The locations covered by most pollutant-concentration data systems are chosen to be characteristic rather than representative (table 5-3 ). Thus, most of the National Air Monitoring Stations or water-system quality sites of the National Stream Quality Accounting Network are in densely populated areas. These data systems contain detailed information on the location of the monitoring site, and samples are collected frequently enough to represent short periods. However, site selection is not based on detailed information about the population, the area, or the distribution of exposures among individuals, and the positioning of a station does not necessarily reflect the most likely route of human exposure. For instance, some air-monitoring stations are on the tops of buildings, and water-quality assessments are performed at the outflow pipes of water-treatment facilities, not at residential taps. Those locations might yield informative data on relative exposures, but may not represent either the distribution of concentrations in the environment or the actual exposures of people.
Pollutant-concentration data systems are probably underused for ecologic studies. These systems contain detailed geographic data, and, although few pollutants may be assessed, the analytic methods tend to be relatively stable over time, and exposure is generally measured at or integrated over short intervals.
Although most data on pollutant concentration are from monitoring systems, data from "response epidemiologic studies" are increasing. Response (or applied) epidemiologic studies are designed to respond quickly to expressed concerns regarding the potential for exposure or adverse health effects. Examples of response epidemiologic programs are the health-assessment studies of the Agency for Toxic Substances and Disease Registry (ATSDR) and the health-hazard evaluations of the National Institute for Occupational Safety and Health (NIOSH). In these studies, environmental concentrations of various pollutants are regularly assessed. However, study sites are often selected because the potential exposure is considered high or because of complaints about symptoms, so sites are not characteristic of ordinary population exposures. These studies do, however, attempt to characterize explicitly the scope or potential for human exposure in these presumably extreme settings, and they may contribute information on the relation between the environmental distribution of pollutants and human exposure or internal dose.
Data on human exposure (table 5-4) are the least developed of the classes considered here, and generalizations to larger groups of people or