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=AL RELEASE DDJC Tracy been exposed, ATSDR made assumptions about exposure duration and concentrations. ATSDR assumed EPA's national upper-bound limit at one residence of 30 years for an exposure duration for an adult and 6 years exposure for a child and that individuals drank the maximum concentration of carbon tetrachloride detected over the entire 30-year exposure duration for adults. Actual exposures were probably much less because the residents have been supplied with bottled water and/or a carbon filtration system and no one is thought to have been consistently using water containing the highest levels of contaminants for this extended period of time. ATSDR estimated a daily exposure dose of carbon tetrachloride (adult: 0.00005 mg/kg/day; child: 0.0001 mg/kg/day) from drinking private well water are approximately 7 to 14 times less than EPA's chronic RM of 0.0007 mg/kg/day. Because the estimated exposure doses for both the adult and child are less than the RfD for carbon tetrachloride, ATSDR does not expect that drinking water form private wells, even over a 30-year period, would lead to an increased likelihood of developing noncancer health effects. To account for dermal (skin contact) and inhalation(breathing vapors) exposures that might occur during domestic use of the water, ATSDR assumed that these exposures each equal the exposure from drinking the water. In applying this assumption, ATSDR multiplied the estimated oral dose by three (to consider all three routes: ingestion, inhalation of vapors, dermal contact). The resulting overall estimated doses are still less than the RID for carbon tetrachloride, respectively. Cancer EPA has classified carbon tetrachloride as a probable human carcinogen. The link between TCE and cancer at relatively low levels is controversial, however. Although TCE has been shown to cause cancer in laboratory animals given large doses, available human studies for WE are inconclusive and the data are inadequate to establish a causal fink. Because of these uncertainties, EPA is currently reviewing the scientific literature pertaining the carcinogenicity of TCE. As a conservative screening measure, ATSDR used a previously derived CPF for TCE of 0.11 mg/kg/day''. This approach provides a conservative evaluation of the likelihood of past exposure to TCE. Using the CPFs for carbon tetrachloride and for TCE, ATSDR derived excess cancer risk TCE from drinking the private well water. The resulting cancer estimates are 9 x 10' for TCE and carbon 3 x 10-6for carbon tetrachloride, or roughly three new cancer cases over a lifetime in 1,000,000 exposed persons. ATSDR believes that even if people drank water containing the highest detected levels of WE or carbon tetrachloride for a very long time, it is unlikely that the level of exposure would lead to cancer. C-4