WHO Guidelines for Drinking-Water Quality 3 – Chapter 8 DRAFT 11 March, 2003 8. CHEMICALS ASPECTS In general, approaches to the management of chemical hazards vary between those where the source water quality is a significant contributor (with control effected through source water inputs or through treatment or blending), and those from materials and chemicals used in the production and distribution of drinking-water, (controlled by process optimisation or product specification). Most chemicals arising from source waters are of health concern only after extended exposure. The principal exception is nitrate. Typically changes in water quality occur progressively except for those substances that are discharged or leach intermittently to flowing surface waters or groundwater supplies, for example, from contaminated landfill sites In some cases, there are groups of chemicals which arise from the same sources for example the disinfection by-products, and it may not be necessary to set standards for all of the substances for which there are guideline values. If chlorination is practised, the trihalomethanes, of which chloroform is the major component, are likely to be the main disinfection by-products, together with the chlorinated acetic acids in some instances. In some cases, control of chloroform levels and, where appropriate, trichloroacetic acid levels will also provide an adequate measure of control over other chlorination by-products. Several of the inorganic elements for which guideline values have been recommended are recognized to be essential elements in human nutrition. No attempt has been made here to define a minimum desirable concentration of such substances in drinking-water. 8.1 Chemical hazards in drinking-water A number of chemical contaminants have been shown to cause adverse health effects in humans as a consequence of prolonged exposure through drinking-water. However, this is only a very small proportion of the chemicals that may reach drinking-water from various sources. The lists of chemicals addressed in these Guidelines do not imply that all of these chemicals will always be present, or that other chemicals not addressed will not be present. The substances considered here have been assessed for possible health effects and guideline values have only been proposed on the basis of health concerns. Additional consideration of the potential effects of chemical contaminants on the acceptability of drinking-water to consumers is included in chapter 10: Acceptability Aspects. Some substances of health concern have aesthetic effects that would normally lead to rejection of water at concentrations significantly lower than those of health concern. For such substances, health- based reference values are needed, for instance, for use in interpreting data collected in response to customer complaints. In section 1.2 it is indicated that “In developing national drinking-water standards based on these guideline values, it will be necessary to take into account a variety of geographical, socioeconomic, dietary and other conditions affecting potential exposure. This may lead to national standards that differ appreciably from the guideline values.” This is particularly 189 WHO Guidelines for Drinking-Water Quality 3 – Chapter 8 DRAFT 11 March, 2003 applicable to chemical contaminants, for which there is a long list, and setting standards for, or including, all of them in monitoring programmes is neither feasible nor desirable. It is, important that chemical contaminants are prioritised so that the most important are considered for inclusion in national standards or monitoring programmes. The scientific basis for guideline values has been made as clear as possible in the summaries presented here. This is particularly important in helping to modify guideline values to suit national requirements or in assessing the significance for health of concentrations of a contaminant that are greater than the guideline value. Chemical contaminants in drinking-water may be categorised in various ways, however, the most appropriate is to consider the primary source of the contaminant i.e. to group chemicals according to where control may be effectively exercised. This aids in the development of approaches that are designed to prevent or minimise contamination, rather than those that rely primarily on the measurement of contaminant levels in final waters. Sources of chemicals in drinking-water related to health are therefore divided into six major source groups, see table 8.1 Table 8.1 Categorisation of source of chemical constituents Source of chemical constituents Examples of sources Naturally occurring Rocks, soils and the effects of the geological setting and climate Industry and human settlements Mining (extractive industries) and manufacturing and processing industries, sewage, solid wastes, urban runoff, fuel leakages Agricultural activities Manures, fertilisers, intensive animal practices and pesticides Water treatment and distribution systems Coagulants, disinfection by-products, and piping materials Larvicides used in water for Public Health Larvaecides used in the control of insect vectors Cyanobacteria Eutrophic lakes Some contaminants may fall into more than one category but the primary category is the one under which the guideline summary evaluation is listed. The categories are not always clear cut. The group of naturally occurring contaminants includes many of the inorganic chemicals that are found in drinking-water as a consequence of release from rocks and soils by rainfall, some of which may become problematical only where there is environmental disturbance such as in mining areas. The criteria use to decide whether a Guideline Value is established for a particular chemical constituent are: i. the chemical is of significant international concern; ii. there is credible evidence of occurrence of the chemical in drinking-water; iii. there is evidence of actual toxicity combined with evidence of occurrence in drinking – water at concentrations close to, or above those of health concern; or 190 WHO Guidelines for Drinking-Water Quality 3 – Chapter 8 DRAFT 11 March, 2003 iv. the chemical is being considered, or is included in the WHO-PES programme (approval programme for direct application to drinking-water for control of insect vectors of disease). 8.2 Derivation of chemical guideline values Guideline Values are derived for many chemical constituents of drinking-water. A Guideline Value represents the concentration of a constituent which does not result in any significant risk to health over a lifetime of consumption There are two principal sources of information on health effects resulting from exposure to chemicals that can be used in deriving guideline values. The first is investigation in human populations. The value of such studies for many substances is limited, owing to lack of quantitative information on the concentration to which people are exposed or on simultaneous exposure to other agents. However, for some substances such studies are sometimes the primary basis on which guidelines are developed. The second, and most frequently used source of information is toxicity studies using laboratory animals. The limitations of toxicology studies include the relatively small number of animals used and the relatively high doses administered, which creates uncertainty as to the relevance of particular findings to human health. This is because there is a need to extrapolate the results to the low doses to which human populations are usually exposed. In most cases, the study used to derive the guideline value is supported by a range of other studies, including human data, and these are also considered in carrying out a health risk assessment. In order to derive a guideline value to protect human health, it is necessary to select the most suitable study or studies. Data from well-conducted studies, where a clear dose-response relationship has been demonstrated, are preferred. Expert judgement was exercised in the selection of the most appropriate study from the range of information available. 8.2.1: Approaches taken To approaches to the derivation of Guideline Values are used: one for “threshold chemicals” and the other for “non-threshold chemicals” (mostly genotoxic carcinogens). It is generally considered that the initiating event in the process of genotoxic chemical carcinogenesis is the induction of a mutation in the genetic materials (DNA) of somatic cells (i.e. cells other than ova or sperm), and that there is a possibility of risk at any exposure (i.e. no threshold). On the other hand, there are carcinogens that are capable of producing tumours in animals or humans without exerting a genotoxic activity, but acting through an indirect mechanism. It is generally believed that a threshold dose exists for non-genotoxic carcinogens. In deriving Guideline Values for carcinogens, consideration was given to the potential mechanism/s by which the substance may cause cancer, in order to decide whether a threshold or non-threshold approach should be used (see 8.2.1 for Threshold chemical effect and 8.2.6 Non-threshold chemicals). The evaluation of the potential carcinogenicity of chemical substances is usually based on long-term animal studies. Sometimes data are available on carcinogenicity in humans, however mostly from occupational exposure. 191 WHO Guidelines for Drinking-Water Quality 3 – Chapter 8 DRAFT 11 March, 2003 On the basis of the available evidence, IARC categorises chemical substances with respect to their potential