Veterans and Agent Orange Update 2010 (2012) / Chapter Skim
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3 Exposure to the Herbicides Used in Vietnam
3 Exposure to the Herbicides Used in Vietnam
Pages 54-75
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From page 54... ...
and is the most toxic congener of the tetrachlorodibenzo-p-dioxins. It then reviews several key methodologic issues in human population studies; disease latency, possible misclassification based on exposure, and exposure specificity required for scientific evaluation of studies.
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From page 55... ...
Such biologic markers of exposure integrate absorption from all exposure routes, but their interpretation requires knowledge of pharmacokinetic processes. All those exposure-assessment approaches have been used in studies of Vietnam veterans.
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From page 56... ...
sodium Acid, 65% active -- 25,650 L 1962-1964 cacodylate ingredient; salt, 70% active ingredient Blue aqueous 21% sodium cacodylate + cacodylic acid to Acid weight, 360 g/L 4,255,952 L 4,715,731 L 1964-1971 solution yield at least 26% total acid equivalent by (1,124,307 gal) weight Total, all -- -- -- 67,789,844 L 76,954,766 L (including formulations (17,908,238 gal)
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From page 57... ...
Agent Pink, Agent Green, Agent Purple, Agent Orange, and Agent Orange II all contained 2,4,5-T and were contaminated to some extent with TCDD. Agent White contained 2,4-D and picloram.
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From page 58... ...
reacts with chloroacetic acid.
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From page 59... ...
EXPOSURE OF VIETNAM VETERANS Determination of exposures of US military personnel who served in Vietnam has been perhaps the greatest challenge in the study of health effects associated with herbicides and TCDD. Some military personnel stationed in cities or on large bases may have received little or no herbicide exposure, whereas troops who moved through defoliated areas soon after treatment may have been exposed through soil contact, drinking water, or bathing.
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From page 60... ...
In accord with Congress's mandated presumption of herbicide exposure of all Vietnam veterans, VAO committees have treated Vietnam-veteran status as a proxy for some herbicide exposure when no more specific exposure information is available. Exposure of Herbicide Handlers Military personnel who came into direct contact with the herbicidal chemi cals through mixing, loading, spraying, and clean-up activities had relatively high exposures to them.
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From page 61... ...
. Blood samples collected from 50 Vietnam veterans in 1996 showed an association between those who reported spraying herbicides and higher serum TCDD concentrations; this finding was confirmed in a follow-up study of a larger fraction of the cohort (Kang et al., 2006)
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From page 62... ...
proposed further work on exposure reconstruction and development of a model that could be used to categorize exposures of ground troops. The committee cautioned that serum TCDD measurements not be regarded as a "gold standard" for exposure, that is, as a fully accurate measure of herbicide exposure.
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From page 63... ...
, Vietnamese researchers have made a number of attempts to characterize the herbicide exposure of residents of Vietnam in the process of trying to assess adverse reproductive out comes. Some compared residents of the South with residents of the unsprayed North, and others endeavored to compare South Vietnamese people who lived in sprayed and unsprayed villages, as determined by observed defoliation.
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From page 64... ...
The above studies are not directly relevant to this committee's task, but they may prove useful in future epidemiologic studies of the Vietnamese population and in the development of risk-mitigation policies. MODELS FOR CHARACTERIZING HERBICIDE EXPOSURE IOM, following up on the recommendations contained in the original VAO report (IOM, 1994)
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From page 65... ...
. The report concluded that "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 support ing informative epidemiologic studies of herbicides and health among Vietnam veterans and that it should be used to conduct studies." A different perspective has been put forth by Young and colleagues in a se ries of papers (Young et al., 2004a,b)
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From page 66... ...
Considerable work has been done by National Academies commit tees and others to develop ground-troop exposure assessments based on num bers, patterns, and timing of aerial spray missions combined with troop-location information. Although previously recommended by earlier VAO committees, the Stellman model has not yet been applied in a study evaluating the health of ground troops.
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From page 67... ...
We focus here on several key methodologic issues that com plicate development of accurate estimates of exposure of the Vietnam-veteran population and the other study populations discussed in this report: the latent period between exposure and disease, exposure misclassification, and exposure specificity. Latency The temporal relationship between exposure and disease is complex and often difficult to define in studies of human populations.
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From page 68... ...
The information in Tables 3-2 and 3-3 represents the committee's current thinking with respect to their relevance and has helped to guide its evaluation of epidemiologic studies. Earlier VAO committees did not address the issue of exposure specificity in exactly this manner.
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From page 69... ...
TABLE 3-3 Current Committee Guidance for the Classification of Exposure Information in Epidemiologic Studies That Focus on Exposure to Dioxin-like Chemicals and Relevance of the Information to the Committee's Charge Specificity of Exposure Relevance to Reported in Study Additional Information Committee's Charge Dioxin-like chemicals Exposure to PCBs or polychlorinated Limited relevance dibenzofuron (PCDF) Dioxin-like chemicals Results expressed in terms of (total)
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From page 70... ...
, and dicamba -- has been factored into the committee's deliberations with somewhat less weight. The very few epidemiologic findings on exposure to picloram or cacodylic acid have been regarded as highly relevant.
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From page 71... ...
1989b. Comparison of Serum Levels of 2,3,7,8-Tetrachlorodibenzo-p-Dioxin with Indirect Es timates of Agent Orange Exposure Among Vietnam Veterans: Final Report.
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From page 72... ...
2008. The Utility of Proximity-Based Herbicide Exposure Assessment in Epidemiologic Studies of Vietnam Veterans.
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From page 73... ...
2001. US Army Chemical Corps Vietnam Veterans Health Study: Preliminary results.
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From page 74... ...
Presentation to the Institute of Medi cine Committee to Review the Health Effects in Vietnam Veterans of Exposure to Herbicides. December 8, 1992.
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From page 75... ...
2004a. Assessing possible exposures of ground troops to Agent Orange during the Vietnam War: The use of contemporary military records.
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