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' Human Health Risk Assessment Former Mobil Oil Bulk Plant 04-343 500 East Grant Line Road Tracy, California 6.0 RISK CHARACTERIZATION 6.1 INTRODUCTION Risk characterization yields upper-bound estimates of carcinogenic and noncarcinogenic health risks by combining the quantitative exposure and dose-response estimates. The estimation of risk for a potential carcinogen is calculated by multiplying the Cancer Slope Factor by the ' quantitative estimate of exposure, the LADD. Potential human exposure to benzene was evaluated using this approach. The estimation of noncarcinogenic health risks is evaluated through ft.. calculation of a Hazard Index. The Hazard Index is the ratio of the quantitative estimate of ambient concentration to the Reference Concentration. Toluene, ethylbenzene, and xylenes were evaluated using the Hazard Index approach. The intent of the risk and hazard index estimates is to yield reasonable quantitative values of potential exposure without using such extreme assumptions that estimates do not make sense. In risk assessment of human exposures to carcinogenic chemicals, the (le- minimis (insignificant) risk levels of 1 in 10,000 to 1 in 1,000,000 (lW to 10-6) are often used as benchmarks for an acceptable risk level (Travis et al., 1987). A Hazard Index of 1.0 or less indicates that no adverse nonearcinogenic health risks are expected to occur. In virtually every case, the reasonable lower-bound risk estimate is zero. 6.2 RISK MANAGEMENT Risk assessment and risk management are integral components of the regulatory activities of federal, state, and local agencies. Risk assessment involves the quantification of the potential for adverse health effects in a potentially exposed population, and risk management refers to the ' selection and implementation of the most appropriate regulatory action. The regulatory action may be based on the results of the risk assessment, available control technology, cost-benefit analysis, asymptotic limit, acceptable risk, and social and political factors. 3G-ui:�6-1 i 6-1