INTRODUCTION TO THE CHEMICAL SUBSTANCES General Information quately protect all workers. Some individuals may ® experience discomfort or even more serious adverse The TLVs are guidelines to be used by profess- health effects when exposed to a chemical sub- sional industrial hygienists. The values presented in stance at the TLV® or even at concentrations below this book are intended for use only as guidelines or the TLV®. There are numerous possible reasons for recommendations to assist in the evaluation and increased susceptibility to a chemical substance, control of potential workplace health hazards and for including age, gender, ethnicity, genetic factors no other use (e.g., neither for evaluating or control- (predisposition), lifestyle choices (e.g., diet, smoking, ling community air pollution; nor for estimating the abuse of alcohol and other drugs), medications, and toxic potential of continuous, uninterrupted expo- pre-existing medical conditions (e.g., aggravation of sures or other extended work periods; nor for pro- asthma or cardiovascular disease). Some individuals ving or disproving an existing disease or physical may become more responsive to one or more chem- condition in an individual). Further, these values are ical substances following previous exposures (e.g., not fine lines between the safe and dangerous con- sensitized workers). Susceptibility to the effects of ditions and should not be used by anyone who is not ® chemical substances may be altered during different trained in the discipline of industrial hygiene. TLVs periods of fetal development and throughout an are not regulatory or consensus standards. individual’s reproductive lifetime. Some changes in Editor’s note: The approximate year that the cur- susceptibility may also occur at different work levels rent Documentation was last substantially reviewed (e.g., light versus heavy work) or at exercise — and, where necessary, updated may be found fol- situations in which there is increased cardiopulmo- lowing the CAS number for each of the adopted nary demand. Additionally, variations in temperature entries in the alphabetical listing, e.g., Aldrin [309-00- (e.g., extreme heat or cold) and relative humidity 2] (2006). The reader is advised to refer to the “TLV may alter an individual’s response to a toxicant. The Chronology” section in each Documentation for a Documentation for any given TLV® must be re- ® brief history of the TLV recommendations and viewed, keeping in mind that other factors may notations. modify biological responses. Although TLVs® refer to airborne levels of Definition of the TLVs® chemical exposure, dermal exposures may possibly ® occur in the workplace (see “Skin” in the Definitions Threshold limit values (TLVs ) refer to airborne and Notations section starting on page v). concentrations of chemical substances and repre- ® sent conditions under which it is believed that nearly Three categories of TLVs are specified: time- all workers may be repeatedly exposed, day after weighted average (TWA); short-term exposure limit day, over a working lifetime, without adverse health (STEL); and a Ceiling (C). For most substances, a effects. TWA alone or with a STEL is relevant. For some ® Those who use the TLVs MUST consult the substances (e.g., irritant gases), only the TLV– ® latest Documentation to ensure that they under- Ceiling is applicable. If any of these TLV types are stand the basis for the TLV® and the information exceeded, a potential hazard from that substance is used in its development. The amount and quality of presumed to exist. the information that is available for each chemical Threshold Limit Value–Time-Weighted substance varies over time. Average (TLV–TWA): The TWA concentration for a Chemical substances with equivalent TLVs® conventional 8-hour workday and a 40-hour work- (i.e., same numerical values) cannot be assumed to week, to which it is believed that nearly all workers have similar toxicologic effects or similar biologic may be repeatedly exposed, day after day, for a potency. In this book, there are columns listing the working lifetime without adverse effect. Although TLVs® for each chemical substance (that is, airborne calculating the average concentration for a work- concentrations in parts per million [ppm] or milli- week, rather than a workday, may be appropriate in grams per cubic meter [mg/m3]) and critical effects some instances, ACGIH® does not offer guidance produced by the chemical substance. These critical regarding such exposures. effects form the basis of the TLV®. Threshold Limit Value–Short-Term Exposure ACGIH® recognizes that there will be consider- Limit (TLV–STEL): A 15-minute TWA exposure that able variation in the level of biological response to a should not be exceeded at any time during a work- particular chemical substance, regardless of the air- day, even if the 8-hour TWA is within the TLV–TWA. borne concentration. Indeed, TLVs® do not represent The TLV–STEL is the concentration to which it is a fine line between a healthy versus an unhealthy believed that workers can be exposed continuously work environment or the point at which material im- for a short period of time with-out suffering from 1) pairment of health will occur. TLVs® will not ade- irritation, 2) chronic or irreversible tissue damage, 3) ACGIH® © 2007 Introduction: Chemical Substances – i dose-rate-dependent toxic effects, or 4) narcosis of amount that depends on the geometric standard devi- sufficient degree to increase the likelihood of acci- ation. In the lognormal distribution, the geometric dental injury, impaired self-rescue, or materially re- standard deviation (sdg) is the antilog of the standard duced work efficiency. The TLV–STEL will not deviation of the sample value logarithms, and 68.26% necessarily protect against these effects if the daily of all values lie between mg/sdg and mg × sdg. TLV–TWA is exceeded. The TLV–STEL is not a If the short-term exposure values in a given situ- separate, in-dependent exposure guideline; rather, it ation have a geometric standard deviation of 2.0, 5% supplements the TLV–TWA where there are recog- of all values will exceed 3.13 times the geometric nized acute effects from a substance whose toxic mean. If a process displays variability greater than effects are primarily of a chronic nature. Exposures this, it is not under good control, and efforts should above the TLV–TWA up to the TLV–STEL should be be made to restore control. less than 15 minutes, should occur no more than The approach is a considerable simplification of four times per day, and there should be at least 60 the lognormal concentration distribution concept but minutes between successive exposures in this is considered more convenient. If exposure excur- range. An averaging period other than 15 minutes sions are maintained within the recommended limits, may be recommended when this is warranted by the geometric standard deviation of the concentra- observed biological effects. tion measurements will be near 2.0, and the goal of Threshold Limit Value–Ceiling (TLV–C): The the recommendations will be accomplished. It is concentration that should not be exceeded during recognized that the geometric standard deviations of any part of the working exposure. If instantaneous some common workplace exposures may exceed measurements are not available, sampling should be 2.0 (Buringh and Lanting, 1991). If such distributions conducted for the minimum period of time sufficient are known and workers are not at increased risk of to detect exposures at or above the ceiling value. adverse health effects, recommended excursion ® ® ACGIH believes that TLVs based on physical limits should be modified, based upon workplace- irritation should be considered no less binding than specific data. When the toxicologic data for a those based on physical impairment. There is in- specific substance are available to establish a TLV– creasing evidence that physical irritation may initiate, STEL or a TLV–C, these values take precedence promote, or accelerate adverse health effects over the excursion limit. through interaction with other chemical or biologic agents or through other mechanisms. TWA and STEL versus Ceiling (C) Excursion Limits A substance may have certain toxicological properties that require the use of a TLV–C rather For many substances with a TLV–TWA, there is than a TLV–TWA excursion limit or a TLV-STEL. no TLV–STEL. Nevertheless, excursions above the The amount by which the TLVs® may be exceeded TLV–TWA should be controlled, even where the 8- for short periods without injury to health depends hour TLV–TWA is within recommended limits. Ex- upon a number of factors such as the nature of the cursion limits apply to those TLV–TWAs that do not contaminant, whether very high concentrations — have TLV–STELs. even for short periods — produce acute poisoning, whether the effects are cumulative, the frequency Excursions in worker exposure levels may with which high concentrations occur, and the exceed 3 times the TLV–TWA for no more duration of such periods. All factors must be taken than a total of 30 minutes during a workday, into consideration in arriving at a decision as to and under no circumstances should they whether a hazardous condition exists. exceed 5 times the TLV–TWA, provided that Although the TWA concentration provides the the TLV–TWA is not exceeded. most satisfactory, practical way of monitoring air- The approach here is that the maximum recom- borne agents for compliance with the TLVs®, there mended excursion should be related to the variability are certain substances for which it is inappropriate. generally observed in actual industrial processes. In In the latter group are substances that are predom- reviewing large numbers of industrial hygiene sur- inantly fast-acting and whose TLV® is more appro- veys conducted by the U.S. National Institute for priately based on this particular response. Sub- Occupational Safety and Health, Leidel et al.