This should include sampling of tap water, estimating outgassing from showers as relevant, absorption through cooking or eating, and other residential uses, and surface soil. Epidemiology is not merely a passive science, cataloguing exposures and effects. It is an active tool for evaluating the link between potentially hazardous exposures and disease and for directing interventions to prevent further exposures. Where the evaluation of human exposures and health effects associated with hazardous-waste sites is not integrated into early site evaluation and interim remediation decisions, the real contributions of epidemiology are lost.

This is of grave concern because hazardous wastes have constituted a significant public health hazard to specific populations at specific sites, summarized in Table 1-1. Further, in 1988, ATSDR reported that conditions at about 11 percent of all 951 NPL sites constituted ongoing or probable public health concerns. During 1990, 32 percent of a specially reviewed subgroup of sites were identified as requiring some kind of protective health action. The health of the public has remained in jeopardy at many sites long after the risks could have—and should have—been identified.

As Chapter 3 reports, repositories of potentially dangerous substances can be found at a number of hazardous-waste sites. Dangerous substances also have been generated by leaking underground storage tanks, non-point sources such as agricultural pesticide runoff, automobile emissions, agricultural, mining, storage, and other activities. Information about these materials generally reflects the data requirements of environmental engineering and site remediation, rather than public health considerations. Accordingly, whether these materials pose a risk to public health cannot readily be determined in the absence of more detailed information about potential human exposures.

Improvements must be made in the ability to assess the more than 600 chemical compounds identified at hazardous-waste sites, along with the hundreds or thousands of unidentified pollutants that could enter the groundwater. The potential for exposure is of such magnitude that researchers who develop exposure assessment strategies will have to direct their attention not solely to an analysis of the contaminants at a hazardous-waste site, but to off-site migration and public exposure as well. In this context, measures of personal exposure, including the use of biologic markers and biologic monitoring as discussed in Chapter 7, along with personal sampling, although often difficult and time consuming, must receive greater scientific attention if appropriate associations are to be made between contaminants, exposures, and health effects.

Enough hazardous-waste sites have been identified where the popu-



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