ground, where they eventually entered the subsurface water resources that the community used for its domestic water. Measurements of the concentrations in the air and water can help public health practitioners determine the total possible exposure.

The characterization of exposure in the community requires an understanding of all of the potential pathways by which pollutant releases may result in exposure. This includes direct pathways such as through the air and drinking water and less obvious pathways such as through uptake by food sources. Studies of this nature need to be sensitive to the ways in which differences in behavior, employment, and lifestyles among subgroups in the population may result in differences in exposure. For example, among the Alutiiq, Yup'ik, and Inupiat Alaskan Native peoples, the yearly intake of wild foods per person is between 171 and 272 kilograms (375 and 600 pounds). Increasing evidence of certain contaminants such as mercury in the wild food supply of these Alaskan Natives has been exhibited by methyl mercury levels that exceed those provisionally established as safe by the World Health Organization. Research that did not account for the variable ingestion of contaminated foodstuffs would fail in determining the relative effects of exposures across populations.

As the focus of analysis moves from sources and releases to environmental concentrations, the quality and quantity of the data currently available decrease significantly. Of the more than 60,000 chemicals in commercial use in the United States, data on the concentrations of chemicals in the environment are available for only a small number (National Research Council, 1984; Roe et al., 1997). One systemic gap is measures of indoor concentrations, even though the U.S. population spends much of its time indoors (Environmental Protection Agency, 1993); most monitoring systems focus on the concentrations outdoors or in the workplace.

A more accurate description of the amount of toxicants absorbed requires data on actual doses received by humans. Analysis of biological samples (e.g., urine or blood samples) from affected populations can lead to more accurate descriptions of exposure to an environmental health hazard (National Research Council, 1989b), but the samples can be difficult to obtain. Methods for tissue analysis are also limited.

The environmental toxicant for which the largest amount of data is available is lead (National Research Council, 1993). It causes a variety of health problems, including neurodevelopmental effects in infants and children and cardiovascular effects in adults. Exposure to lead can be determined by measuring the level of lead in blood. Harmful health effects have been identified for levels of lead in the blood that are greater than or equal to 10 µg/dl. There are multiple sources of lead exposure, with lead-based paint being the most common. Consequently, exposure is high in urban areas, where housing is often old and likely to be coated with lead-based paint. Blood lead levels are consistently higher for poor and minority children and for residents of central cities (Brody et al., 1994; Centers for Disease Control and Prevention, 1991, 1997).

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