To conduct an epidemiologic study of the effects of a local exposure source on a residential population, one needs to know the demographics of the population at risk, the extent of the exposure of concern, and information about other risk factors for disease, such as occupation, diet, and socioeconomic status. If the population is large enough and if a sufficient number of people are exposed to the disease agent, as defined by statistical criteria, then these data can be used in a traditional epidemiologic design (e.g., a cohort, case-control, or cross-sectional study). These epidemiologic studies help determine whether those people with higher exposures are or were more likely to develop disease when the effects of other risk factors for disease are removed or adjusted for. That is, one can compare residents with disease to those without disease in terms of exposure while making adjustments (either statistically or in the design) for other risk or lifestyle factors.
However, in most minority and low-income communities, such data are not readily available (Environmental Protection Agency, 1992; Sexton et al., 1993). As noted in an Environmental Protection Agency report on this issue, ''Environmental and health data are not routinely collected and analyzed by income and race" (Environmental Protection Agency, 1992, p. 1). When such data are collected, they are not always available to researchers at the community or neighborhood level. For example, for some cancer incidence studies, states have been willing to provide municipal-level data and, in special circumstances, individual-level data. For studies in other states, data have been limited to the county level. Furthermore, data on lifestyle and behavior are not generally available except for regional data based on statistical sample surveys (e.g., the Centers for Disease Control and Prevention's Behavioral Risk Factor Surveillance System). Therefore, to undertake epidemiologically reliable studies of these communities, substantial data collection efforts would be required, but data collection is an extremely costly and resource-intensive enterprise.
In addition, many of the minority and low-income communities with environmental justice concerns are extremely small, in epidemiologic terms. Studies of small populations may not have adequate statistical power to detect a significant effect even if one exists. For example, Zimmerman reports that communities with at least 2,500 residents and at least one inactive hazardous-waste site on the National Priorities List (NPL) have a 1990 median population of less than 18,000 people (Zimmerman, 1993). To appreciate the limitations of studying a population this small or smaller, consider the following hypothetical calculations. For a cancer with an incidence rate of 1 in 30,000 people per year, the typical rate of childhood leukemia, one would expect to see fewer than one case per year in over half of the communities with NPL sites. Even if 5 years of