chronic diseases may not be expressed until older ages. If diseases occurring in older members of the population can be linked to identified episodes of past pollution, they could provide information needed to prevent future disease in those who are now young, but such links are hard to discern in a mobile population.
For many chronic diseases, the time of first diagnosis may not be important. Rather, a measure of cumulative incidence (approximated by prevalence in nonfatal conditions) could be just as informative, thus increasing the usefulness of data that may mark the presence of chronic disease but not the date of diagnosis.
The Agency for Toxic Substances and Disease Registry was created by Congress by the Comprehensive Environmental Response, Compensation, and Liability Act of 1980 (CERCLA) to address possible public-health effects of environmental exposures to hazardous substances from waste sites and chemical spills (NRC, 1991b). CERCLA requires ATSDR, in cooperation with the states, to establish national registries of persons who have been exposed to hazardous substances and later develop serious disease or illness (ATSDR, 1988a). While disease registries have not yet been established, the National Exposure Registry is further developed and is the focus of this discussion.
The stated purpose of ATSDR's Exposure Registry is ''to aid in assessing long-term health consequences of exposure to Superfund-related hazardous substances" (ATSDR, 1988a, p. 7). To facilitate epidemiologic research, ATSDR intends to design and create its data systems for both hypothesis generation (identifying possible adverse health outcomes) and hypothesis testing (of suspected adverse health outcomes). Other goals are to facilitate state and federal health-surveillance programs and to provide information that can be used to assess the effects of an exposure on a population.
ATSDR's data system will contain subregistries created in 4 phases. First, it narrows down potential sites for inclusion to a workable number using criteria similar to those presented in table 5-7. Second, site files are requested from EPA, the US Geological Survey, and agency personnel associated with a remediation project. At this time, additional secondary criteria are evaluated, including assessment of participation, existing biomonitoring data, number of secondary or potential confounding contaminants, and reported health problems. During the third phase, site visits are conducted with local and state departments of health and environment and other interested officials. Affected neighborhoods are inspected, and special characteristics, including susceptible or transient