The NRC report (1991) on human exposure assessment for air-borne pollutants, which reviews progress in addressing total human exposure, is particularly valuable for pursuing those objectives. The report describes the framework and specific methods for exposure assessment. It recommends that scientists and regulators consistently use its definitions of exposure and exposure assessment to ensure standardization across disciplines. This approach has special significance for studies of possible adverse health outcomes associated with hazardous-waste sites because of the potential for multiple chemical exposure, the wide range of pathways for transport of contaminants, and the complex temporal characteristics of exposure.

The NRC report (1991) summarized the requisite entities to be determined in exposure assessment:

  • concentration distributions in time and space for different environmental media;

  • populations or groups at high and low risk;

  • chemical and physical contributions of various sources;

  • factors that control contaminant release into environmental media, routes of environmental transport, and routes of entry into humans.

ROUTES OF EXPOSURE

In general, all routes of exposure and all environmental media should be assessed to determine their relative contribution to the overall exposure associated with a waste site. Such work has been done, but generally not in the context of epidemiologic investigation. Likely media of exposure from hazardous-waste sites include air, water, food, and soil (Figure 3-4). Exposure to toxic chemicals would most likely occur through contaminated groundwater that has leached or run off from waste sites to enter the drinking water supply. Other sources of exposure include direct contact with contaminated sediment; accidental ingestion of contaminated soil or surface water; release of volatile agents into the air; and ingestion of contaminated vegetables, fruit, meat, poultry, or fish.

Exposure to contaminated water can derive from showering or bathing, from drinking water, and from using water in food preparation. Those routes of exposure have received considerable attention as a result of EPA's Total Exposure Assessment Methodology (TEAM) studies (Wallace et al., 1986, 1987, 1988), the work of Andelman et al. (1986, 1990), and the work of Lioy and co-workers (Jo et al., 1990a,b). The results of the investigations illustrate the importance of indoor



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