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r The EPA has developed various types of Rf )s depending on the exposure route (ingestion or inhalation), the critical effect, and the length of exposure being evaluated (chronic or subchronic). The EPA bases the RtD on the most sensitive animal species tested (i.e., the species that experiences adverse effects at the lowest dose). The EPA defines a chronic RfD as an estimate of a daily exposure level for the human population that is unlikely to result in deleterious effects during a lifetime (70 years, according to EPA guidance). A chronic Rf ) is used to evaluate the potential noncarcinogenic hazards associated with long-term contaminant exposures (7 years to a lifetime). Subchronic RfDs have been developed to characterize potential noncarcinogenic hazards 4-- associated with short-term contaminant exposures. The EPA defines subchronic exposure as periods ranging from two weeks to seven years (EPA 1989b). Subchronic RfDs tend to be higher,generally by one order of magnitude, than chronic RfDs because of the shorter exposure duration. s Both chronic and subchronic RfDs are presented for the chemicals of concern. The subcronic RfDs are used to evaluated noncarcinogenic effects for the 6-year childhood groundwater ingestion, dermal exposure, and inhalation scenarios. 5.3 TOXICITY ASSESSMENT FOR CARCINOGENIC EFFECTS In estimating the risk posed by potential carcinogens, it is the common practice of the EPA and other regulatory agencies to assume that any exposure level is associated with a finite probability, however minute, of producing a carcinogenic response. EPA assumes that a small number of molecular events can evoke changes in a single cell that can lead to uncontrolled cellular proliferation. This mechanism for carcinogenicity is referred to as "non-threshold" since there is theoretically no level of exposure for such a substance that does not pose a small probability of producing a carcinogenic response. There is increasing evidence that this assumption is not correct for several chemicals. That is, it does appear that for at least some chemicals, there is a threshold below which the contaminant will not increase the risk of cancer. The EPA uses an evaluation process in which the substance is assigned a weight-of-evidence classification. This S:\LDC\YELLO.RYr May 4, 1995 5-4 ar