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SECTIONFIVE 5.9 PURPOSE OF THE TOMMY ASSESSMEMT The purpose of the toxicity assessment is to evaluate the toxicity of source-related cher ucals of concern and to estimate the dose-response relationship for each of these chemicals Chemicals of concern at the Shop 10 site were identified by the application of EPA, DTSC, and RWQCB guidance and professional judgement The list of the chemicals of concern appears in Section 3 The information obtained in the toxicity assessment is combined with estimated contaminant intakes calculated as part of the exposure assessment to estimate the potential excess lifetime canner nsk and potential noncarcinogennc health hazards(EPA, 1989b) Noncarcinogenic responses are generally characterized by a threshold a certain mwmum intake of substance below which adverse effects will not occur Above the threshold, protective mechanisms of the organism may be overwhelmed and effects may occur Carcinogenic responses are assumed to have no threshold Tlus assumption means that there is some flute cancer risk no matter how small the dose,the smaller the dose,the smaller the risk of cancer(EPA, 1989b) The two principal indexes of toxicity are the Reference Dose (RM) for noncaranogenic health effects, and Slope Factor (SF) for carcinogenic health effects These values are derived by the EPA for the most commonly occurrmng and the most toxic chenucals generally associated with chemical releases to the environment An RfD is an intake or contaminant dose per unit of body weight per day that is unlikely to result in toxic effects to human populations, including sensitive subgroups (e g, the very young or old), even if exposure occurs every day for a lifetime The RfD allows for the existence of a threshold and is used for the assessment of potential noncarcuiogenic effects(EPA, 1989b) The SF is used to estimate an upper-bound probability of an individual developing cancer as a result of exposure to a potential carcinogen Available RfDs and SFs for each contaminant of concern are presented in Table 5-1 The RiDs and SFs values used in this nsk assessment were obtained from the following sources 0 DTSC's Cancer Potency Factor Tables(DTSC, 1996) 0 Cal EPA MTBE Update(Cal EPA, 1998) 0 EPA's Integrated Risk information System(IRIS)(EPA,1998) Internet database system o EPA's Health Effects Assessment Summary Tables(BEAST)(EPA, 1997) o EPA Region IX Preliminary Remediation Goals Toxicity Tables(EPA, 1998) ® 711997197300ONA%STOCKTO"EPORi�.SHOREP#1 DOCO+MS•M73009NMWA 5-1