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SECTIQNSI X Risk Charaderizatiou The resulting sum is referred to by EPA as the hazard index (H.I) The H I approach assumes that multiple subthreshold exposures to several chemicals could result in an adverse health effect Where a receptor may be subject to chemicals of concern via multiple exposure pathways (e g, inhalation of volatilies and ingestion of water), the individual hazard indexes from all relevant pathways are summed to obtain the total H I for that receptor For multiple chemical exposures, the total H I might exceed 10 even if no single chemical intake exceeds its RfD If the total RI is less than 10, cumulative exposures to the chemicals of concern at the site are judged unlikely to result in adverse noncarcinogenic health effects If the sum is greater than 10, a more detailed and critical evaluation of potential noncarcmogenic health hazards may be warranted Such additional evaluation may include the consideration of the specific target organ(s)affected and mechanisms)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 SF as follows 1 Risk = Chemical Intake(mg/kg-day)x SF(mg/kg-day)"' EPA states that carcinogenic risks estimated using this approach are upper-bound estimates This 1 means that the actual risk is likely to be less than the predicted risk(EPA, 1989b) EPA policy must be considered when interpreting the significance of cancer risk estimates In the National Oil and Hazardous Substances Pollution Contingency Plan (NCP) (40 CFR Part 300), EPA states that "For known or suspected carcinogens, acceptable exposure levels are generally concentration levels that represents an excess upper-bound lifetime cancer risk to an individual between 10'4and 10'6 " The EPA further states, in the preamble to the NCP, that the 1 x 10-6nsk level should be used as a point of departure for establishing remediation goals for chemicals at specific hazardous � � ® T11097W73WONAWOCKTQMREPORT%SHOREpoI D0G13-FE84XW3000NA%SNA 6-2