The Utility of Proximity-Based Herbicide Exposure Assessment in Epidemiologic Studies of Vietnam Veterans (2008) / Chapter Skim
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5 Recommendations Regarding Epidemiologic Studies Using the Exposure Assessment Model
5 Recommendations Regarding Epidemiologic Studies Using the Exposure Assessment Model
Pages 84-112
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From page 84... ...
The exposure assessment model is applicable to the population of ultimate interest, namely Vietnam veterans; and (2) most previous studies of this population have been so severely limited with respect to exposure assessment that a more accurate yet imperfect method would advance the current understanding of whether herbicide exposure is associated with adverse health outcomes among Vietnam vet 84
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From page 85... ...
", but rather the narrow one, "Did herbicide exposure affect the health of Vietnam veterans? " Because the herbicide exposures experienced by most Vietnam veterans are imperfectly characterized and presumably low compared with occupationally exposed populations, studies on these veterans can make only limited contributions to answering global questions regarding herbicides and health.
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From page 86... ...
Although the actual proportion of Vietnam veterans who had potential for meaningful herbicide exposure is not known, given the temporal and geographic distribution of spraying, it seems likely that exposure was not universal or equal for all those who served. For instance, the Ranch Hand and Army Chemical Corps veterans, who handled the herbicides and presumably had higher exposures on average than other troops, can be studied with confidence as being exposed populations.
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From page 87... ...
The exposure assessment hierarchy discussed in Chapter 3 helps to illustrate both the potential contributions and the limitations of the model's exposure metrics. It is very likely that exposure metrics based on proximity to herbicide spraying provide a better estimate of exposure than does simple presence in Vietnam during certain years.
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From page 88... ...
into an exposure opportunity score, even with measurement error, there are further gains in accuracy and statistical power. We expect that the exposure opportunity score, even if imprecise, will better identify the highly exposed individuals, improving the specificity of the exposure assignment and therefore reducing the exposure misclassification bias.
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From page 89... ...
As a result, service members may have herbicide exposures that cannot be captured by the model's exposure metrics. Another consideration is that many of the herbicides used in Vietnam, or products similar to them, were also in widespread use at lower concentrations in the United States beginning before the Vietnam War, so veterans may have had some exposure to them and any TCDD contaminants before or after their military service.
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From page 90... ...
Even with improvements in exposure assignment, their experience is less readily documented than that of some other groups that have already been studied. On the other hand, where the current body of research on specific herbicides and health outcomes is less extensive -- which is often the case for health outcomes other than cancer -- the application of the Stellman team's exposure assessment model to Vietnam veterans does hold potential for advancing the larger body of knowledge concerning herbicides and health.
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From page 91... ...
GENERAL CONSIDERATIONS FOR CONDUCTING STUDIES To make the best use of the Stellman team's exposure assessment model and produce meaningful epidemiologic findings for Vietnam veterans, several issues need to be considered that bear on the feasibility of studies and the information value of their results. Representativeness of Vietnam Veterans The committee has assumed that the main purpose of conducting studies using the exposure assessment model is to learn more about if and how Vietnam veterans' health was affected by herbicide spraying.
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From page 92... ...
Breast cancer is also in this category, but studying it poses special challenges because of the small number of women who served in Vietnam. In selecting health outcomes for study, researchers may want to take into account the ability of the Stellman team's exposure assessment software to generate separate exposure metrics for each of the herbicides used in Vietnam, including the arsenical cacodylic acid of Agent Blue and picloram in Agent White. The biennial reviews carried out by IOM contain careful reviews of the toxicologic and epidemiologic literature on the herbicides and can offer a source of guidance on outcomes of potential interest.
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From page 93... ...
Completeness and Timeliness of Health Data Studying the relationship between herbicide exposure in Vietnam and health outcomes that are often not fatal, such as diabetes or prostate cancer, might be strengthened by using data on morbidity in addition to mortality data. In these cases it is important to determine health outcomes accurately and comprehensively both for the sake of precision (to maximize the n umber of cases)
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From page 94... ...
Confounding Another major consideration in designing an epidemiologic study to evaluate possible adverse health effects of herbicide exposures in Vietnam veterans is the need to control for potential confounders, such as tobacco, alcohol, or drug consumption. Some Vietnam veterans, particularly among those with posttraumatic stress disorder, are likely to have had high rates of alcohol or drug use (e.g., McFall et al., 1992)
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From page 95... ...
Although a "gold standard" measure of exposure is highly desirable for estimating the bias in a relative risk estimate due to exposure measurement error, it is not necessary for obtaining a plausible range of relative risk estimates associated with different exposure measures obtained from the model's sensitivity analysis. The examination of this range of relative risk estimates will increase the likelihood of identifying associations of potential importance, and it will also help to reflect more accurately the uncertainty inherent in all of the indices of herbicide exposure.
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From page 96... ...
for a 3-year project to support work on -- and to facilitate researchers' access to -- the exposure assessment model and the data on herbicide spraying and unit and troop locations that have been assembled to date (Stellman, 2007a)
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From page 97... ...
for each grid location, as the Stellman team plans to do, would go far toward making the workings of the herbicide exposure assessment model reproducible by other researchers. Similarly, their plans to make publicly available the compiled, cleaned, and checked databases of unit locations already developed would facilitate the work of future researchers.
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From page 98... ...
Even a retrospective study, though likely to be less expensive than a prospective study, will still be expensive because of the need to conduct extensive manual searching to obtain the necessary troop assignment information for linkage with the Stellman team's exposure assessment model. Because of the costs that are likely to be associated with conducting an entirely new cohort study, the most practical approach may be to start with study groups that have already been assembled.
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From page 99... ...
The committee is aware of the following potential study populations that should be considered for studies using the Stellman team's model. VA Study Populations and Registries One source of frustration for researchers in the years following the Vietnam War was the lack of a comprehensive list of U.S.
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From page 100... ...
. The investigators were able to establish whether the study subjects had served in Vietnam and to gather information on dates of service, military occupational specialty codes, and unit identifications for each assignment by reviewing military service records at the National Personnel Records Center in St.
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From page 101... ...
If so, this combination would introduce a bias into any study of the health effects associated with herbicide exposure that was performed on a population selected from this registry. The CDC Agent Orange Study In the 1980s, the Centers for Disease Control and Prevention (CDC)
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From page 102... ...
The committee notes that the Air Force Ranch Hand personnel and Army Chemical Corps personnel are two groups of veterans that are not suitable study populations when the Stellman team's model is to be used, even though their exposures to herbicides are likely to be among the highest of all veterans. Most of their herbicide exposures were a direct result of duties that required handling or applying herbicides.
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From page 103... ...
veterans, service in Vietnam has often constituted the exposure criterion in Australian studies. Thus Australian veterans would seem to present another opportunity for application of the Stellman team's exposure assessment model.
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From page 104... ...
ONGOING VA WORK TO APPLY THE EXPOSURE ASSESSMENT MODEL Dr. Han Kang and colleagues at VA have several projects under way to explore use of the Stellman team's model (VA, 2007)
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From page 105... ...
How much time and resources are required to conduct a health study of Vietnam veterans using the model? The model will be applied to three existing databases: the Agent Orange Registry of 409,000 Vietnam veterans, Marine Corps Vietnam Veterans, and Women Vietnam Veterans.
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From page 106... ...
CONCLUSIONS The committee reached the following specific conclusions regarding the use of the Stellman team's herbicide exposure assessment model in epidemiologic studies: 1. Epidemiologic studies of health outcomes among Vietnam veterans that use the exposure opportunity index are capable of characterizing the health of veterans in relation to their proximity to herbicide spraying.
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From page 107... ...
If positive, such studies would add support to the model's potential value, but, if negative, the model's value would not be disproved because the levels of exposure may be lower than would cause adverse health effects or the study's power may be insufficient to address the question adequately. RECOMMENDATIONS The committee's conclusions from its consideration of the Stellman team's herbicide exposure assessment model led it to make the following recommendations: 1.
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From page 108... ...
1985. Exposure assessment for the Agent Orange study.
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From page 109... ...
Unpublished document submitted to the IOM Committee on Making the Best Use of the Agent Orange Reconstruction Model, July 11.
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From page 110... ...
Stellman's herbicide exposure reconstruction model. http://www.va.gov/wriisc-dc/research/studies_ongoing.
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From page 111... ...
2006. [Final report: Dynamic study for defoliant (Agent Orange)
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