ARCHIVED REPORTS XR0006421 - Laserfiche WebLink

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At the concentrations currently found at this site, the risk of most importance is the chronic or long- term risk Two types of chronic risk have to be assessed cancer risk and non-cancer chronic exposure risk These two types of risk are treated differently because of the way they effect people Carcinogens are modeled as if any amount of the compound will produce some possibility of causing cancer Non carcinogens will not cause health effects until they reach a threshold value at which concentration they may have some adverse effect on the population Below the threshold value, a high-risk compound may even have a beneficial effect, e g , vitamin A and sodium chloride (table salt) Because of these different effect models, the risk is calculated differently The cancer risk is calculated as a probability of causing cancer using a slope factor (SF) The SF expresses the probability of cancer from the intake of 1 mg of compound per kg of body weight over a 70-year lifetime The non-cancer risk is calculated by comparing the chance of exceeding the threshold limit of the compound In order to insure that no sensitive person will be effected by the chemical, the Reference Dose (Rfd) that is used to calculate the risk of exposure is given an uncertainty factor of 10 to 1000 times less than the No-Observed-Adverse-Effect-Level (NOAEL) The NOAEL is the concentration at which no adverse effect was found in human and/or animal studies The non-cancer risk is calculated by dividing the long-term ingestion rate by the Rfd If the result is less than one, the threshold limit above which health effects will occur should not be reached TOXICITY ASSESSMENT Of the compounds currently found at the site, the primary driver of risk at this site is the benzene Cancer Toxicity Benzene, a class A carcinogen, has been shown in work place studies to cause Leukemia in humans The current EPA Cancer risk slope factor (SF) is 0 029 (1/(mg/kg-day)) Non-cancer Toxicity Five compounds were considered for the non-cancer toxicity, Benzene, Ethlybenzene, Hexane, MTBE, Toluene, and Xylenes (which includes, meta, para and orto xylenes) See Appendix G of Update Status Report #53, RBCA Spreadsheet, RBCA Chemical Data, and Toxicity Data for current EPA Hexane was used to represent the alkane (non-aromatic) portion of gasoline Hexane was chosen because its properties represent the light, faster moving, end of the alkanes and as such represents the worst case for alkane migration 11 R603 Update#54/CLOSURE 3 12198