Veterans and Agent Orange Update 1998 (1999) / Chapter Skim
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4 Methodologic Considerations in Evaluating the Evidence
4 Methodologic Considerations in Evaluating the Evidence
Pages 124-134
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From page 124... ...
whether a statistical association with herbicide exposure exists, taking into account the strength of the scientific evidence and the appropriateness of the statistical and epidemiologic methods used to detect the association; 2. the increased risk of each disease among those exposed to herbicides during Vietnam service; and 3.
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From page 125... ...
Committee staff examined the reference lists of major review articles, books, and reports for relevant citations. Reference lists of individual articles were also scanned for pertinent citations.
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From page 126... ...
The committee necessarily focused on a pragmatic question: What is the nature of the relevant evidence for or against a statistical association between exposure and the health outcome? The evidentiary base that the committee found to be most helpful derived from epidemiologic studies of populations that is, investigations in which large groups of people are studied to determine the association between the occurrence of particular diseases and exposure to the substances at issue.
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From page 127... ...
if there is evidence for a positive association between exposure and a health outcome, concerns the likely magnitude of the association in Vietnam veterans exposed to herbicides. The most desirable evidence in answering this type of question involves knowledge of the rate of occurrence of the disease in those Vietnam veterans who were actually exposed to herbicides, the rate in those who were not exposed (the "background" rate of the disease in the population of Vietnam
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From page 128... ...
Indeed, most of the evidence on which the findings in this report are based comes from studies of people exposed to dioxin or herbicides in occupational and environmental settings rather than from studies of Vietnam veterans. When the available data do not permit a meaningful statement regarding risk among Vietnam veterans, no conclusion is drawn and the reader is referred to the appropriate section in an earlier report for additional discussion.
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From page 129... ...
Although studies in which transformed human cell lines are employed to study AhR biology minimize the inherent error associated with species extrapolations, caution must be exercised because the extent to which transformation itself influences toxicity outcomes has yet to be fully defined. In addition, while it is generally accepted that genetic susceptibility plays a key role in determining the adverse effects of environmental chemicals, the impact of different genetic backgrounds on AhR function is not yet completely understood.
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From page 130... ...
For instance, the analytical data available from individuals known to have been exposed to herbicides during the Vietnam War constitute a valuable resource for the study of TCDD-related disease, with documented TCDD body burdens providing a quantitative bridge between experimental studies and human epidemiology. Taken together, experimental studies and epidemiologic investigations provide complementary perspectives from which to view human health effects of exposure to herbicides.
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From page 131... ...
In general, however, for reports of overall associations with exposure, the committee did not consider the risk of publication bias to be high among studies of herbicide exposure and health risks. The committee took this position because there are numerous published studies showing no positive association; because it examined a substantial amount of unpublished material; and because the committee felt that publicity surrounding the issue of exposure to herbicides, particularly regarding Vietnam veterans, has been so intense that any studies showing no association would be unlikely to be viewed as unimportant by the investigators.
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From page 132... ...
A1though there is no evidence that exposure to herbicides is related to bladder cancer, relative risks in largest groups of exposed individuals under study tended to be greater than 1. This new information led the committee to change the classification of bladder cancer from limited/suggestive evidence of no association to inadequate or insufficient evidence to determine whether an association exists.
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From page 133... ...
For example, if studies fail to control for confounding, contain inadequate exposure assessment, or have inadequate sample size, this may constitute inadequate/ insufficient evidence to determine whether an association exists. · Limited/Suggestive Evidence of No Association There are several adequate studies, covering the full range of exposure levels that humans are known to encounter, that are mutually consistent in not showing a positive association between exposure to herbicides and the outcome at any level of exposure.
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From page 134... ...
1997. Characterizing Exposure of Veterans to Agent Orange and Other Herbicides Used in Vietnam: Scientific Considerations Regarding a Request for Proposals for Research.
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