Second National Health and Nutrition Examination Survey was later identified as a 37% decrease in the population mean blood lead level as a result of a decrease in the use of lead in gasoline (Annest et al., 1983). This 37% decrease nationwide was used to justify the elimination of lead from gasoline.

Detecting the effects of monitored changes in disease and exposure also often depends on the alertness of researchers. The 2.5-fold increase in mortality in the London smog episode in 1952 became apparent when an investigator compiled weekly mortality data. Similarly, hospital admissions for asthma in children were cut in half in a Utah valley when a steel mill closed down, and admissions returned to their previous level when it reopened; this was also not detected by clinicians or the state health department but required the examination of hospitalization data by an investigator (Pope, 1991).

Developing Relevant Exposure Gradients

Exposures to synthetic organic chemicals and other modern products cannot be accurately segregated by source, such as air, water, or soil. Rather, modern exposure scenarios often model multimedia, multi-temporal levels of many complex chemical compounds. Further, physical and biologic characteristics of other environmental factors can influence uptake and total dose of chemicals. Thus, heat, meteorologic conditions, humidity, and particle size affect the extent of uptake of airborne contaminants, and water hardness, pH, acidity, volatility of contaminants, and other natural background factors affect exposure to materials in water.

Much work in the past has relied on assumed dichotomous, yes-no exposures, but it is not always possible to find and study populations that are entirely unexposed to some environmental agents. Epidemiologists must work closely with exposure analysts to generate meaningful gradients of exposures for such populations, including the use of models to improve exposure estimates. These models need to include environmental and biologic fate, population activity patterns, biomonitoring, and biomarkers. Wherever possible, models should be validated by monitoring carefully selected subsamples of the population under study. This will allow more-refined estimates of individual exposure to be used in population-based studies, as is discussed in chapters 3 and 6.

The Role of Public-Health Departments in Environmental-Epidemiology Research

Many issues in environmental epidemiology are in the domain of departments of public health. A group of residents near a hazardous-waste

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