for most sensitive tumour 1800 28 × 10 site in mice [Harderian gland])

3 –6 1.7 mg/m (95% LCL of TC05 for most 1300 38 × 10 sensitive tumour site in mice)

3 –6 6.7 mg/m (TC05 for most sensitive tumour 5200 9.6 × 10 site in rats [mammary gland])

3 –6 4.7 mg/m (95% LCL of TC05 for most 3600 14 × 10 sensitive tumour site in rats)

3 3 –5 6.4 µg/m (95th percentile 1.7 mg/m (TC01 for leukaemia in humans) 270 3.7 × 10

for area affected by 3 –5 2.3 mg/m (TC05 for most sensitive tumour 360 14 × 10 industrial point source) site in mice [Harderian gland])

3 –5 1.7 mg/m (95% LCL of TC05 for most 270 19 × 10 sensitive tumour site in mice)

3 –5 6.7 mg/m (TC05 for most sensitive tumour 1000 5.0 × 10 site in rats [mammary gland])

3 –5 4.7 mg/m (95% LCL of TC05 for most 730 6.8 × 10 sensitive tumour site in rats)

Table 7: Comparison of estimates of potency for non-cancer effects with exposure levels.

Margin between effect level and

Exposure Potency (BMC05) exposure

3 3 1.0 µg/m (95th percentile for all sites 0.57 mg/m (BMC05 for ovarian atrophy in mice) 570 in Canada) 3 0.44 mg/m (95% LCL of BMC05 for ovarian atrophy in mice) 440

3 3 1.3 µg/m (95th percentile for 0.57 mg/m (BMC05 for ovarian atrophy in mice) 440

reasonable worst-case scenario) 3 0.44 mg/m (95% LCL of BMC05 for ovarian atrophy in mice) 340

3 3 6.4 µg/m (95th percentile for area 0.57 mg/m (BMC05 for ovarian atrophy in mice) 90 affected by industrial point source) 3 0.44 mg/m (95% LCL of BMC05 for ovarian atrophy in mice) 70

The most limiting aspect of the exposure assess- Higher concentrations of butadiene have been ment is the lack of sufficient data on the concentrations measured in indoor air where ETS was known to be of butadiene in indoor air. This is an important short- present. However, the data on concentrations of buta- coming, since humans spend significantly more time in diene in ETS-contaminated indoor air are highly variable indoor environments than outdoors. In the absence of and are not sufficient to reasonably define the range of indoor sources, it is reasonably certain that concentra- mean concentrations. Nevertheless, there is a high tions of butadiene in indoor environments are similar to degree of certainty that non-smokers spending a the concentrations in the local ambient air. considerable proportion of their time in indoor environments where ETS is present are exposed to

38 1,3-Butadiene: Human health aspects higher concentrations of butadiene than are non- Although the assessment of the exposure of the smokers who are not exposed to ETS. There is a high critical cohort of workers is likely one of the most degree of certainty that smokers are exposed to higher comprehensive published to date, there is also uncer- concentrations of butadiene and have significantly tainty in the estimates of carcinogenic potency derived higher daily intakes than do non-smokers. on the basis of this study, due primarily to the fact that the estimates of exposure are based on few actual histori- There is somewhat less certainty that butadiene cal monitoring data.1 For example, when the exposure of monomer is not released in detectable amounts from workers at one plant was re-examined, there were 2- to 3- consumer products (e.g., synthetic materials) incorpor- fold changes in the estimates for several job groups ating this compound in their production. Although there (with a 10-fold increase for one job group). In addition, may be contributions to indoor concentrations of buta- with the exception of incorporating exposure to styrene diene from certain cooking activities, the data are not as a stratification variable in the analyses, potential sufficient to identify specific sources or activities or to interactions between various occupational exposures identify a range of emissions of butadiene during could not be taken into account in the derivation of the cooking. carcinogenic potency based on the observations in this cohort. It has also been demonstrated that genetic poly- Although data on levels of butadiene in foodstuffs morphism for several of the enzymes involved in metabo- are scarce, based on the physical and chemical lism of butadiene affects sensitivity to toxic effects properties of the substance and the fact that it is induced by the substance. Also, since information on released primarily to ambient air (where it is likely to genotype for the relevant enzymes was not available for remain without partitioning to other media), there is a this large cohort and only a small amount of information reasonable degree of certainty that food does not on the distribution in the general population has been represent a major source of exposure. Similarly, although identified, it is not possible to determine how represen- the database for concentrations of butadiene in drinking- tative the study cohort is of the genetic susceptibility to water is limited, there is a reasonable degree of certainty butadiene of the general public. that drinking-water is not an important source of exposure for the general public in Canada, based on the With respect to the quantitation of exposure– volatility and release patterns of the compound. response and derivation of potency estimates based on the epidemiological data, the inability of any of the There is some degree of uncertainty that the models to consistently predict leukaemia rates in the weight of epidemiological evidence for the association validation study contributes to additional uncertainty. In between butadiene and leukaemia satisfies criteria for addition, the small number of leukaemia cases being causality. In particular, the need for coherence is modelled contributes to model instability. However, the seemingly not addressed, since the observed increase in fact that the range of potency estimates for the four mortality due to leukaemia in styrene-butadiene rubber models is narrow (i.e., 1.4–4.3 mg/m3) increases the workers was not observed in the cohorts of monomer confidence in the calculated potencies. There is also workers (although there was some evidence of an some uncertainty associated with the fact that the association with other forms of lymphohaematopoietic potency estimates were based only on cases in which cancer, particularly in short-term workers). This may be leukaemia was considered the underlying cause of death, related to the nature of exposure to both butadiene and rather than all cases, which could result in an underesti- other substances in these two industries. However, in mation of leukaemogenic potency. view of the overwhelming evidence of carcinogenicity and genotoxicity in experimental animals, available infor- In view of the likely variability in metabolism of mation on species differences in sensitivity likely being butadiene across the human population related to related to differences in metabolism, and the potential for genetic polymorphism for relevant enzymes, estimates of considerable interindividual variability in metabolism to carcinogenic potency as well as benchmark putatively toxic metabolites in the human population, concentrations for non-cancer effects based on studies along with the limited evidence of genotoxicity in occu- in mice are considered justifiably conservative. However, pationally exposed populations, there is a high degree of confidence that butadiene is likely to be carcinogenic in humans. Based on the extensive database on the geno- 1 Although it has not been possible to quantitatively toxicity of butadiene and its principal metabolites both in characterize uncertainty regarding these estimates of vitro and in vivo in both somatic and germ cells, confi- exposure and the impact of this uncertainty upon the dence that butadiene induces tumours (and possibly estimates of carcinogenic potency, data being collected other effects) through direct interaction with genetic currently may permit a more quantitative characterization material is high. in future (personal communication [correspondence dated 20 March 1998] from J. Lynch, Consultant, Rumson, NJ, to Health Canada).

39 Concise International Chemical Assessment Document 30 because of the high mortality in the study in mice in of only moderate or marked severity would result in only which exposure–response could best be characterized a 1.5- or 3-fold difference in the measure of risk.) How- and the limitations in the study in rats (high mortality at ever, in view of the weight of evidence of causality for the higher of only two widely spaced exposure levels), the association between butadiene and these effects in there is a moderate degree of uncertainty in estimates of mice and the relatively low value for the measure of carcinogenic potency derived on the basis of dose–response compared with that for other types of investigations in experimental animals. In addition, since effects, additional investigation in this area is deemed to the available PBPK models were considered inadequate, be of high priority. the potency estimates developed here are based on an inhaled concentrations exposure metric; i.e., none of the available information on species differences in metabolism is taken into account. It is noteworthy that if 12. PREVIOUS EVALUATIONS BY the calculated margins between exposure and INTERNATIONAL BODIES carcinogenic potency presented above that serve to characterize risk were derived on the basis of the 95% LCLs of the TC s for tumours in mice, the values would 05 An IARC Working Group that convened in 1998 differ by only 1.4- to 3.3-fold (i.e., within the same order has classified 1,3-butadiene as probably carcinogenic of magnitude) from those calculated on the basis of the to humans (Group 2A) based on limited evidence of point estimates; similarly, use of the 95% LCLs of the carcinogenicity in humans and sufficient evidence of TC s for tumours in rats would result in a 1.1- to 6.4-fold 05 carcinogenicity for 1,3-butadiene and 1,2:3,4-diepoxy- difference in the margins between exposure and potency. butane in experimental animals (IARC, 1999). In addition, although confidence in the use of the potency estimate for lymphomas in mice is low, due to the inherent sensitivity associated with the presence of an endogenous retrovirus, it is noteworthy that the TC05 for this tumour would not be limiting, as it falls within the range of values determined for cancers at other sites. Also, it should be noted that, although these margins and measures of risk presented above were based on comparison of the 95th percentile of the exposure data for each scenario, use of the median concentration (i.e., the 50th percentile) and either the point estimates of carcinogenicity or the associated 95% LCLs would result in a 5-fold difference in the resulting values for the general population and a 10-fold difference in values for those in an area influenced by a point source.

There is uncertainty about the relevance of the ovarian atrophy observed in mice to humans, based on lack of data on the relative role of butadiene in the etiology of these lesions. Although the observed effects in the 2-year bioassay may have been related to senile changes, possibly exacerbated by butadiene, atrophy of the ovaries was detected in these mice as early as 9 months, and there is consistent evidence in other chronic and subchronic studies that the ovaries are tar- gets of toxic effects induced by butadiene or its epoxide metabolites. As a result of this uncertainty, quantitative measures of dose–response developed on the basis of ovarian atrophy must necessarily be interpreted with caution. In addition, the BMC05 presented above was based on inclusion of atrophy of all severities, including “minimal” severity, the biological significance of which is unclear. If only lesions of moderate or marked severity are considered, the resulting BMC05 and hence the cal- culated margin between exposure and effect level and risk estimates would differ by about fivefold. (Use of the

95% LCLs of the BMC05s for atrophy of all severities or

40 1,3-Butadiene: Human health aspects

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Walk R-A, Jenderny J, Röhrborn G, Hackenberg U (1987) Chromo- Inquiry Centre somal abnormalities and sister-chromatid exchange in bone marrow of Environment Canada mice and Chinese hamsters after inhalation and intraperitoneal Main Floor, Place Vincent Massey administration: I. Diepoxybutane. Mutation research, 182:333–342. 351 St. Joseph Blvd. Hull, Quebec Walles SAS, Victorin K, Lundborg M (1995) DNA damage in lung cells in Canada K1A 0H3 vivo and in vitro by 1,3-butadiene and nitrogen dioxide and their photochemical reaction products. Mutation research, 328:11–19. or on the Internet at:

Ward DE, Hao WM (1992) Air toxic emissions from the burning of biomass globally — preliminary estimates. Presented at the 85th Annual www.ec.gc.ca/cceb1/eng/public/index_e.html Meeting of the Air and Waste Management Association, Kansas City, MO. Unpublished supporting documentation for the health effects assessment, which presents additional information, is Ward EM, Fajen JM, Ruder AM, Rinsky RA, Halperin WE, Fessler- available upon request from: Flesch CA (1995) Mortality study of workers in 1,3-butadiene production units identified from a chemical workers cohort. Environmental health Environmental Health Centre perspectives, 103(6):598–603. Room 104 Health Canada Ward EM, Fajen JM, Ruder AM, Rinsky RA, Halperin WE, Fessler- Tunney’s Pasture Flesch CA (1996) Mortality study of workers employed in 1,3-butadiene Ottawa, Ontario production units identified from a chemical workers cohort. Toxicology, Canada K1A 0L2 113:157–168.

Sections of the Assessment Report and supporting Ward JB Jr, Ammenheuser MM, Bechtold WE, Whorton EB Jr, Legator MS (1994) hprt mutant lymphocyte frequencies in workers at a 1,3- documentation on genotoxicity and reproductive and butadiene production plant. Environmental health perspectives, developmental toxicity were reviewed by D. Blakey and W. 102(Suppl. 9):79–85. Foster, respectively, of the Environmental and Occupational Toxicology Division of Health Canada. A review of the exposure Ward JB Jr, Ammenheuser MM, Whorton EB Jr, Bechtold WE, Kelsey assessment included in the critical epidemiological studies was KT, Legator MS (1996) Biological monitoring for mutagenic effects of prepared under contract by M. Gerin and J. Siemiatycki of the occupational exposure to butadiene. Toxicology, 113:84–90. Institut Armand-Frappier, University of Quebec.

Wiencke JK, Kelsey KT (1993) Susceptibility to induction of In the first stage of external review, sections of the chromosomal damage by metabolites of 1,3-butadiene and its supporting documentation pertaining to human health were relationship to “spontaneous” sister chromatid exchange frequencies in considered by the following individuals, primarily to address human lymphocytes. In: Sorsa M, Peltonen K, Vainio H, Hemminki K, eds. Butadiene and styrene: Assessment of health hazards. Lyon, adequacy of coverage: J. Aquavella, Monsanto Company; M. International Agency for Research on Cancer, pp. 265–273 (IARC Bird, Exxon Biomedical Sciences, Inc.; J.A. Bond, Chemical Scientific Publications No. 127). Industry Institute of Toxicology; I. Brooke, United Kingdom Health and Safety Executive; G. Granville, Shell Canada Ltd.; Wiencke JK, Pemble S, Ketterer B, Kelsey KT (1995) Gene deletion of R. Keefe, Imperial Oil Ltd.; A. Koppikar, US Environmental glutathione S-transferase 2: correlation with induced genetic damage Protection Agency; R.J. Lewis, Exxon Biomedical Sciences, Inc.; and potential role in endogenous mutagenesis. Cancer epidemiology K. Peltonen, Finnish Institute of Occupational Health; and F. biomarkers and prevention, 4:253–259. Ratpan, Nova Chemicals

Wilson AL, Colome SD, Tian Y (1991) Air toxics microenvironmental In the second stage of external review, accuracy of exposure and monitoring study. Final report. Prepared for South Coast reporting, adequacy of coverage, and defensibility of Air Quality Management District, El Monte, CA, and US Environmental conclusions with respect to hazard characterization and Protection Agency by Integrated Environmental Services, Irvine, CA. exposure–response analyses were considered in written review by

Xi L, Zhang L, Wang Y, Smith MT (1997) Induction of chromosome- BIBRA International and the following individuals: R.J. Albertini, specific aneuploidy and micronuclei in human lymphocytes by University of Vermont; J.A. Bond, Chemical Industry Institute of metabolites of 1,3-butadiene. Carcinogenesis, 18(9):1687–1693. Toxicology; I. Brooke, United Kingdom Health and Safety Executive; J. Bucher, US National Toxicology Program; B. Xiao Y, Tates AD (1995) Clastogenic effects of 1,3-butadiene and its Davis, US National Toxicology Program; E. Delzell, University of metabolites 1,2-epoxybutene and 1,2,3,4-diepoxybutane in splenocytes Alabama at Birmingham; B.J. Divine, Texaco; A.A. Elfarra, and germ cells of rats and mice in vivo. Environmental and molecular University of Wisconsin-Madison; E. Frome, Oak Ridge National mutagenesis, 26:97–108. Laboratory; B.D. Goldstein, Environmental and Occupational Health Sciences Institute; R.F. Henderson, Lovelace Respiratory Research Institute; R.D. Irons, University of Colorado Health Sciences Center; A. Koppikar, US Environmental Protection Agency; J. Lubin, US National Cancer Institute; J. Lynch, Exxon Biomedical Sciences, Inc. (retired); R.L. Melnick, US National Toxicology Program; K. Peltonen, Finnish Institute of Occupational Health; A.G. Renwick, University of Southampton;

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J. Siemiatycki, Institut Armand-Frappier; L.T. Stayner, US APPENDIX 2 — CICAD PEER REVIEW National Institute for Occupational Safety and Health; J.A. Swenberg, University of North Carolina; R. Tice, Integrated Laboratory Systems, Inc.; and J.B. Ward, Jr., University of Texas Medical Branch. The draft CICAD on 1,3-butadiene was sent for review to institutions and organizations identified by IPCS after contact In the third and final stage of external expert review, with IPCS national Contact Points and Participating Institutions, adequacy of incorporation of the comments received during the as well as to identified experts. Comments were received from: second stage was considered at a final meeting of a panel of the following members convened by Toxicology Excellence in Risk M. Baril, International Programme on Chemical Safety/ Assessment (TERA) in November 1998: H. Clewell, K.S. Crump Institut de Recherche en Santé et en Sécurité du Travail Division of ICF Kaiser; M.L. Dourson, TERA; and L. Erdreich, du Québec, Canada Bailey Research Associates, Inc. R. Benson, Drinking Water Program, US Environmental The health-related sections of the Assessment Report Protection Agency, USA were reviewed and approved by the Health Protection Branch Risk Management meeting. The entire Assessment Report was T. Berzins, National Chemicals Inspectorate (KEMI), reviewed and approved by the Environment Canada/Health Sweden Canada CEPA Management Committee. R. Cary, Health and Safety Executive, United Kingdom Concurrent with review of the draft CICAD, there was also a public comment period for the source national assessment, in R. Chhabra, National Institute of Environmental Health which the Priority Substances List Assessment Report was made Sciences, National Institutes of Health, USA available for 60 days (2 October to 1 December 1999). A summary of the comments and responses is available on the P. Edwards, Department of Health, United Kingdom Internet at www.ec.gc.cceb1/eng/public/index_e.html. H. Gibb, National Center for Environmental Assessment, US Environmental Protection Agency, USA

R. Hertel, Federal Institute for Health Protection of Consumers and Veterinary Medicine, Germany

J. Heuer, Federal Institute for Health Protection of Consumers and Veterinary Medicine, Germany

J. Jinot, US Environmental Protection Agency, USA

C. Kimmel, US Environmental Protection Agency, USA

A.M. Koppikar, US Environmental Protection Agency, USA

S. Kristensen, National Industrial Chemicals Notification and Assessment Scheme (NICNAS), Australia

N. Moore, BP Amoco Chemicals (commented through Department of Health, United Kingdom)

H. Nagy, National Institute of Occupational Safety and Health, USA

S. Tarkowski, Nofer Institute of Occupational Medicine, Poland

L. Vodickova, National Institute of Public Health, Centre of Industrial Hygiene and Occupational Diseases, Czech Republic

P. Yao, Institute of Occupational Medicine, Chinese Academy of Preventive Medicine, People’s Republic of China

K. Ziegler-Skylakakis, GSF-Forschungszentrum für Umvelt und Gesundheit, Germany (transmitted comments from BUA and industry representatives)

50 1,3-Butadiene: Human health aspects

APPENDIX 3 — CICAD FINAL REVIEW BOARD Observer Helsinki, Finland, 26–29 June 2000 Dr R.J. Lewis (representative of European Centre for Ecotoxicology and Toxicology of Chemicals), Epidemiology and Health Surveillance, ExxonMobil Biomedical Sciences, Inc., Members Annandale, NJ, USA

Mr H. Ahlers, Education and Information Division, National Institute for Occupational Safety and Health, Cincinnati, OH, Secretariat USA Dr A. Aitio, International Programme on Chemical Safety, World Dr T. Berzins, National Chemicals Inspectorate (KEMI), Solna, Health Organization, Geneva, Switzerland (Secretary) Sweden Dr P.G. Jenkins, International Programme on Chemical Safety, Dr R.M. Bruce, Office of Research and Development, National World Health Organization, Geneva, Switzerland Center for Environmental Assessment, US Environmental Protection Agency, Cincinnati, OH, USA Dr M. Younes, International Programme on Chemical Safety, World Health Organization, Geneva, Switzerland Mr R. Cary, Health and Safety Executive, Liverpool, United Kingdom (Rapporteur)

Dr R.S. Chhabra, General Toxicology Group, National Institute of Environmental Health Sciences, Research Triangle Park, NC, USA

Dr H. Choudhury, National Center for Environmental Assessment, US Environmental Protection Agency, Cincinnati, OH, USA

Dr S. Dobson, Centre for Ecology and Hydrology, Monks Wood, Abbots Ripton, United Kingdom (Chairman)

Dr H. Gibb, National Center for Environmental Assessment, US Environmental Protection Agency, Washington, DC, USA

Dr R.F. Hertel, Federal Institute for Health Protection of Consumers and Veterinary Medicine, Berlin, Germany

Ms K. Hughes, Priority Substances Section, Environmental Health Directorate, Health Canada, Ottawa, Ontario, Canada

Dr G. Koennecker, Chemical Risk Assessment, Fraunhofer Institute for Toxicology and Aerosol Research, Hanover, Germany

Ms M. Meek, Existing Substances Division, Environmental Health Directorate, Health Canada, Ottawa, Ontario, Canada

Dr A. Nishikawa, Division of Pathology, Biological Safety Research Centre, National Institute of Health Sciences, Tokyo, Japan

Dr V. Riihimäki, Finnish Institute of Occupational Health, Helsinki, Finland

Dr J. Risher, Agency for Toxic Substances and Disease Registry, Division of Toxicology, US Department of Health and Human Services, Atlanta, GA, USA

Professor K. Savolainen, Finnish Institute of Occupational Health, Helsinki, Finland (Vice-Chairman)

Dr J. Sekizawa, Division of Chem-Bio Informatics, National Institute of Health Sciences, Tokyo, Japan

Dr S. Soliman, Department of Pesticide Chemistry, Faculty of Agriculture, Alexandria University, Alexandria, Egypt

Ms D. Willcocks, National Industrial Chemicals Notification and Assessment Scheme, Sydney, NSW, Australia

51 Concise International Chemical Assessment Document 30

APPENDIX 4 — QUANTITATION OF RR = O/E = g(D(t)) EXPOSURE–RESPONSE FOR CRITICAL where RR is the rate ratio, O and E are the observed and EFFECTS ASSOCIATED WITH EXPOSURE TO expected numbers of leukaemia deaths, D(t) is cumulative 1,3-BUTADIENE butadiene exposure up to time t, and g is the exposure–response model, which is constrained to pass through one at zero Tumorigenic concentrations based on exposure. Four different models, discussed in more detail below, epidemiological data were fitted to the data. At the model-fitting stage, the expected number of deaths is calculated on the basis of the non-exposed Methods person-years in the cohort, and not from population background rates. The raw study data1 for the six plants investigated by Delzell et al. (1995) were used to calculate the potency Lifetime probability of death due to leukaemia estimates. The data consisted of the cumulative occupational exposures to butadiene and styrene at each year of each Once the fitted exposure–response model was obtained, subject’s life (person-year), beginning with his entry into the the lifetime probability of death due to leukaemia was cohort and terminating with death or other exit from the cohort. computed using lifetable methods taking into account the death The data also contained information on race, age, calendar rates in the Canadian population. The derivation of the formula year, and years since hire of each subject. used for the lifetime probability of death due to leukaemia proceeds as follows. The response of interest was cases of death due to all forms of leukaemia, as information on the specific type of Let d(t) represent the exposure concentration of butadiene leukaemia was insufficient; only cases in which leukaemia was in ppm at age t years, and let D(t) denote the cumulative considered the underlying cause of death were considered in exposure in ppm-years, with: these analyses. Exposure estimates were cumulative t occupational exposures in ppm-years assumed to be incurred for D(t) = ò d ( x ) dx 8 h/day, 240 days/year over a 45-year working life. 0

The objective of this exposure–response analysis was to This formulation of cumulative exposure allows for the possibility compute the carcinogenic potency, expressed as the TC01, or the of non-constant exposure scenarios. concentration of butadiene associated with a 1% excess probability of dying from leukaemia. This analysis involved two At a cumulative exposure of D(t) ppm-years, the stages. First, the relationship between exposure and the death probability of dying from leukaemia by age t is given by: rate due to leukaemia within the cohort was modelled. This was t accomplished by collapsing (or stratifying) the data into discrete P ( D (t ); t ) = h ( D ( x ); x ) S ( x ) dx (1) exposure categories and then modelling the mean exposure in ò R each category versus the death rates due to leukaemia. In the 0 second stage of analysis, the TC01 was calculated based on this where hR(D(t);t) is the mortality rate from leukaemia at age t exposure–response relationship and the background mortality given a cumulative exposure to butadiene of D(t), and S(t) is the rates in the Canadian population. probability of survival up to age t. Equation (1) follows from the argument that the probability of death by age t is equal to the Exposure–response modelling probability of death at age t multiplied by the probability of surviving up until age t. In lifetable analysis, the mortality and In addition to stratifying by exposure, the data were survival rates are constant for each year, so the integral in (1) stratified by race, age, calendar year, years since hire, and can be replaced by a summation over year. styrene exposure in order to incorporate this information into the exposure–response relationship. Each of these variables was Exposure to butadiene is assumed to augment the back- collapsed into a small number of discrete categories in order to ground rate of leukaemia in a multiplicative fashion. In other reduce the number of strata, thereby improving model stability. words, the mortality rate, given exposure to butadiene, is equal These variables and their categories are presented in Table A-1. to the background exposure rate multiplied by the excess risk Exposure, defined as the mean cumulative exposure per person- due to exposure to butadiene. This is known as the “proportional year, was calculated for person-years falling into each possible hazard” model and is expressed as: combination of the stratification variables. The data were imported to Epicure (1993)2 for exposure– = ´ H R ( D (t ); t ) h (t ) [g (D (t )) ] (2) response modelling. All fitted models were of the form: where h(t) is the background mortality rate from leukaemia in the population, calculated from Canadian age-specific death rates3 1 due to leukaemia, and g(D(t)) is the fitted exposure–response The cooperation of the sponsors and researchers for model, or excess risk at age t. the Delzell et al. (1995) study in the provision of these data is gratefully acknowledged. The survival rate, S(t), appearing in equation (1) is computed from Canadian age-specific death rates due to all 2 causes, where the reported Canadian leukaemia mortality rate is Epicure is a collection of interactive programs used to fit models to epidemiological data. The specific program used to model the data for this cohort of styrene- 3 butadiene rubber workers is called AMFIT, which is Mortality data were provided to Health Canada by specially designed to model hazard functions for Statistics Canada. The cooperation of the registrars of censored cohort survival data. The strength of Epicure vital statistics in the provinces and territories of Canada lies in its ability to easily allow the background rate to who make mortality data available to Statistics Canada depend on user-specified strata, such as age, calendar under federal–provincial agreements is gratefully period, and race. acknowledged.

52 1,3-Butadiene: Human health aspects

Table A-1: Stratification variables for exposure–response modelling of epidemiological data from Delzell et al. (1995).

Variable Categories Cumulative butadiene exposure (ppm-years) 0, >0–4, 5–9, 10–19, 20–29, 30–49, 50–99, 100–199, 200+

Cumulative styrene exposure (ppm-years) 0, >0–3, 4–6, 7–9, 10–19, 20–39, 40–59, 60–79, 80+

Race black, white, other

Age 40–44, 45–49, ..., 75–79, 80+ Calendar period 1940–44, 1945–49, ..., 1990–95

Years since hire 0–4, 5–9, …, 50–55

replaced by the modelled rate in order to incorporate exposure 1000 times to characterize the variability due to the random to butadiene. The formula describing the probability of survival splitting process. If the models provided consistent fits, then the up to age i is given by: likelihood ratio test would be expected to reject at a rate equal to the desired significance level of the test (i.e., at a significance é i ù level of 0.05, the fitted parameters should be significantly S = exp ê - h * - h + h g ú different 1 in 20 times). If the tests are significant more often i å j j j j (3) than this, the confidence in the predictive power of the models is ë j = 1 û reduced. * where hj and hj are the Canadian mortality rates due to all causes and due to leukaemia at age j, respectively, and gj = Results g(D(j)) is the excess risk at age j. Exposure–response modelling Substituting equation (2) into (1), the lifetime probability of death due to leukaemia is given by: Four different exposure–response models were examined 70 and are presented in Table A-2. These models are identical to P ( D ( 70 ); 70 ) = å h i g i S i - 1 those fitted in the Delzell et al. (1995) report except that model i = 1 2 is more general and flexible than the square root model used by those authors. Preliminary analysis indicated that all where 1–70 years is the standard lifetime for a human. stratification variables except race significantly affected the model fit. Since race was only marginally insignificant, all Cancer potency (TC 01) variables were used to stratify the data prior to model fitting.

The TC01 is computed by determining the exposure D(t) at The four models were fitted while stratifying on race, age, which the excess risk is equal to 0.01. That is, calendar year, years since hire, and styrene exposure. The results of the model fitting are displayed in Table A-2. (Note: A P ( D ( t ); t ) - P ( 0 ; t ) smaller deviance roughly indicates a better fit.) A graphic = 0 .01 representation of the data and the fitted models is shown in 1 - P ( 0 ; t ) Figure A-1. Judging from the model deviances and the shape of the curves relative to the data, especially in the low-dose region, If a constant exposure d is assumed for an individual from model 1 provides the best fit to the data. birth to age 70 years, then d(t) = d ppm and the cumulative For purposes of comparison, the same models were fitted exposure D(t) = d × t ppm-years. The TC01 is then the ambient exposure level d (in ppm) at which the excess risk equals 0.01 at using the median exposure as per the Delzell et al. (1995) t = 70 years. report. These analyses indicated that there is little difference between using mean or median exposures. Models including (@ Lagged exposure analysis age as a multiplying factor of e age instead of as a stratification variable were also fitted, but these models did not fit as well. In separate analyses, exposures were lagged by n = 2, 5, Since cumulative exposure and years since hire may be 10, 15, 20, and 25 years to determine if the models would confounded, their interaction was examined. The interaction provide better fits if the most recent n years of exposure were was not significant for any of the models. The same models were ignored. An n-year lag was achieved by resetting an individual’s refitted excluding the largest exposure group (200+ ppm-years), cumulative exposure at each year to be equal to the exposure but this did not significantly affect any of the parameter he had accumulated n years prior. In so doing, the last n years of estimates. The four models were also refitted allowing for exposure do not affect the probability of developing leukaemia. different background rates for control and exposed populations. The data were first stratified on unlagged cumulative exposure, Different background rates might be necessary in occupational and then the individual exposures were lagged. Thus, the studies where lifetime non-exposed workers may differ number of strata remains constant when using different lag fundamentally from exposed workers as a result of differences in periods, and models with different lags may be directly jobs and work areas. Results of this analysis indicated that compared (Preston et al., 1987). different background rates are not necessary for these data.

Validation study The parameter estimates obtained in the present analysis are also not significantly different from those presented in the Delzell et al. (1995) report. The differences in parameter To assess the predictive power of the exposure–response estimates are likely due to the different levels used in the models, a validation study was performed in which individuals in stratification variables. Table A-2 compares the parameter the cohort were divided randomly into two groups. The models estimates obtained in this analysis with those of the Delzell et al. were fit separately to both groups, and then a likelihood ratio (1995) report. test was performed to determine if the estimated parameters were equal. The process of dividing and fitting was repeated

53 Concise International Chemical Assessment Document 30

Table A-2: Parameter estimates and model deviances for each of four models fit to mean cumulative exposure per person-year for Delzell et al. (1995) study and comparison with parameter estimates from Delzell et al. (1995) analyses.

Parameter Parameter estimates from Model estimates Standard error Deviance Delzell et al. (1995) study p-value a 1) RR = (1 + dose)" " = 0.2850 SE(") = 0.0976 171.5 " = 0.2028 0.39

2) RR = 1 + $@dose" " = 0.3999 SE(") = 0.2733 172.0 " = 0.5000b 0.62 $ = 0.4558 SE($) = 0.8222 $ = 0.1293

3) RR = e$@dose $ = 0.0029 SE($) = 0.0014 176.7 $ = 0.0041 0.38

4) RR = 1 + $@dose $ = 0.0099 SE($) = 0.0065 174.7 $ = 0.0068 0.63 a p-value of likelihood ratio test of equality of parameters. b For the Delzell et al. (1995) analysis, " was fixed at 0.5, and only $ was estimated.

Mean cumulative butadiene exposure per person year (ppm-years), adjusted for age, calendar period, race, years since hire and styrene exposure

Figure A-1. Observed rate ratios and fitted curves for leukaemia in Delzell et al. (1995) study.

Cancer potency (TC 01) Lagged exposure analysis

The TC01s were calculated for each model using the lifetable The same four models were fitted when exposures were lagged methods described above, and the resulting ambient occupational by 2, 5, 10, 15, 20, and 25 years. The resulting model fits are displayed exposures per person-year were converted to environmental exposures in Table A-4. Since the deviances are similar for each lag period, this by assuming that the exposures occurred for 8 h/day, analysis indicates that lagging exposures does not dramatically improve

240 days/year. This amounts to multiplying the TC01 by: the fit of any of the four models. In fact, TC01s for all four models and all lag periods ranged from 0.8 to 4.3 mg/m3. 8 h 240 days ´ 24 h 365 days Validation study

p To convert the ambient exposures from ppm to mg/m3, the With respect to model validation, the -values for the tests of equality of the parameters are displayed in Table A-5. If the TC01s are further multiplied by 2.21, the conversion factor for models were providing consistent fits between the two halves, butadiene. The occupational and equivalent environmental TC01s are p " presented in Table A-3. Environmental TC01s for each of the four the proportion of -values less than the significance level of 3 models ranged from 1.4 to 4.3 mg/m . TC01s calculated excluding the would be ". The results of the simulation study indicate that the largest exposure group were slightly smaller, ranging from 0.6 to 1.6 test is rejecting more often than would be expected if the mg/m3, while those calculated on the basis of median exposures were similar, ranging from 0.4 to 5.0 mg/m3.

TC01s were also calculated using the parameter estimates from the Delzell et al. (1995) report and are compared with the TC01s developed here in Table A-3. They ranged from 3.1 to 14.3 mg/m3.

54 1,3-Butadiene: Human health aspects

Table A-3: Carcinogenic potency estimates (TC 01s) for models fit to mean cumulative exposure per person-year based on Delzell et al. (1995) study and comparison with estimates from Delzell et al. (1995) analyses.

Current analysis Delzell et al. (1995) analysis

Occupational TC 01 Environmental TC 01 Environmental TC 01 Model (mg/m3) (mg/m3) (mg/m3)

1) RR = (1 + dose)" 7.8 1.7 14.3

2) RR = 1 + $@dose" 6.5 1.4 6.4

3) RR = e$@dose 19.8 4.3 3.1 4) RR = 1 + $@dose 13.8 3.0 4.5

models were providing the same fits to both halves of the data. In the NTP study, mice were exposed to 0, 6.25, 20, 62.5, 200, For model 1, the test was rejected at a significance level of 1% or 625 ppm (0, 13.8, 44.2, 138, 442, or 1383 mg/m3) butadiene for 6 in 7.4% of the runs, whereas a rejection rate of 1% of the runs h/day, 5 days/week, for 103 weeks. Survival of mice decreased with would be expected if the model was fitting consistently. The increasing exposure concentration; therefore, to minimize the effect of results of this analysis reduce the confidence in the power of the the high mortality rate, the poly-3 adjusted data (Bailer & Portier, 1988; models to predict leukaemia mortality rates. Portier & Bailer, 1989) presented in the NTP (1993) report were used in these calculations. For some tumour types, the adjusted data still demonstrated downward curvature at the highest concentration. In these Summary cases, the high-exposure group was excluded in the determination of

the TC05. The TC05s were calculated for these end-points by first fitting It is noteworthy that the choice of the exposure–response model a multistage model to the data. The multistage model is given by: does not have a large impact on the resulting TC01; as indicated in Table A-3, the values are similar, ranging from 1.4 to 4.3 mg/m3. However, if - q - q d - ... - q d k a best model must be chosen, it would be model 1, owing to the smaller P ( d ) = 1 - e 0 1 k deviance (Table A-2), the shape of the curve relative to the data in the low-dose region (Figure A-1), and the fact that it has one fewer where d is dose, k is the number of dose groups in the study minus parameter than model 2, which provides a similar fit. The TC01 for model one, P(d) is the probability of the animal developing a tumour at dose d, 1 is 1.7 mg/m3. and qi > 0, i = 1,..., k are parameters to be estimated.

It is difficult, though, to assess how well any of these models The models were fitted using GLOBAL82 (Howe & Crump, 1982), truly describes the data. It is noted that the plot in Figure A-1 provides and a chi-square lack of fit test was performed for each model fit. The only a rough indication of the shape of the data, since each point on the degrees of freedom for this test are equal to k minus the number of qi’s plot is an average of data in many strata. The results of the validation whose estimates are non-zero. A p-value less than 0.05 indicates a study reduce confidence in the ability of the models to predict significant lack of fit. The lower confidence limits presented are leukaemia mortality. approximate, based on output from GLOBAL82. Results from the model fitting are displayed in Table A-6. Plots of the data and the fitted The choice of exposure lag does not greatly improve the fit of models are shown in Figure A-2. any of the four models, and it does not affect the resulting TC01.

Including all lagged models, the range of TC01s is still from 0.8 to 4.3 3 TC05s were determined as the doses D (in mg/m ) that satisfy mg/m3. P ( D ) - P ( 0 ) For comparison with these values, potency estimates were also = 0 . 05 calculated on the basis of the recent case–control study of styrene- 1 - P ( 0 ) butadiene rubber workers by Matanoski et al. (1997). Although workers were from plants subsumed in the Delzell et al. (1995) study, exposure and then adjusted by multiplying by: was independently characterized. Treating the odds ratio presented by these authors as a rate ratio (since leukaemia is a rare disease) and 2 using their model and parameter estimates as well as the same lifetable 6h/ day 5days/ week wweeks wweeks methods described above, the TC01 for environmental exposure was ´ ´ ´ calculated to be 0.4 mg/m3. It is reassuring, therefore, that this value is only slightly lower than the estimates derived on the basis of the Delzell 24 h/ day 7 days/ week 104 weeks 104 weeks et al. (1995) cohort study data. where, in the first term, which amortizes the dose to be constant over Tumorigenic concentrations based on data from the lifetime of a mouse, w is the duration of the experiment (103 studies in experimental animals weeks). The second factor was suggested by Peto et al. (1984) and corrects for an experiment length that is unequal to the standard lifetime. Since tumours develop much more rapidly later in life, a Estimates of carcinogenic potency were calculated on the basis greater than linear increase in the tumour rate is expected when animals of the incidences of malignant lymphomas, histiocytic sarcomas, are observed for tumours longer than their standard lifetime (or the cardiac haemangiosarcomas, alveolar/bronchiolar adenomas or reverse, when animals are observed for a period shorter than their carcinomas, hepatocellular adenomas or carcinomas, squamous cell standard lifetime). (Application of this factor does not impact greatly on papillomas or carcinomas of the forestomach, adenomas or carcinomas the final values, since it is very close to one.) The selected TC05 values of the Harderian gland, granulosa cell tumours of the ovaries, and for this study and their 95% lower confidence limits (LCLs) are adenoacanthomas, carcinomas, or malignant mixed tumours of the presented in Table A-6 and range from 2.3 mg/m3 (95% LCL = 1.7 mammary gland observed in B6C3F 1 mice in the chronic bioassay mg/m3) or 1.1 ppm (95% LCL = 0.79 ppm) for Harderian gland tumours conducted by the NTP (1993) and the mammary gland tumours, in males to 99 mg/m3 (95% LCL = 23 mg/m3) or 45 ppm (95% LCL = 10 pancreatic exocrine adenomas, Leydig cell tumours, Zymbal gland ppm) for malignant lymphomas in males. carcinomas, thyroid follicular cell adenomas or carcinomas, and uterine sarcomas in Sprague-Dawley rats reported by Hazleton Laboratories Europe Ltd. (1981a). (The tumour incidence data for each of the sites considered are presented in Tables 2 and 3.)

55 Concise International Chemical Assessment Document 30

Table A-4: Parameter estimates and model deviances for each of four lagged-exposure models fitted to median cumulative exposure per person-year.

Model Lag Parameter estimates Standard error Deviance 1) RR = (1 + dose)" None " = 0.2850 SE(") = 0.0976 171.5

2 years " = 0.2852 SE(") = 0.0982 171.6

5 years " = 0.2883 SE(") = 0.0995 171.6

10 years " = 0.3064 SE(") = 0.1034 171.1 15 years " = 0.2955 SE(") = 0.1079 172.4

20 years " = 0.2891 SE(") = 0.1141 173.6

25 years " = 0.2898 SE(") = 0.1334 175.4 2) RR = 1 + $@dose" None " = 0.3999 SE(") = 0.2733 172.0

$ = 0.4557 SE($) = 0.8219

2 years " = 0.3992 SE(") = 0.2739 172.0

$ = 0.4602 SE($) = 0.8279 5 years " = 0.4024 SE(") = 0.2737 172.0

$ = 0.4647 SE($) = 0.8288

10 years " = 0.4245 SE(") = 0.2755 171.4 $ = 0.4693 SE($) = 0.8345

15 years " = 0.4835 SE(") = 0.3397 172.6

$ = 0.2878 SE($) = 0.5846

20 years " = 0.4720 SE(") = 0.3558 173.9 $ = 0.3243 SE($) = 0.6572

25 years " = 0.2960 SE(") = 0.2833 175.3

$ = 0.9293 SE($) = 1.5710 3) RR = e$@dose None $ = 0.0029 SE($) = 0.0014 176.7

2 years $ = 0.0029 SE($) = 0.0015 176.8

5 years $ = 0.0031 SE($) = 0.0015 176.7

10 years $ = 0.0034 SE($) = 0.0016 176.4 15 years $ = 0.0035 SE($) = 0.0018 177.0

20 years $ = 0.0033 SE($) = 0.0022 178.2

25 years $ = 0.0033 SE($) = 0.0022 178.2

4) RR = 1 + $@dose None $ = 0.0099 SE($) = 0.0065 174.7 2 years $ = 0.0102 SE($) = 0.0067 174.7

5 years $ = 0.0109 SE($) = 0.0072 174.6

10 years $ = 0.0137 SE($) = 0.0089 173.8 15 years $ = 0.0158 SE($) = 0.0106 174.1

20 years $ = 0.0179 SE($) = 0.0129 175.7

25 years $ = 0.0179 SE($) = 0.0129 175.7

Estimates of carcinogenic potency were also calculated a multistage model was fit to the data for rats using GLOBAL82 based on the results of the bioassay in Sprague-Dawley rats and adjusted to account for study duration (w) by multiplying by: (Hazleton Laboratories Europe Ltd., 1981a). In this study, rats 2 were exposed to 0, 1000, or 8000 ppm (0, 2212, or 17 696 6h/ day 5days/ week w weeks wweeks mg/m3) for 6 h/day, 5 days/week, for 105 (males) or 111 (females) ´ ´ ´ weeks. A high mortality rate was observed at the higher 24 h/ day 7 days/ week 104 weeks 104 weeks concentration; therefore, this exposure group was excluded from the analysis, except for the potency estimates for pancreatic where the duration of the experiment was 105 weeks for males exocrine adenomas in males (for this end-point, exclusion of the and 111 weeks for females. The exposure–response curves and high-exposure group would have resulted in the estimated adjusted TC05 values based on this study in rats are exposure–response relationship curving downwards). As for mice, presented in Figure A-3 and Table A-6, respectively. The

56 1,3-Butadiene: Human health aspects

Table A-5: Model validation p-values for Delzell et al. (1995) study.

Proportion of p-valuesa that are Model less than 0.01 less than 0.05 less than 0.1

1) RR = (1 + dose)" 0.074 0.167 0.252

2) RR = 1 + $@dose" 0.084 0.19 0.286 3) RR = e$@dose 0.08 0.188 0.264

4) RR = 1 + $@dose 0.103 0.214 0.303 a p-value of likelihood ratio test of equality of parameters fitted to each half of the data.

concentrations of butadiene estimated to be associated with a 6 h / day 5 days / week 3 ´ 5% increased incidence of tumours ranged from 6.7 mg/m (95% 24 h / day 7 days / week LCL = 4.7 mg/m3) or 3.0 ppm (95% LCL = 2.1 ppm) to 4872 mg/m3 (95% LCL = 766 mg/m3) or 2203 ppm (95% LCL = 346 Resulting BMC05s and lack of fit information for all models fit are ppm) for tumours of the mammary gland and Zymbal gland in displayed in Table A-8. female rats, respectively. Although the available data for analysis of exposure–response were more limited for rats than for The model fitted to all six exposure groups exhibited a mice, it is interesting to note the similarity in estimates of significant lack of fit, likely due to the fact that the curve rises 3 potency for mammary gland tumours (i.e., 6.7 mg/m in both sharply and then plateaus at the three highest exposure groups. species). Plots of the data and fitted model are displayed in Figure A-4.

Since a good fit in the range of the BMC05 (in the vicinity of 6.25 Based on modelling (using THC; Howe, 1995a) of the ppm [13.8 mg/m3]) is desired, the model was refitted omitting incidence of micronucleated polychromatic erythrocytes in the two highest exposure groups. This model again indicates a B6C3F1 mice exposed to butadiene for up to 15 months in the marginal lack of fit. The graph of this model (Figure A-5) NTP bioassay, BMC05s for somatic cell mutations were very indicates that this model provides a reasonable visual fit to the similar to the lower end of the range of the TC05s for tumour data, but the resulting BMC05 is uncertain due to lack of fit of the induction. model. Benchmark concentrations for ovarian atrophy The BMC05 for the model excluding the two highest dose in mice groups was calculated to be 0.57 mg/m3, with a 95% LCL of 0.44 mg/m3. Benchmark concentrations (BMC05s) for ovarian atrophy were derived on the basis of the chronic bioassay conducted by If only those animals that had moderate or marked the NTP (1993) in which B6C3F1 mice were exposed to ovarian atrophy from all exposure groups were included, the 3 3 concentrations of 0, 6.25, 20, 62.5, 200, or 625 ppm (0, 13.8, resulting BMC05 would be 9.6 mg/m (95% LCL = 7.6 mg/m ), 44.2, 138, 442, and 1383 mg/m3) butadiene for up to 2 years. although there is again a significant lack of fit (Figure A-6). If the

There was a concentration-related increase in the incidence as highest exposure group is excluded, the BMC05 for moderate or well as the severity of ovarian atrophy, as summarized in Table marked ovarian atrophy becomes 3.1 mg/m3, with a 95% LCL of A-7. 2.5 mg/m3 (Figure A-7).

The exposure–response relationship for ovarian atrophy from this study was quantified by fitting the following model to the dose–response data (Howe, 1995b):

ì ï q0 if d £d0 P(d)=í k -q1(d-d0 )-...-qk (d-d0 ) îïq0 +(1-q0)×[1-e ] if d £d0 where d is dose, k is the number of dose groups in the study minus one, P(d) is the probability of the animal developing the effect at dose d, and qi > 0, i =1,..., k and d0 are parameters to be estimated. The models were fit using THRESH (Howe, 1995b), and the BMC05s were calculated as the dose D that satisfies:

P ( D ) - P ( 0 ) = 0 .05 1 - P ( 0 )

A chi-square lack of fit test was performed for each of the model fits. The degrees of freedom for this test are equal to k minus the number of qi’s whose estimates are non-zero. A p- value less than 0.05 indicates a significant lack of fit.

The BMC05 was then amortized to be constant over the standard life of a mouse by multiplying by:

57 a Table A-6: Carcinogenic potency estimates (TC 05s) of butadiene based on results of bioassays in experimental animals.

Males Females

TC 05 95% TC 05 95% LCL Tumour type (mg/m3) LCL P 2 b dfc pd (mg/m3) (mg/m3) P 2 b dfc pd (mg/m3) Mice (from NTP, 1993) Alveolar/bronchiolar adenomas or carcinomas 2.4 1.4 1.0 3 0.79 5.2 3.2 9.1 4 0.06 Histiocytic sarcomas 12 8.4 7.6 5 0.18 21 12 5.4 4 0.25 Cardiac haemangiosarcomas 14 6.4 0.34 3 0.95 7.6 5.2 18 4 0.00 Forestomach squamous cell papillomas or carcinomas 29 13 6.1 3 0.11 14 8.1 4.3 4 0.36 Ovarian granulosa cell tumours – – – – – 6.7 4.4 5.0 3 0.17 Mammary gland adenoacanthomas, carcinomas, or malignant mixed – – – – – 6.7 4.9 13 4 0.01 tumours Hepatocellular adenomas or carcinomas 3.2 1.9 2.8 2 0.24 5.4 3.2 0.8 3 0.85 Harderian gland adenomas or carcinomas 2.3 1.7 0.5 2 0.77 4.7 2.7 1.5 2 0.47 Malignant lymphomase 99 23 3.3 3 0.35 23 6.9 3.9 3 0.27 Rats (from Hazleton Laboratories Europe Ltd., 1981a) Mammary gland adenomas or carcinomas – – – – – 6.7 4.7 0 0 – Pancreatic exocrine adenomas 597 316 1.1 1 0.29 – – – – – Leydig cell tumours 161 96 0 1 – – – – – – Thyroid follicular cell adenomas or carcinomas – – – – – 142 113 0 1 – Uterine sarcomas – – – – – 189 113 0 0 – Zymbal gland carcinomas 1023 905 1 1 0.32 4872 766 0.06 2 0.97 a Values have been adjusted for lifetime exposure. b Chi-squared goodness of fit statistic. c Degrees of freedom. d p-value of goodness of fit test (p-value < 0.05 indicates significant lack of fit). e Values for malignant lymphomas presented here only for comparison; potency estimates for these tumours not considered relevant to humans due to the greater sensitivity of these mice to induction of this effect associated with the presence of an endogenous retrovirus. Figure A-2: Exposure-response analysis for butadiene-induced tumours in mice

59 Figure A-2 (continued).

60 Fiure A2 Continued).

61 Fig A-2 (continued).

62 Figure A-3: Exposure-response analysis for butadiene-induced tumours in rats

63 Figure A-3 (continued).

64 1,3-Butadiene: Human health aspects

Table A-7: Incidence and severity of ovarian atrophy observed in 2-year bioassay in mice (NTP, 1993).

Exposure level Number of animals All Minimal Mild Moderate Marked (ppm) examined severities severity severity severity severity 0 49 4 1 2 1 0

6.25 49 19 0 15 4 0

20 48 32 1 23 8 0

62.5 50 42 3 18 21 0 200 50 43 0 9 34 0

625 79 69 0 19 47 3

Table A-8: Benchmark concentrations for ovarian atrophy.

95% LCL on 95% LCL on

BMC05 BMC05 BMC05 BMC05 Chi- Ovarian atrophy (ppm) (ppm) (mg/m3) (mg/m3) square df p-value All severities 2.5 1.9 5.6 4.1 61 4 0.00

All severities, excluding 0.25 0.20 0.57 0.44 7.0 2 0.03 top two dose groups

Moderate/marked 4.3 3.4 9.6 7.6 37.1 4 0.00 severity

Moderate/marked 1.4 1.1 3.1 2.5 2.2 3 0.55 severity, excluding top dose group

65 Concise International Chemical Assessment Document 30

Figure A-4: Exposure–response analysis for ovarian atrophy in mice

(* BMC05 and BMCL05 unadjusted for lifetime dosing).

Figure A-5: Exposure–response analysis for ovarian atrophy in mice excluding two highest dose groups

(* BMC05 and BMCL05 unadjusted for lifetime dosing).

66 1,3-Butadiene: Human health aspects

Figure A-6: Exposure–response analysis for moderate/marked ovarian atrophy

(* BMC05 and BMCL05 unadjusted for lifetime dosing).

Figure A-7: Exposure–response analysis for moderate/marked ovarian atrophy, excluding high-dose group

(* BMC05 and BMCL05 unadjusted for lifetime dosing).

67 1,3-BUTADIENE 0017 April 2000 CAS No: 106-99-0 Divinyl RTECS No: EI9275000 Vinylethylene

UN No: 1010 (stabilized) C4H6 / CH2=(CH)2=CH2 EC No: 601-013-00-X Molecular mass: 54.1

TYPES OF HAZARD/ ACUTE HAZARDS/SYMPTOMS PREVENTION FIRST AID/FIRE FIGHTING EXPOSURE

FIRE Extremely flammable. NO open flames, NO sparks, and Shut off supply; if not possible and NO smoking. no risk to surroundings, let the fire burn itself out; in other cases extinguish with water spray, powder, carbon dioxide.

EXPLOSION Gas/air mixtures are explosive. Closed system, ventilation, In case of fire: keep cylinder cool by explosion-proof electrical equipment spraying with water. and lighting. Prevent build-up of electrostatic charges (e.g., by grounding) if in liquid state.

EXPOSURE AVOID ALL CONTACT! AVOID EXPOSURE OF (PREGNANT) WOMEN!

Inhalation Cough. Sore throat. Dizziness. Ventilation, local exhaust, or Fresh air, rest. Refer for medical Headache. Drowsiness. Blurred breathing protection. attention. vision. Nausea. Unconsciousness.

Skin ON CONTACT WITH LIQUID: Cold-insulating gloves. ON FROSTBITE: rinse with plenty FROSTBITE. of water, do NOT remove clothes. Refer for medical attention.

Eyes Redness. Pain. Blurred vision. See Safety goggles. First rinse with plenty of water for Skin. several minutes (remove contact lenses if easily possible), then take to a doctor.

Ingestion Do not eat, drink, or smoke during work.

SPILLAGE DISPOSAL PACKAGING & LABELLING

Evacuate danger area! Consult an expert! F+ Symbol Do not transport with food and Ventilation. NEVER direct water jet on liquid. T Symbol feedstuffs. Remove all ignition sources. Chemical protection R: 45-12 suit including self-contained breathing apparatus. S: 53-45 Note: D UN Hazard Class: 2.1

EMERGENCY RESPONSE STORAGE

Transport Emergency Card: TEC (R)-20G41 Fireproof. Cool. Separated from food and feedstuffs. NFPA Code: H2; F4; R2

Prepared in the context of cooperation between the International IPCS Programme on Chemical Safety and the European Commission International © IPCS 2000 Programme on Chemical Safety SEE IMPORTANT INFORMATION ON THE BACK. 0017 1,3-BUTADIENE

IMPORTANT DATA Physical State; Appearance Routes of exposure COLOURLESS COMPRESSED LIQUEFIED GAS, WITH The substance can be absorbed into the body by inhalation. CHARACTERISTIC ODOUR. Inhalation risk Physical dangers A harmful concentration of this gas in the air will be reached The gas is heavier than air and may travel along the ground; very quickly on loss of containment. distant ignition possible. As a result of flow, agitation, etc., electrostatic charges can be generated. The vapours are Effects of short-term exposure uninhibited and may form polymers in vents or flame arresters The substance irritates the eyes and the respiratory tract. Rapid of storage tanks, resulting in blockage of vents. evaporation of the liquid may cause frostbite. The substance may cause effects on the central nervous system, resulting in Chemical dangers lowering of consciousness. The substance can under specific circumstances (exposure to air) form peroxides, initiating explosive polymerization. The Effects of long-term or repeated exposure substance may polymerize due to warming with fire or explosion The substance may have effects on the bone marrow, resulting hazard. Shock-sensitive compounds are formed with copper in leukemia. This substance is probably carcinogenic to and its alloys (see Notes). The substance decomposes humans. May cause heritable genetic damage in humans. explosively on rapid heating under pressure. Reacts vigorously Animal tests show that this substance possibly causes toxic with oxidants and many other substances, causing fire and effects upon human reproduction. explosion hazard.

Occupational exposure limits TLV: (as TWA) 2 ppm; A2 (ACGIH 1999). MAK: class 2 (1999)

PHYSICAL PROPERTIES

Boiling point: -4°C Relative vapour density (air = 1): 1.9 3,47 (1.87) Melting point: -109°C Flash point: -76°C Relative density (water = 1): 0.6 Auto-ignition temperature: 414°C Solubility in water: none (0.1 g/100 ml) Explosive limits, vol% in air: 1.1-16.3 Vapour pressure, kPa at 20°C: 245 Octanol/water partition coefficient as log Pow: 1.99

ENVIRONMENTAL DATA

NOTES Piping material for this gas must not contain over 63% of copper. Use of alcoholic beverages enhances the harmful effect. The odour warning when the exposure limit value is exceeded is insufficient.

ADDITIONAL INFORMATION

LEGAL NOTICE Neither the EC nor the IPCS nor any person acting on behalf of the EC or the IPCS is responsible for the use which might be made of this information

©IPCS 2000 Concise International Chemical Assessment Document 30

RÉSUMÉ D’ORIENTATION moins des différences quantitatives notamment en ce qui concerne la proportion de métabolites présumés toxiques qui se forment; ainsi chez la souris le métabolisme Ce CICAD sur le 1,3-butadiène a été préparé par la oxydatif en mono-, puis en diépoxyde est plus important Direction de l’Hygiène de l’Environnement de Santé que chez le rat ou l’Homme. Par ailleurs, il peut également Canada sur la base d’une documentation préparée y avoir des variations interindividuelles dans la simultanément dans le cadre du Programme d’évaluation capacicité de métabolisation chez l’Homme, variations des substances prioritaires, en application de la Loi qui s’expliquent par le polymorphisme génétique des canadienne sur la protection de l’environnement (LCPE). enzymes en cause. Les évaluations sanitaires des substances prioritaires effectuées en application de cette loi portent sur les Le 1,3-butadiène présente une faible toxicité aiguë effets potentiels que ces produits pourraient avoir sur la chez les animaux de laboratoire. On a toutefois observé santé humaine en cas d’exposition indirecte dans une atrophie ovarienne à toutes les concentrations chez l’environnement général. Cette mise au point prend en des souris exposées à ce composé pendant une période compte des données allant jusqu’à fin avril 1998. prolongée. D’autres effets sur les ovaires ont également L’appendice 1 donne des informations sur la nature de été relevés lors d’études à court terme. Chez les souris l’examen par des pairs ainsi que sur les sources mâles, on a constaté que le 1,3-butadiène provoquait une documentaires utilisées. Des renseignements sur atrophie testiculaire, mais à des concentrations supéri- l’examen de ce CICAD par des pairs sont donnés à eures à celles qui étaient toxiques pour les femelles. Les l’appendice 2. Ce CICAD a été approuvé en tant données limitées dont on dispose ne permettent pas de qu’évaluation internationale lors de la réunion du Comité conclure que ce composé est tératogène pour la progéni- d’évaluation finale qui s’est tenue à Helsinki (Finlande) ture des animaux de laboratoire mâles ou femelles qui y du 26 au 29 juin 2000. La liste des participants à cette ont été exposés, ni qu’il est véritablement toxique pour le réunion figure à l’appendice 3. La fiche internationale sur foetus à des concentrations inférieures à celles qui sont la sécurité chimique (ICSC 0017) du 1,3-butadiène, établie toxiques pour la mère. par le Programme international sur la sécurité chimique (IPCS, 1993) est également reproduite dans le présent Le 1,3-butadiène exerce également divers effets sur document. le sang et la moelle osseuse de la souris; en ce qui con- cerne le rat, les données, bien que limitées, ne mettent Le 1,3-butadiène (No CAS 106-99-0) résulte de la pas en évidence d’effets de ce genre. combustion incomplète de certains produits lors de processus naturels ou d’activités humaines. C’est L’ensemble des études montre qu’une fois inhalé, également un produit chimique industriel principalement le 1,3-butadiène se révèle fortement cancérogène chez la utilisé pour la préparation de polymères, notamment du souris, provoquant des tumeurs de localisations polybutadiène et des caoutchoucs et latex styrène- multiples à toutes concentrations. Il se révèle également butadiène ou des élastomères acrylonitrile-butadiène. Le cancérogène chez le rat à toutes les concentrations, 1,3-butadiène pénètre dans l’environnement avec les gaz selon la seule étude adéquate dont on dispose. Chez d’échappement des véhicules à essence ou à gazole, lors cette espèce, les seules concentrations étudiées étaient de l’utilisation de combustibles à d’autres fins que le beaucoup plus fortes que pour la souris, mais il apparaît transport, de la combustion de la biomasse ou encore néanmoins que le rat est l’espèce la moins sensible, si lors de son utilisation sur des sites industriels. l’on prend comme élément de comparaison l’incidence des tumeurs. Il est probable que la plus grande Le 1,3-butadiène ne persiste pas dans l’environne- sensibilité observée chez les souris est liée aux ment mais il est néanmoins répandu dans tout le milieu différences interspécifiques mentionnées plus haut urbain du fait de la présence généralisée des sources de concernant le métabolisme, et notamment le métabolisme combustion où il prend naissance. C’est dans les villes qui conduit à la formation d’époxydes actifs. et à proximité immédiate des installations industrielles que sont mesurées les concentrations atmosphériques Le 1,3-butadiène a une action mutagène sur les les plus fortes. cellules somatiques de la souris et du rat, avec une activité plus forte chez la souris. On a également L’exposition de la population générale au 1,3- constaté d’autres lésions génétiques dans les cellules butadiène est due principalement à sa présence dans l’air somatiques de cette dernière espèce, à l’exclusion de extérieur ou intérieur. Comparativement, la contribution celles du rat. Le composé s’est révélé systématiquement des autres véhicules, comme les aliments ou l’eau de génotoxique pour les cellules germinales de la souris, boisson, reste négligeable. En revanche, celle de la mais apparemment pas pour celles du rat, selon la seule fumée de tabac peut être importante. étude retrouvée.

Le métabolisme du 1,3-butadiène se révèle être de Toutefois il n’y a apparemment pas de différence même nature d’une espèce à l’autre, mais il y a néan- interspécifique pour ce qui est de la sensibilité aux effets

70 1,3-Butadiene: Human health aspects génétiques dus aux époxydes résultant de la métabolisa- RESUMEN DE ORIENTACIÓN tion du 1,3-butadiène. Selon certaines données con- cernant l’exposition professionnelle mais qui restent toutefois limitées, le 1,3-butadiène serait également Este CICAD sobre el 1,3-butadieno, preparado por génotoxique pour l’Homme et provoquerait des lésions la Dirección General de Higiene del Medio del Ministerio mutagènes et clastogènes dans les cellules somatiques. de Salud del Canadá, se basó en la documentación preparada al mismo tiempo como parte del Programa de Il existe, entre l’exposition au 1,3-butadiène sur le Sustancias Prioritarias en el marco de la Ley Canadiense lieu de travail et certaines leucémies, une corrélation qui de Protección del Medio Ambiente (CEPA). El objetivo répond à plusieurs des critères d’une relation de cause à de las evaluaciones sanitarias de las sustancias priori- effet. L’étude la plus vaste et la plus exhaustive tarias en el marco de la CEPA es valorar los efectos effectuée jusqu’ici, et qui a porté sur une cohorte de potenciales de la exposición indirecta en el medio travailleurs employés dans de multiples usines, a mis en ambiente general para la salud humana. En este examen évidence un parallélisme entre une augmentation se utilizaron los datos obtenidos hasta el final de abril de constatée de la mortalité par leucémie et l’exposition 1998. La información relativa al carácter del examen cumulée estimative au 1,3-butadiène dans les industries colegiado y a la disponibilidad del documento original se produisant des élastomères styrène-butadiène. La presenta en el apéndice 1. La información sobre el correction pour tenir compte de l’exposition au benzène examen colegiado de este CICAD figura en el apéndice 2. et au styrène n’a pas fait disparaître cette corrélation, qui Este CICAD se aprobó como evaluación internacional en se manifestait d’ailleurs le plus fortement dans les sous- una reunión de la Junta de Evaluación Final, celebrada groupes potentiellement les plus exposés. On a en Helsinki (Finlandia) del 26 al 29 de junio de 2000. La également observé une corrélation entre l’exposition au lista de participantes en esta reunión figura en el 1,3-butadiène et les leucémies lors d’une étude cas- apéndice 3. La Ficha internacional de seguridad química témoins menée indépendamment de l’enquête précitée (ICSC 0017) para el 1,3-butadieno, preparada por el sur une population de travailleurs en grande partie Programa Internacional de Seguridad de las Sustancias identique. Par contre, on n’a pas constaté Químicas (IPCS, 1993), también se reproduce en el d’augmentation de la mortalité par leucémie chez des presente documento. travailleurs employés à la production de butadiène monomère mais non exposés simultanément à certaines El 1,3-butadieno (CAS Nº 106-99-0) se produce por des autres substances présentes dans l’industrie des una combustión incompleta en procesos naturales y élastomères styrène-butadiène, en dépit d’éléments actividades humanas. Es también un producto químico d’appréciation limités concernant la possibilité d’une industrial que se utiliza fundamentalmente en la association avec la mortalité par lymphosarcomes ou producción de polímeros, en particular polibutadieno, réticulosarcomes dans certains sous-groupes. cauchos y látex de estireno-butadieno y cauchos de nitrilo-butadieno. El 1,3-butadieno se incorpora al medio Au vu des données épidémiologiques et toxicolo- ambiente a partir de las emisiones de los gases de escape giques, le 1,3-butadiène est cancérogène pour l’Homme de los vehículos con motor de gasolina y diésel, de la et pourrait également être génotoxique. Le pouvoir combustión de combustibles fósiles no utilizados en el cancérogène (concentration qui entraîne une augmen- transporte, de la combustión de biomasa y de usos tation de 1% de la mortalité par leucémie) a été évalué à industriales sobre el terreno. 1,7 mg/m3 en s’appuyant sur les résultats de l’enquête épidémiologique la plus vaste et la mieux conduite sur El 1,3-butadieno no es persistente, pero sí ubicuo des travailleurs exposés. Cette valeur correspond à en el medio ambiente urbano, debido a la presencia l’extrémité inférieure de la série de concentrations generalizada de sus fuentes de combustión. Las tumorigènes déterminée par des études sur des concentraciones atmosféricas más altas se han medido rongeurs. Le 1,3-butadiène présente également une en el aire de las ciudades y en las cercanías de fuentes toxicité génésique chez les animaux de laboratoire. Ce industriales. potentiel toxique peut s’évaluer par la concentration de référence de 1,3-butadiène obtenue dans le cas des effets La población general está expuesta al 1,3-buta- ovariens et qui est égale à 0,57 mg/m3. dieno fundamentalmente mediante el aire del ambiente y de los espacios cerrados. En comparación, otros medios, Les effets sanitaires du 1,3-butadiène et le mode entre ellos los alimentos y el agua de bebida, contrib- d’action de ce composé ont été étudiés en détail, mais uyen de manera insignificante a la exposición a él. El d’importants travaux de recherche continuent de lui être humo del tabaco puede producir cantidades significa- consacrés afin de tenter de lever les incertitudes qui tivas de1,3-butadieno. subsistent dans la base de données. El metabolismo del 1,3-butadieno parece ser cualitativamente semejante en todas las especies, aunque hay diferencias cuantitativas en los metabolitos

71 Concise International Chemical Assessment Document 30 supuestamente tóxicos que se forman; los ratones La asociación entre la exposición al 1,3-butadieno parecen oxidar el 1,3-butadieno a monoepóxido y en el entorno de trabajo y la leucemia cumple varios de después a diepóxido en mayor medida que las ratas o las los criterios tradicionalmente establecidos para la personas. Sin embargo, la capacidad metabólica para el causalidad. En el estudio más amplio y completo 1,3-butadieno puede variar de unas personas a otras, en realizado hasta el momento, utilizando una cohorte de función del polimorfismo genético de las enzimas trabajadores de diversas instalaciones, la mortalidad por correspondientes. leucemia aumentó con una exposición acumulativa estimada al 1,3-butadieno en la industria del caucho de El 1,3-butadieno tiene una toxicidad aguda baja en estireno-butadieno; esta asociación se mantuvo tras el animales experimentales. Sin embargo, la exposición control de la exposición al estireno y al benceno y prolongada de ratones a este producto se asoció con la alcanzó la intensidad máxima en los subgrupos con el aparición de atrofia ovárica con todas las concentra- mayor potencial de exposición. Análogamente, en un ciones utilizadas en los ensayos. En estudios de expo- estudio de casos y testigos independiente basado prác- sición más breve se observaron también efectos en los ticamente en la misma población de trabajadores se ovarios. En los ratones machos se detectó asimismo observó una asociación entre la exposición al 1,3-buta- atrofia testicular a concentraciones superiores a las dieno y la leucemia. Sin embargo, no se produjo un asociadas con efectos en las hembras. Sobre la base de aumento de la mortalidad a causa de la leucemia en los los limitados datos disponibles, no hay pruebas conclu- trabajadores de la producción de monómeros de buta- yentes de que el 1,3-butadieno sea teratogénico en los dieno que no estaban simultáneamente expuestos a animales experimentales tras la exposición materna o algunas de las otras sustancias presentes en la industria paterna o de que induzca una toxicidad fetal importante a del caucho de estireno-butadieno, aunque en algunos concentraciones inferiores a las que provocan toxicidad subgrupos se obtuvieron pruebas limitadas de materna. asociación con la mortalidad por linfosarcoma y reticulosarcoma. El 1,3-butadieno indujo también diversos efectos en la sangre y en la médula ósea de ratones; aunque los Los datos epidemiológicos y toxicológicos dispon- datos son limitados, no se han observado efectos seme- ibles demuestran que el 1,3-butadieno es carcinogénico, jantes en las ratas. y también puede ser genotóxico, para las personas. Se calculó una potencia carcinogénica (concentración que El 1,3-butadieno inhalado es un potente carcinó- produce un aumento del 1% de la mortalidad por leu- geno en ratones, induciendo tumores en lugares cemia) de 1,7 mg/m3, sobre la base de los resultados de la múltiples a todas las concentraciones utilizadas en todos mayor investigación epidemiológica bien realizada en los estudios identificados. Fue también carcinogénico en trabajadores expuestos. Este valor es semejante al nivel ratas a todos los niveles de exposición en el único más bajo de la gama de concentraciones tumorígenas estudio disponible al respecto; aunque en las ratas sólo determinadas a partir de los estudios realizados con se ensayaron concentraciones mucho más altas que en roedores. El 1,3-butadieno indujo también toxicidad los ratones, las ratas parecen ser la especie menos reproductiva en animales experimentales. Como medida sensible, tomando como base la comparación de los de su potencia para inducir efectos reproductivos, se datos sobre la incidencia de tumores. La mayor obtuvo una concentración de referencia de 0,57 mg/m3 sensibilidad de los ratones que las ratas a la inducción para la toxicidad ovárica en los ratones. de estos efectos por el 1,3-butadieno probablemente se debe a diferencias entre las especies con respecto al Aunque se han investigado a fondo los efectos en metabolismo de formación de epóxidos activos. la salud asociados con la exposición al 1,3-butadieno y el mecanismo de acción para la inducción de estos efectos, El 1,3-butadieno es mutagénico en células se siguen realizando numerosas investigaciones sobre somáticas tanto de ratones como de ratas, aunque la esta sustancia, a fin de tratar de abordar algunas de las potencia mutagénica fue mayor en los primeros. De la incertidumbres asociadas con la base de datos. misma manera, el 1,3-butadieno indujo otros daños genéticos en células somáticas de ratones, pero no en las de ratas. Fue también sistemáticamente genotóxico en células germinales de ratones, pero no en la única valoración identificada en ratas. Sin embargo, no se observaron diferencias aparentes en la sensibilidad de las especies a los efectos genéticos inducidos por los metabolitos epóxidos del 1,3-butadieno. Hay también pruebas limitadas de poblaciones expuestas en el lugar de trabajo de que el 1,3-butadieno es genotóxico para las personas, induciendo daños mutagénicos y clastogénicos en las células somáticas.

72 THE CONCISE INTERNATIONAL CHEMICAL ASSESSMENT DOCUMENT SERIES

Azodicarbonamide (No. 16, 1999) Benzoic acid and sodium benzoate (No. 26, 2000) Benzyl butyl phthalate (No. 17, 1999) Biphenyl (No. 6, 1999) 2-Butoxyethanol (No. 10, 1998) Chloral hydrate (No. 25, 2000) Crystalline silica, Quartz (No. 24, 2000) Cumene (No. 18, 1999) 1,2-Diaminoethane (No. 15, 1999) 3,3'-Dichlorobenzidine (No. 2, 1998) 1,2-Dichloroethane (No. 1, 1998) 2,2-Dichloro-1,1,1-trifluoroethane (HCFC-123) (No. 23, 2000) Diphenylmethane diisocyanate (MDI) (No. 27, 2000) Ethylenediamine (No. 15, 1999) Ethylene glycol: environmental aspects (No. 22, 2000) 2-Furaldehyde (No. 21, 2000) HCFC-123 (No. 23, 2000) Limonene (No. 5, 1998) Manganese and its compounds (No. 12, 1999) Methyl chloride (No. 28, 2000) Methyl methacrylate (No. 4, 1998) Mononitrophenols (No. 20, 2000) Phenylhydrazine (No. 19, 2000) N-Phenyl-1-naphthylamine (No. 9, 1998) 1,1,2,2-Tetrachloroethane (No. 3, 1998) 1,1,1,2-Tetrafluoroethane (No. 11, 1998) o-Toluidine (No. 7, 1998) Tributyltin oxide (No. 14, 1999) Triglycidyl isocyanurate (No. 8, 1998) Triphenyltin compounds (No. 13, 1999) Vanadium pentoxide and other inorganic vanadium compounds (No. 29, 2001)

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