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1 Executive Summary New knowledge about disinfection and disinfectant by-products has led to changes in procedures for disinfecting drinking water in the 6 years since the Safe Drinking Water Committee last reviewed the potential health effects of these practices (Volumes 2 and 3 of Drinking Water and Health). This report, prepared by the Safe Drinking Water Committee's Subcom- mittee on Disinfectants and Disinfectant By-Products, examines these innovations and assesses their implications for human health. The predominant method of drinking water disinfection practiced in the United States today is chlorination. Studies of the toxicity of the by- products of disinfectants have focused on the trihalomethanes (THMs), which are formed during chlorination and for which considerable data on carcinogenicity have been developed. The level of total THMs in finished drinking water, currently regulated at 100 micrograms (fig) per liter, should be reduced. Noting that chloroform is the principal THM produced by chlorination, the subcommittee found this level to be unsupportable on the basis of the risk values for chloroform developed in this review. Other, nonvolatile by-products of chlorination may be important in contributing mutagenic properties to drinking water, especially when the natural water being treated contains high levels of organic matter. Short- term animal skin tests, although not conclusive, provide indications that organic concentrates from chlorinated water are tumorigenic under some experimental conditions. Studies by routes other than dermal application have not shown such an effect. The subcommittee has developed risk assessments and recommended SNARLs for some of these by-products based on the available data. 1

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2 DR! N K! NO WATER AND H EALTH Unfortunately, many by-products of chlorination and other disinfection practices have not been identified. Consequently, the risks of ingesting these by-products cannot be quantified at present, but are potentially high enough to warrant continued efforts to analyze them. Further studies of reaction mechanisms, controlling factors, and by-product identification are needed. Improved methods for characterizing the nonvolatile products are also needed to support such studies. Methods should be sought to follow the risk associated with multiple chlorination by-products even in the absence of individually quantifiable compound risks. The fact that THM levels may also indicate the presence of unidentified by-products of chlorination is further reason to reduce the total THMs in finished drinking water whenever possible. The use of alternative methods of drinking water disinfection is in- creasing, largely due to health and regulatory concerns about trihalometh- anes. Thus, the nature and toxicity of the by-products of some other widely used water treatments (chloramination, ozonation, and chlorine dioxide) are also evaluated in the report to the extent allowed by available data. Research is also needed to improve understanding of their relative efficacy in eliminating the currently most resistant viruses and protozoan cysts and the major factors affecting such efficacy under treatment plant operating conditions. To prevent overestimation of the degree of disinfection achieved by alternative practices (especially chloramination, which is becoming widely used), methods must be developed for fully quantifying both or- ganic nitrogen precursors of toxic by-products and an organic chloramine fraction in the presence of inorganic monochloramine. Recognizing the paucity of information on these alternative practices, the subcommittee urges that the direct and indirect implications of their potential widespread use be investigated more thoroughly. A major health concern is the chronic ingestion of low levels of dis- infection by-products. In some epidemiological studies of the effects of chlorination, investigators have found increased rates of bladder cancer associated with trends in the levels of certain contaminants in water sup- plies. Interpretation of these studies is hampered by a lack of control for confounding variables (e.g., age, sex, individual health, smoking history, other exposures). Nevertheless, the subcommittee recommended that ep- idemiologists continue to improve protocols and conduct such studies, particularly of drinking water and bladder cancer, wherever exposure data can be obtained directly from individuals rather than by estimation from exposure models. Humans may also be exposed to disinfectants and their by-products from sources other than drinking water and routes other than ingestion. For example, cooking, showers, bathing, swimming, and other activities could provide additional toxic exposures through inhalation or skin ab

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Executive Summary 3 sorption. Given the absence of data on these noningestion routes of ex- posure and the lack of methods for estimating the magnitude of such exposures, the subcommittee declined to include them in its risk estimates. Recognizing their potential for producing toxic effects, however, it rec- ommends that methods be developed for estimating both noningestion exposures to contaminants in drinking water and exposures to the same contaminants in other media, such as food, air, and dust, so that total exposure can be considered by regulators in setting acceptable levels of contaminants in water. The subcommittee calculated quantitative risk assessment for disinfec- tants or their by-products when there were sufficient data. These assess- ments include four distinct components: hazard identification, exposure assessment, dose-response assessment, and characterization of human risk at projected levels and patterns of exposure. The first six volumes of this series provided such assessments for approximately 220 drinking water contaminants. Some of those were disinfectants that are reevaluated in this volume along with other compounds not reviewed previously. .~