These systems provide broad-scale information on specified populations for which epidemiologic analyses can be conducted. Surveillance systems are generally designed to attain complete or nearly complete coverage of every identified instance of certain defined conditions in a defined population. Thus, they can be used to estimate the background incidence and prevalence of adverse effects, and trends can be analyzed across time and between populations or geographic areas.
Surveillance systems can identify increases or decreases in the occurrence of deaths from specific diseases and thus suggest or test hypotheses related to environmental exposures. For example, observations of a decline in age-adjusted stomach-cancer rates over time in the United States have stimulated the development of hypotheses about changes in dietary habits in the population as a whole, as well as about changes in the use of food preservatives and refrigeration (Howson et al., 1986) that might explain these trends. Similarly, after postmenopausal estrogen use fell in the United States, rates of endometrial carcinoma declined in women over age 65, lending support to an inference drawn from case-control studies that postmenopausal estrogen use increased the risk of endometrial cancer (Austin and Roe, 1982). In another instance, surveillance data from the National Center for Health Statistics suggested that a fall in blood-lead levels in US children was linked to a drop in gasoline-lead levels (Annest et al., 1983).
As public-health agencies have expanded the scope of surveillance systems (see chapter 5), it has become feasible to study the relationships between disease patterns and variations in environmental factors. Surveillance systems are expected to become increasingly common because the quality of their data is rising, statistical methods are improving, and costs are declining. The Agency for Toxic Substances and Disease Registry (ATSDR) has devised several surveillance systems to monitor the health of persons believed to have incurred exposure to such substances as trichloroethylene and dioxin. No results are yet available from those systems. If these exposure registries are to produce valuable results, they will need to include sufficient numbers of persons over a long enough period for diseases of interest to manifest themselves in numbers sufficient to demonstrate that some problem exists or that the problem is unlikely to exist and be large enough to cause serious concern.
Ecologic studies explore the statistical connection between disease and estimated exposures in population groups rather than individuals. They combine data from vital records, hospital discharges, or disease registries