LETTERS
Drinking-water standards Dear Sir: The perspective and approach for drinking-water standards suggested by Munro and lkavis (ES&I: August 1986, pp. 768-69) is too narrow. Their article does not mention all the information available to develop standards, it ignores uncertainties in the risk calculations, and it neglects differences between calculation methods. One could, as they do, arbitrarily select one calculation model and arrive at some nominal “risk” levels, but then these levels must be thought of as hypothetical values and not as point estimates of actual risks. The Safe Drinking Water Act (SDWA) requires that drinking-water contaminants be regulated on the basis of health effects, taking cost into account, Thus EPA must consider not only risk, but also the kinds of health effects and the cost and feasibility of monitoring and treatment. In general, the nominal risk levels associated with the standards for different contaminants vary and do not involve a single individual risk level that might be considered acceptable. Table 1 in their article is misleading. The values listed by Munro and Travis are the potential upper limits of population risks, using a particular calculation model, although the range of uncertainty is known from other sources to be quite large. In general, for the volatile organic compounds the range of uncertainty is four to six orders of magnitude (a factor of 10,OOO to 1 million), and the lower risk limit approaches zero. At the trihalomethane standard level, the more recent risk estimates are in the range of iO-10-iO-5 excess cancers per lifetime, not 5 x excess cancers per lifetime. The contaminant with the largest potential risk level as a drinking-water contaminant is radon, which was not 1068 Environ. Sci. Technol., Vol. 20,No. 11, 1986
mentioned in the article. Drinking water is, on the average, a minor source of total indoor air radon, but in some homes it is the major source. About one-half of the drinking-water supplies in the United States could theoretically exceed the nominal 10-4/lifetime risk level for radon because the gas is frequently inhaled. EPA does use individual and totalpopulation risk information in its regulatory analyses as suggested by Munro and Travis ( I , 2). The nominal population risks for drinking-water contaminants have been calculated for existing levels in drinking water, as shown in Table 1, Reference 2. The standards for drinking-water contaminants promulgated by EPA will not bk consistent for the individual or population risk level. This is because of different health effects end points, variation of costs and feasibility of monitoring and treatment, and different routes of exposure (ingestion, inhalation, and dermal contact). The range of uncertainty in exposure levels, type and toxicity of health effects, and cost and feasibility of monitoring and treatment for drinking-water contaminants leads to a wide range of nominal risk levels. Indeed, methods for calculating risk levels for many nonradionuclide substances are so varied and the results so imprecise that it is questionable whether one sh~uldeven speak in terms of equivalent risks other than for purposes of formalized impact assessments. Regarding the opinion that drinkingwater standards would be more rational if they considered individual and population risks, our reading of SDWA does not find quantitative risks to be a deciding factor for maximum contaminant level goals (MCLGs) or maximum contaminant levels (MCLs). For probable
human carcinogens our conciilsions are that recommended maximum contaminant levels (RMCLs), which are MCLGs in the new law, should be zero as an ideal and that the MCLs should be as close to the MCLGs as is technically and economically feasible. Calculated risk values for radionuclides cannot be compared with those for the organics because the methodologies employed are so different. We believe that the values for radionuclides also are much more reliable than are those for the organics. C. Richard Cothern Joseph A. Cotruvo Office of Drinking Water Environmental Protection Agency Washington, D.C. 20460
References (1) Cothern, C. R.; Coniglio, W. A , ; Marcus, W.L. “Techniques for the Assessment of the Carcinogenic Risk to the U.S. Population Due to Exposure from Selected Volatile Organic Compounds from Drinking Water Via the Ingestion, Inhalation and Dermal Routes,” EPA 570/9-85-001, NTIS PBB4 213941; National Technical Information Service: Springfield, Va., 1984. (2) Cothern, C. R. “Techniques for the Assessment of Carcinogenic Risk Due to Drinking Water Contaminants,’’ CRC Crit. Rev., in press. The thoughts and ideas expressed in this letter are those of the authors and are not necessarily those of the U.S. Environmental Protection Agency.
R e authors reply: Our estimates of individual lifetime risk (ILR) associated with existing and proposed drinking-water standards are based on EPA carcinogenic potency and risk estimates (Fed. Regist. , .WednesContinued on p . 1091
