cal and physical agents from hazardous-waste sites and adverse consequences to human health, obtaining valid measures or estimates of exposure is essential.

The field of exposure assessment entails numerous techniques to measure or estimate the contaminant, its source, the environmental media of exposure, avenues of transport through each medium, chemical and physical transformations, routes of entry to the body, intensity and frequency of contact, and its spatial and temporal concentration patterns. It also includes estimations of total exposure to different compounds and mixtures. Exposure assessment has proved difficult, because epidemiologic research typically involves retrospective studies. Records of ambient pollutant concentrations can sometimes provide a surrogate for exposure, but these surrogates are not always available, and direct measures of past exposures have not usually been recorded and must be estimated with models.

Within the past decade, estimates of the number of potential NPL sites have grown dramatically. OTA concludes that there could be as many as 439,000 candidate sites. These sites include mining waste sites, leaking underground storage tanks, pesticide-contaminated sites, federal facilities, radioactive release sites, underground injection wells, municipal gas facilities, and wood-preserving plants, among others. As of December 1988, one ATSDR report concluded, 109 NPL sites (11 percent of the total) were associated with a risk to human health because of actual exposure (11 sites) or probable exposure (98 sites) to hazardous chemical agents that could cause harm to human health. Chiefly on the basis of exposure assessments, these sites were placed in the categories of “urgent public health concern ” or “public health concern.”

Repositories of potentially dangerous substances can be found at a number of hazardous-waste sites that have been generated by a wide range of activities. Information about the materials generally reflects the data requirements of environmental engineering and site remediation, rather than public health considerations. Accordingly, whether the materials pose a risk to public health cannot readily be determined in the absence of more detailed information about potential human exposures.

The focus of many studies has been on site-specific characterization, even though pollutants do not respect such boundaries. Given the potential for movement of materials in groundwater and air and the importance of multiple routes of exposure, efforts need to proceed to estimate plume characteristics and groundwater staging to improve the ability to anticipate the movement of pollutants and ultimately to prevent greater exposures. Similarly, exposure from domestic water is not limited to ingestion, but includes airborne ex-