assigned to each job or work task, and time spent on each job or task is calculated. Such metrics are able to incorporate exposure mitigation factors, such as process changes, engineering controls, or the use of protective clothing. The production-worker cohort analysis conducted by the US National Institute for Occupational Safety and Health (NIOSH) is a good example of a study that has used those methods.

Many environmental-exposure studies use proximity to a contaminant source as the primary means of exposure classification. If, for example, an industrial facility emits a chemical contaminant, investigators may create geographic zones around the facility and assign exposure categories to individuals on the basis of location of residence. That approach was taken in the case of a serious industrial accident in Seveso, Italy, that contaminated nearby areas with TCDD. Assessments of this kind are often refined to include knowledge of exposure pathways (how chemicals move from the source through the environment) and personal behavior, and sometimes include measurements of chemicals in environmental samples such as soil.

Biomarkers of exposure can provide crucial information for both occupational and environmental studies, in that a quantitative exposure estimate can be assigned to each individual in the study. The most important biomarker in the context of Vietnam veterans' exposure to Agent Orange is the measurement of TCDD in serum. Studies of the absorption, distribution, and metabolism of TCDD have been conducted over the last 20 years. In the late 1980s, the Centers for Disease Control (CDC) developed a highly sensitive assay to detect TCDD in serum and demonstrated a high correlation between serum TCDD and TCDD in adipose tissue (Patterson et al., 1986, 1987). The serum TCDD assay is now used extensively to evaluate TCDD exposure in Vietnam veterans and other populations.

OCCUPATIONAL EXPOSURE TO HERBICIDES AND TCDD

The committee reviewed many epidemiologic studies of occupationally exposed groups for evidence of an association between health risks and exposure to TCDD and the herbicides used in Vietnam; primarily the phenoxy herbicides 2,4-dichlorophenoxyacetic acid (2,4-D), 2,4,5-trichlorophenoxyacetic acid (2,4,5-T) and chlorophenols. In reviewing the studies, the committee explicitly considered two types of exposure: exposure to TCDD itself and exposure to the various herbicides, particularly 2,4-D and 2,4,5-T. Separate consideration was necessary because of the possibility that, for example, some health effects may be associated with exposure to 2,4-D in agriculture and forestry. TCDD is an unwanted byproduct of 2,4,5-T production, but not of 2,4-D, although small quantities of other dioxins can be found in 2,4-D.



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