The present report follows this model, summarizing the content of the two previous reports and providing detailed reviews of the most recent research.
In conducting these studies, the committee operated independently of the DVA and other government agencies. The committee was not asked to and did not make judgments regarding specific cases in which individual Vietnam veterans have claimed injury from herbicide exposure; this was not part of its congressional charge. Rather, the studies provide scientific information for the Secretary of Veterans Affairs to consider as the DVA exercises its responsibilities to Vietnam veterans. Update 1996 contains a summary of the literature addressed in VAO and in-depth reviews of the scientific studies and other information developed during the intervening time. The present report follows this model, summarizing the content of the two previous reports and providing detailed reviews of the most recent research.
In fulfilling its charge of judging whether each of a set of human health effects is associated with exposure to herbicides or dioxin, the committee concentrated primarily on reviewing and interpreting epidemiologic studies. The committee began its evaluation presuming neither the presence nor the absence of association. It sought to characterize and weigh the strengths and limitations of the available evidence. These judgments have both quantitative and qualitative aspects. They reflect the nature of the exposures, health outcomes, and populations exposed; the characteristics of the evidence examined; and the approach taken to evaluate this evidence. To facilitate independent assessment of the committee's conclusions, Chapter 5 of VAO describes as explicitly as possible the methodological considerations that guided the original committee's review and its process of evaluation. This methodology was subsequently adopted by successor committees. It is summarized in Chapter 4 of this report.
In reviewing the literature, the committee found that the existing epidemiologic data base is severely lacking in quantitative measures of individual exposure to herbicides and dioxin. Assessment of the intensity and duration of individual exposures is a key component in determining whether specific health outcomes are associated with exposure to dioxin or other chemicals found in the herbicides used in Vietnam. Although different approaches have been employed to estimate exposure in Vietnam veterans and others exposed occupationally or environmentally, each approach is limited in its ability to determine precisely the degree and level of individual exposure. The available quantitative and qualitative evidence about herbicide exposure, summarized in Chapter 5, suggests that Vietnam veterans as a group had substantially lower exposure to herbicides and dioxin than the subjects in many occupational studies. Participants in Operation Ranch Hand are a known exception to this pattern, and it is likely that others among the approximately 3 million men and woman who served in Vietnam were exposed to herbicides at levels associated with health effects. Thus, in the committee's judgment, a sufficiently large range of exposures may exist among Vietnam veterans to conduct a valid epidemiologic study for certain health outcomes.