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5.0 <br /> TOXICITY ASSESSMENT <br /> 5.1 PURPOSE OF THE TOXICITY ASSESSMENT <br /> The purpose of the toxicity assessment is to evaluate the toxicity of source-related <br /> chemicals of concern and to estimate the dose-response relationship for each of these <br /> chemicals. Chemicals of concern at the YFS site were identified by the application of <br /> EPA guidance and professional judgement. The list of the chemicals of concern appears <br /> in Section 3.1. The information obtained in the toxicity assessment is combined with <br /> estimated contaminant intakes calculated as part of the exposure assessment to estimate <br /> the potential excess lifetime cancer risk and potential noncarcinogenic health hazards <br /> (EPA 1989b). <br /> Noncarcinogenic responses are generally characterized by a threshold: a certain <br /> minimum intake of substance below which adverse effects will not occur. Above the <br /> threshold, protective mechanisms of the organism may be overwhelmed and effects may <br /> occur. Carcinogenic responses are assumed to have no threshold. This assumption <br /> means that there is some finite cancer risk no matter how small the dose; the smaller <br /> the dose, the smaller the risk of cancer (EPA 1989b). <br /> The two principal indexes of toxicity are the Reference Dose (RfD) for noncarcinogenic <br /> health effects, and Slope Factor (SF) for carcinogenic health effects. These values are �. <br /> derived by the EPA for the most commonly occurring and the most toxic chemicals <br /> generally associated with chemical releases to the environment. An RfD is an intake or <br /> contaminant dose per unit of body weight per day that is unlikely to result in toxic <br /> effects to human populations, including sensitive subgroups (e.g., the very young or old), <br /> even if exposure occurs every day for a lifetime. The RfD allows for the existence of a w <br /> threshold and is used for the assessment of potential noncarcinogenic effects (EPA <br /> 1989bj. <br /> The SF is used to estimate an upper-bound probability of an individual developing `- <br /> cancer as a result of exposure to a potential carcinogen. Carcinogens with EPA-derived <br /> S:\LDC\YELLO RPT May 4, 1995 5-1 <br />