NHEXAS was published in the Journal of Exposure Analysis and Environmental Epidemiology (1995; 5:229-444).

EPA has made a significant contribution to exposure assessment by the issuance of guidelines for exposure analysis (EPA, 1992). The guidelines describe general concepts of exposure assessment and have application to risk assessment, trends analysis, and epidemiology. Those guidelines and the NRC report Human Exposure Assessment for Airborne Pollutants (NRC, 1991b) are major contributions to the assessment of the impact of toxic agents on potentially exposed populations. They provide a broad overview of the need for exposure assessment. This chapter is not intended to repeat the contents of those documents, but will focus on certain specific issues in exposure assessment for use in environmental epidemiology.

It is relevant to state the assumptions that the committee used as the basis and context for this chapter.

1. The effective application of exposure assessment methods may improve the results of any epidemiologic investigation. As in any line of epidemiologic investigation, an improvement in exposure assessment can reduce bias and improve statistical power to detect adverse effects associated with exposure to environmental contaminants.
2. However, important findings may derive from environmental-epidemiologic investigation even when the exposure assessment uses only simple and crude tools to characterize the exposure of a given population. Overreliance on sampling of exposure of individuals may not be cost-effective and may limit the size of the study, with little improvement over the findings based on indirect methods.

This section reviews some of the basic concepts inherent in exposure assessment. For a more detailed discussion, the reader is referred to the NRC report Human Exposure Assessment for Airborne Pollutants (NRC, 1991b), the EPA guidelines for exposure assessment (EPA, 1992), and the Agency for Toxic Substances and Disease Registry (ATSDR) Guidance Manual (ATSDR, 1994). Epidemiological research uses various exposure metrics. The choice of a specific metric will depend on the type of study in question, the resources available to the investigator, the conceptual framework behind the investigation, and above all, biologic considerations. In deciding which exposure metric is best in a particular study,