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• For adult and child residential exposures it was assumed that <br /> individuals would shower every day that they are at the site. — <br /> 7.4 TOXICITY ASSESSMENT <br /> In general, the available scientific information is insufficient to provide a thorough <br /> understanding of all the potential toxic properties of chemicals to which humans are <br /> potentially exposed. Consequently, varying degrees of uncertainty surround the <br /> assessment of adverse health effects in exposed populations. Source of uncertainty <br /> related directly to toxicity data include: <br /> • Use of dose-response data from experiments on homogeneous,sensitive <br /> animal populations to predict effects in heterogenous human <br /> populations with a wide range of sensitivities <br /> • Extrapolation of data from: (1) high dose animal studies to low-dose <br /> human exposures, (2) acute or subchronic exposure to chronic exposure, <br /> and (3) one exposure route to another (e.g., from ingestion to <br /> inhalation or dermal absorption) <br /> • Use of single-chemical test data that do not account for multiple <br /> exposures or synergistic and antagonistic responses <br /> Intersnecies Extrapolation Use of Animal Studies) <br /> The majority of toxicological knowledge of chemicals comes from experiments with <br /> laboratory animals due to the limited studies available for humans. Experimental animal <br /> data have historically been relied upon by regulatory agencies and other expert groups <br /> to assess the hazards and safety of human exposure to chemicals. Even though this <br /> practice has been supported in general by empirical observations, there are known - <br /> interspecies differences in chemical absorption, metabolism, excretion, and toxic <br /> response. There are also uncertainties concerning the relevance of animal studies using <br /> exposure routes which differ from human exposure routes. In addition, the frequent <br /> necessity to extrapolate results of short-term or subchronic animal studies to humans <br /> exposed over a lifetime has inherent uncertainty. r <br /> Although animal experimental data have many limitations, they are widely believed to <br /> be a necessary part of toxicity assessment, especially in the absence of human <br /> S:1LDCIYELL0.R" May 4,1995 7-5 <br />