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of concern at the site are judged unlikely to result in adverse noncarcinogenic health effects. If the sum is greater than 1.0, a more detailed and critical evaluation of potential noncarcinogenic health hazards may be warranted. Such additional evaluation may include the consideration of the specific target organ(s) affected and mechanism(s) of action of the chemicals of concern, and consideration of exposure assumptions and exposure concentrations used to estimate risk. The assumption of additive effects reflected in the cumulative H.I. is most properly applied to substances that induce the same effect by the same mechanism (EPA 1989b). Consequently, application of the equation to a mixture of substances that are not expected to induce the same type of effects could overestimate the potential for adverse -� health effects. The H.I. provides a rough measure of potential toxicity, but it is conservative and dependent on the quality of the experimental evidence. Since the H.I. -- does not define dose-response relationships, its numerical value cannot be construed as a direct estimate of risk (EPA 1989b), but only as a level of concern. 6.2.2 Carcinogenic Risk Carcinogenic risk is defined as the upperbound incremental probability of an individual developing cancer over a lifetime as a result of exposure to a potential carcinogen. The numerical estimate of excess lifetime cancer risk is calculated by multiplying the chemical intake averaged over a lifetime by the cancer slope factor (SF) as follows: Risk = Chemical Intake (mg/kg-day) x SF (mg/kg-day)"'The EPA slope factors are upper 95th percentile confidence limits of the probability of response per unit uptake of chemical over a lifetime. Slope factors are based on experimental animal data and epidemiological studies when available. A linear nonthreshold mathematical model for low-dose extrapolation, the linearized multistage model, is used to calculate numerical slope factor value. EPA states that carcinogenic risks estimated using this approach are upper-bound estimates. This means that the actual risk is likely to be less than the predicted risk (EPA 1989b). S:\LDC\YELLO.RPT May 4,1995 6-3