B) the increased risk of the disease among those exposed to herbicides during service in the Republic of Vietnam during the Vietnam era; and

C) whether there exists a plausible biological mechanism or other evidence of a causal relationship between herbicide exposure and the disease.

The committee notes that, as a consequence of congressional and judicial history, both its congressional mandate and the statement of task are phrased with the target of evaluation being “association” between exposure and health outcomes, although biologic mechanism and causal relationship are also mentioned as part of the evaluation in Article C. As used technically and as thoroughly addressed in a recent report on decision-making (IOM, 2008), the criteria for causation are somewhat more stringent than those for association. The unique mandate of VAO committees to evaluate association, rather than causation, means that the approach delineated in that report is not entirely applicable here. The rigor of the evidentiary database needed to support a finding of statistical association is weaker than that to support causality; however, positive findings for any of the indicators for causality would enhance conviction that an observed statistical association was reliable. In accord with its charge, the committee examined a variety of indicators appropriate for the task, including factors commonly used to evaluate statistical associations, such as the adequacy of control for bias and confounding and the likelihood that an observed association could be explained by chance; and it assessed evidence concerning biologic plausibility derived from laboratory findings in cell-culture or animal models. The full array of indicators examined was used to categorize the strength of the evidence. In particular, associations that manifest multiple indicators were interpreted as having stronger scientific support. Table 1-1 below presents the cumulative findings through Update 2008 of VAO committees using this approach.

In delivering the charge to the current committee, VA made an additional request arising from the decision-making at VA necessitated by the findings of Update 2008. The sponsor asked that the committee, when summarizing the evidence available to support the association of a health effect with exposure to the components of the herbicides used by the military in Vietnam, address whether all the points that have rather imprecisely become known as the “Hill criteria for causality” (Hill, 1965) had been satisfied by the information available. The committee’s response to that request can be found Chapter 2 in the section “Evaluation of the Evidence.”

Chapter 2 provides details of the committee’s approach to its charge and the methods that it used in reaching conclusions.

ISSUES RAISED IN PUBLIC SESSIONS

It has been the practice of VAO committees to conduct open sessions, not only to gather additional information from people who have particular expertise



The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement