Human Health Risk Assessment - Raytheon

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6 Risk Characterization 44 Human Health Risk Assessment DRAFT This section provides a characterization of the potential risks estimated for identified exposure pathways. Risk characterization integrates the estimated exposure information for site receptors with the representations of the potential toxicity derived for each COPC. Using standard USEPA-recommended approaches, this integration yields quantitative estimates of theoretical excess cancer and non-cancer risk for site-related COPCs. These conservative estimates provide a quantitative representation of the risks associated with the protectively estimated exposures associated with the Site. Risk estimates are calculated for individual COPCs for the complete exposure pathways associated with each assessed area. Per USEPA guidance, risk characterization also includes combining COPC-specific risk estimates across complete exposure pathways to provide overall characterizations of potential site-related risks (i.e., cumulative risks across all exposure pathways). Theoretical Excess Cancer Risks. Theoretical excess cancer risks for receptors are expressed as an estimated upper-bound probability of additional lifetime cancer risk due to exposure to site-related constituents. Thus these estimates do not reflect a receptor’s overall risk of cancer but rather are an upper bound estimate of the incremental risk that could theoretically be attributed to exposure to site COPCs. Theoretical excess cancer risks are calculated for those COPCs identified as potential carcinogens by the USEPA. They are calculated for each COPC for each complete exposure pathway, by receptor. The upper-bound estimate of excess risk related to each COPC is calculated by multiplying the lifetime average daily dose estimated for that COPC by its corresponding route-specific cancer slope factor (USEPA, 1989). where: Riskex = theoretical excess lifetime cancer risk LADD = lifetime average daily dose CSF = cancer slope factor Risk ex = LADD x CSF (6-1) Overall theoretical excess cancer risks for complete pathways and receptors were estimated by summing all COPC-specific risk estimates. This form of summation incorporates the assumption that carcinogenic risks from multiple constituent exposures are additive. This health protective assumption ensures that excess lifetime cancer risks for each COPC, pathway and receptor risk estimates are theoretical upper-bound estimates (i.e., the actual risk is very unlikely to be higher and is expected to be much lower).
45 Human Health Risk Assessment DRAFT USEPA Superfund guidance (USEPA, 1991a) directs that risk managers consider excess cancer risks within the range of one-in-ten thousand (1 x 10 -4 ) and one-in-one million (1 x 10 -6 ) to be acceptable depending upon site-specific considerations. This range of risk is generally referred to as the “acceptable risk range” under the USEPA National Contingency Plan. Under this federal policy, regulators have discretion to require risk management measures. Incremental risks greater than 1 x10 -4 generally oblige regulators to require some form of risk management. In contrast, incremental risks less than 1 x 10 -6 are widely considered to be de miminis risks, not requiring management. A risk level of one in a million is often referred to as a “point of departure” or a level of risk where the estimated level of risk and its attendant exposure assumptions and estimated exposure concentrations are taken into account and the need for risk management is evaluated. In Florida, the risk level of 1 x 10 -6 is the target risk level used in setting health protective cleanup levels. Potential Non-cancer Risks. Potential non-cancer risks for individual COPCs are expressed as hazard quotients (HQs) (USEPA, 1989). HQs are calculated as the ratio of the estimated daily intake of each COPC to the corresponding route-specific RfD. HQs are calculated as follows: where: HQ ADD = average daily dose (mg/kg-day) RfD = reference dose (mg/kg-day) ADD = RfD When the average daily dose estimated from site-associated soil constituents exceeds the protective RfD, the HQ exceeds one. This typically is considered a circumstance requiring further evaluation since it indicates that exposure could be higher than the “no-effect” dose represented by the RfD. An HQ higher than one does not necessarily indicate a significant potential for non-cancer effects, however, since both the average daily dose and RfD are intended to be conservative estimations. An HQ higher than 1.0 suggests that further consideration should be given to the likelihood for potential non-cancer effects. When the HQ does not exceed 1.0, the average daily dose estimated from site-related soil constituents is not greater than the conservative RfD, indicating that exposure is expected to be below the threshold required to produce effects and the likelihood of realizing a non-cancer effect from that COPC is negligible. To summarize potential non-cancer risks for multiple COPCs across complete exposure pathways, and across receptors, HQs are summed to arrive at a Hazard Index (HI). The resulting HI serves as a conservative health protective summary of pathway and receptor risks, since summing all of the individual COPC HQs incorporates the assumption that their risks are all additive. In fact, different COPCs may act through different mechanisms and on different target organs. The overall HIs are useful for rapidly excluding pathways or receptors with (6-2)
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Human Health Risk Assessment - Raytheon

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