that seeks to examine specific populations or communities to clarify the relation between health and physical, biologic, and chemical factors.

Volume 1 of this report (NRC, 1991a) described some limitations and problems in the quantitative estimation of exposure when the focus of a study is possible adverse consequences of chemical exposure from hazardous-waste sites. This chapter reviews some aspects of exposure assessment or analysis that are important in environmental epidemiology and that illustrate the central role of exposure assessment. This chapter also discusses opportunities to improve analysis of exposure.

The importance of exposure assessment has been underscored in several reports (NRC 1988, 1991a,b). An International Society of Exposure Analysis has been formed, and the Science Advisory Board of the Environmental Protection Agency (EPA) recommended that EPA develop a 5-year program on exposure assessment (EPA, 1988).

The National Human Exposure Assessment Survey (NHEXAS) is a federal interagency program to design and implement an exposure surveillance system for the US population. The overall goal is to obtain periodic and systematic measurements of population exposures to multiple chemicals, including data on important environmental media, pathways, and routes, so that we can accurately determine current status, document historical trends, and predict possible future directions for exposures to hazardous chemicals (Sexton, 1991; Sexton et al., 1995) The NHEXAS has 3 specific objectives: (1) to document the occurrence, distribution, and determinants of exposures to hazardous environmental agents, including geographic and temporal trends, for the US population; (2) to understand the determinants of exposure for potentially at-risk population subgroups, as a key element in the development of cost-effective strategies to prevent or reduce exposures (risks) deemed to be unacceptable; and (3) to provide data and methods for linking information on exposures, doses, and health outcomes that will improve environmental health surveillance, enhance epidemiologic investigations, promote development of predictive models, and ultimately lead to better decisions (Sexton et al., 1995). As pointed out by Burke and Sexton (1995), "NHEXAS represents perhaps the most comprehensive exposure surveillance initiative ever undertaken ... it has been designed to address the information needs of regulators and improve the scientific basis for risk assessment, risk management, and risk communication." The "consolidated report" of an EPA-appointed consensus team on NHEXAS concludes as follows: "The implementation of NHEXAS can be considered a turning point in environmental policy. It represents the first concerted effort to understand and track total individual exposures on a national scale'' (Burke et al., 1992). A complete description of the NHEXAS phase I field studies, as well as a summary of the rationale and justification for



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