Finally, as its name—the Agency for Toxic Substances and Disease Registry—implies, Congress intended that the Public Health Service establish registries of populations exposed to hazardous wastes and follow these populations over time to observe associated health effects. Because of the uncertain end points and utility and great expense of starting and maintaining site-specific, open-ended registries, ATSDR has resisted congressional and community pressures to establish them. Instead, the agency has developed specialized registries to study the long-term health effects of exposure to specific chemicals at hazardous-waste sites, with the intention of combining data from several sites where similar exposures have occurred to achieve populations large enough that the associated health effects can be detected.

Other federal statutes have established programs that are much more limited in their capacity to evaluate and control exposures at active and abandoned sites. Although Resource Conservation and Recovery Act (RCRA) amendments authorized ATSDR to conduct health assessments at the request of EPA at landfills and surface impoundments, many of these sites are still held by EPA from entry into the CERCLIS system and will not reach ATSDR's attention for some time. The Safe Drinking Water Act and the Clean Air Act mandate monitoring of a limited set of priority chemicals, but they are not intended to identify or track specific sources of pollution, such as hazardous-waste sites, and are therefore of limited use in characterizing the releases from such sites. In particular, the Safe Drinking Water Act does not include domestic wells, which are frequently the pathway of exposure of greatest concern at NPL sites. Although routine monitoring of drinking water and air quality are state responsibilities, state direction of this monitoring to assess potential emissions from hazardous-waste sites is made even less likely by the fact that EPA's regional offices do not notify the relevant state agencies when off-site migration is suspected or confirmed.

DISTRIBUTION OF EXPOSURE

How well does this system characterize the distribution of human exposures to chemicals released from hazardous-waste sites? For the purposes of environmental epidemiology, this question has two parts: For each site, how well are the potential human exposures characterized in terms of the contamination of media; the identification of onsite human contact, environmental pathways, and off-site migration; of off-site human contact with contaminated media; of populations at risk; and of indicators of human exposure? For hazardous-waste



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