The Utility of Proximity-Based Herbicide Exposure Assessment in Epidemiologic Studies of Vietnam Veterans (2008) / Chapter Skim
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about the best use of this herbicide exposure assessment model. The committee concluded that the model's approach of using an exposure surrogate based on individuals' or military units' proximity in space and time to herbicide spray paths is a reasonable one.
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Despite the shortcomings of the exposure assessment model in its current form and the inherent limitations in the approach, the committee agreed that the model holds promise for supporting informative epidemiologic studies of herbicides and health among Vietnam veterans and that it should be used to conduct studies. The committee offers criteria that VA should draw on as a basis for developing a request for proposals, and it recommends that VA work with the Department of Defense and the National Archives and Records Administration to facilitate access to and interpreta tion of military records for use in the studies.
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The committee found that assignment of exposure based on proximity to herbicide spraying offers the possibility of important improvement in the classification of exposure over most earlier approaches, although there are inherent limitations in the model. Using the model to assess herbicide exposure in epidemiologic studies may permit observation of associations between herbicide exposure and health effects in the Vietnam veteran population that were not identifiable in previous studies.
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VA requested that the committee consider several factors: the relevant IOM recommendations for evaluating such a model; approaches for evaluating the exposure model using existing data on health outcomes associated with herbicide or dioxin exposure among Vietnam veterans; the availability, quality, and usefulness of existing information on Vietnam veterans, including data on troop locations and health outcomes for diseases commonly associated with herbicide exposure as well as those not currently linked to such exposures; and how such information might be used in epidemiologic studies using the exposure assessment model. The committee was also asked to consider the role of epidemiologic studies of Vietnam veterans conducted using the new model in informing the IOM's biennial evaluation of evidence on the association between herbicide exposure and health outcomes.
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and the Herbicide Exposure Assessment–Vietnam (HEA-V) , which is a computerized database engine that facilitates the linking of disparate geo referenced data as well as the calculation of two exposure opportunity metrics at the spatial scale of approximately 1 square kilometer.
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Fourth, with the nature of the model and its metrics clearly in mind, the committee considered the potential contributions and pitfalls of using it in epidemiologic studies. Of particular interest in these deliberations were the potential to study Vietnam veterans directly, the degree to which exposure classification might be improved if the model were to be used, and the appropriate interpretation of the results of any such studies.
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Infrastructure for Proximity-Based Exposure Measures The committee concluded that the Stellman team's approach of using an exposure surrogate based on individuals' or military units' proximity in space and time to herbicide spray paths is a reasonable exposure assessment strategy. This approach is an important improvement over the cruder exposure classification based on service in Vietnam that has been used in many past studies of the health of Vietnam veterans.
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In addition, the committee concluded that it is not feasible to validate the exposure scores produced by the Stellman team's model -- or any other proximity-based model -- by comparisons with biomarkers or soil samples because of the passage of time and the unavailability of archived environmental or biological samples. AVAILABILITY AND ACCESSIBILITY OF DATA ON VETERANS The Stellman team's herbicide exposure assessment model starts from a data infrastructure that focuses on the timing, location, and content of herbicide spraying in Vietnam and the software tools for calculating exposure metrics.
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By contrast, the model is designed to assess the exposure opportunity that would result from unintended proximity to herbicide spraying. EPIDEMIOLOGIC STUDIES USING THE HERBICIDE EXPOSURE ASSESSMENT MODEL Despite recognizing shortcomings in the exposure assessment model in its current form and inherent limitations in the proximity-based approach, the committee concluded that the assessment model holds promise for contributing to informative epidemiologic studies of herbicides and health among Vietnam veterans and that it should be used to conduct such studies.
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Studies of veterans based on the exposure assessment model should include analyses to assess how sensitive an estimated association between the exposure opportunity metric and specific health outcomes is to different parameters and sources of uncertainty in the exposure assessment measure. Sensitivity analyses that summarize the variability in the model's exposure metrics under different assumptions underlying the exposure opportunity model will provide ranges and distributions of exposure opportunity metrics (hits or EOI)
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S pecifically, to make the best use of the exposure assessment model, epidemiologic studies of Vietnam veterans should have the follow ing characteristics: a. The study population should be broadly representative of Viet nam veterans, with care taken to include sufficient numbers of study participants with relatively higher exposure.
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The committee specifically urges attention to the effects of potential inaccuracies in the data on the location of herbicide application or troop presence. It is also important to investigate, especially with any attempt to add refinements to the existing model, the effect of assumptions on factors such as spray swath, the concentration of the TCDD contamination, primary and secondary drift, soil conditions, initial and remaining canopy, and photodegradation of sprayed herbicide.
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