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INITIAL RELEASE DDJC Tracy No chronic MRI. or RfD exists for TCE. Inadequate data exist for evaluating noncancer effects associated with chronic exposures to TCE. Chronic oral exposure studies (e.g., drinking water studies) in animals have focused on cancer endpoints, not noncancer effects. Furthermore, EPA recently withdrew the intermediate MRL TCE. The study on which the intermediate MRL for TCE was based has been questioned because it contains certain flaws and limitations (e.g., the exact amount of TCE-contaminated water consumed by rats in the study is uncertain) (ATSDR, 1997). Therefore, when evaluating possible exposures to TCE, ATSDR considered only cancer. To evaluate cancer effects, ATSDR uses Cancer Potency Factors (CPFs) that define the relationship between oral exposure doses and the increased likelihood of developing cancer over a lifetime. The CPFs are developed using data from animal or human studies and often require extrapolation from high exposure doses administered in animal studies to the lower exposure levels typical of human exposure to environmental contaminants. CPFs represent the upper-bound estimate of the probability of developing cancer at a defined level of exposure; therefore, they tend to be very conservative (i.e., overestimate the actual risk) in order to account for a number of uncertainties in the data used in the extrapolation. ATSDR estimated the potential for cancer to occur using the following equation. (The estimated exposure doses and CPF values for the contaminants of concern are incorporated into the equation): Lifetime Cancer Risk=Estimated exposure dose (mg/kg/day) x CPF(mg/kg/day)-` Although no risk of cancer is considered acceptable, it is impossible to achieve a zero cancer risk. Consequently, ATSDR often uses a range of 104 to 10-6 estimated lifetime cancer risk (or 1 new case in 10,000 to 1,000,000 exposed persons), based on conservative assumptions about exposure, to determine whether a concern regarding cancer effects is valid. In addition to estimating the likelihood of noncancer and cancer effects, ATSDR reviewed the literature to evaluate possible health effects associated with exposure at the doses/concentrations estimated for the pathways described above. Estimated Exposure Dose for Consumption of Contaminants in Private Well Water ATSDR used the following equation to estimate an exposure dose for ingestion of water: Estimated exposure dose=Conc. x CF x IR x EF x ED BW x AT where: C-2