the data were collected; the inclusion criteria; and how exposure was determined. The tables also list the numbers of subjects in the study and comparison populations, and provide a brief description of the study. No studies are evaluated in this chapter; rather, a methodologic framework is provided for the health outcome chapters that follow. Qualitative critique of the study design, population size, methods of data collection, case and control ascertainment, or quality of exposure assessment has been reserved for the individual health outcome chapters in which the results of these studies are discussed.

The text and tables in this chapter are organized into three basic sections— occupational studies, environmental studies, and studies of Vietnam veterans— with subsections included under each heading. The studies address exposures to 2,4-D (2,4-dichlorophenoxyacetic acid); 2,4,5-T (2,4,5-trichlorophenoxyacetic acid) and its contaminant TCDD; cacodylic acid; and picloram. In some cases, the committee examined studies addressing compounds chemically related to the herbicides used in Vietnam, such as 2-methyl-4-chlorophenoxyacetic acid (MCPA), hexachlorophene, and chlorophenols, including trichlorophenol. In other instances, investigators did not indicate specific herbicides to which study participants were exposed or the level of exposure. These complicating factors were considered when the committee weighed the relevance of a study to its findings. Where available, details are given with regard to exposure assessment and how exposure was subsequently used in the analysis.

The occupational section includes studies of production workers, agricultural and forestry workers (including herbicide and pesticide appliers), and paper and pulp workers, as well as case-control studies of specific cancers and the association with exposures to herbicides or related compounds. The environmental section includes studies of populations accidentally exposed to unusual levels of herbicides or dioxins as a result of the location in which they live, for example, the residents of Seveso, Italy; Times Beach, Missouri; and the southern portion of Vietnam. The section on Vietnam veterans includes studies conducted in the United States by the Air Force; the Centers for Disease Control and Prevention (CDC), the Department of Veterans Affairs (DVA, formerly the Veterans Administration [VA]); the American Legion; and the State of Michigan, as well as other groups. Studies of Australian Vietnam veterans are also presented there.

Many cohorts potentially exposed to dioxin and the herbicides used in Vietnam are monitored on an ongoing basis. Studies of the groups that are assessed regularly include the National Institute for Occupational Safety and Health (NIOSH), International Agency for Research on Cancer (IARC), National Cancer Institute (NCI), Seveso, and Ranch Hand cohorts. Typically, the risks between exposure to herbicides and specific health outcomes are updated every 3 to 5 years. For example, the health of the Ranch Hand cohort was assessed in 1982, 1987, 1992, and 1997. For such studies, the committee has chosen to focus on the most recent update, when multiple reports on the same cohort are available. For the sake of thoroughness, the discussion of specific

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