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Exposures of humans to carcinogenic chemicals are modeled mathematically using either animal data or, when available, human data. Many of these models predict a non-threshold and linear dose-response curve which passes through the origin (i.e, a theoretical risk exists at all exposure levels). According to the EPA Guidelines for Carcinogen Risk Assessment (EPA, 1986) and the National Research Council (NAS, 1983), the dose-response estimate should describe and Justify the methods of extrapolation used to predict incidence and should also provide a description of the uncertainty inherent in the methods Exposures of humans to noncarcinogenic chemicals such as diesel fuel and other distillate petroleum hydrocarbons are modeled by an allowable daily exposure level, termed the Reference Dose (RfD) The RfD is the daily dose of a chemical to which a human may be exposed without an appreciable risk of deleterious effects In most cases, the RfD is based on nontoxic exposure levels in animals extrapolated to humans using safety factors This method assumes that these exposures have a threshold, (i e , there is some exposure level below which an adverse effect will not occur in the exposed individual) The Exposure Assessment is the process of measuring or estimating exposures to an agent in the environment. In general, the exposure assessment describes the magnitude, duration, timing, and route of exposure, the size and nature of the populations exposed, and the uncertainties in all estimates The goal of the exposure assessment is to accurately estimate both the dose to which the receptor is being exposed (administered dose) and the dose of the chemical reaching the target tissue on the receptor (target dose) Human exposures are reported as a Lifetime Average Daily Dose (LADD) for carcinogens and an Average Daily Dose (ADD) for noncarcinogens. Risk Charactenzation is the process of obtaining a quantitative estimate of risk (EPA, 1986) The risk characterization step is performed by combining the quantitative exposure and dose-response assessments, including the uncertainties identified in the preceding steps The discussion of the underlying assumptions and associated uncertainties provides insight into the degree to which the numerical estimates are likely to reflect the true magnitude of risk In risk assessments on human exposures, the de minimis (insignificant) risk levels of 1 in 10,000 to 1 in 1,000,000 (1 x 104 to 10-6) are frequently used as benchmarks for an acceptable risk level. In risk assessments on human exposures to noncarcinogenic chemicals, the significant adverse health effects are not expected if exposure levels are below G\O7imAwAFE.FHL%b*S 26 93 1-4 19"