Veterans and Agent Orange Update 2006 (2007) / Chapter Skim
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From page 1... ...
, one form of dioxin, was an unintended contaminant from the production of 2,4,5-T and was present in Agent Orange and some other formulations sprayed in Vietnam; thus, it should be noted that TCDD and Agent Orange are not synonymous. In 1991, because of continuing uncertainty about the long-term health effects of the sprayed herbicides on Vietnam veterans, Congress passed Public Law 102-4 (PL 102-4)
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From page 2... ...
whether a statistical association with herbicide exposure exists, taking into account the strength of the scientific evidence and the appropriateness of the statistical and epidemiological methods used to detect the association; B) the increased risk of disease among those exposed to herbicides during ser vice in the Republic of Vietnam during the Vietnam era; and C)
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From page 3... ...
For many conditions, however, particularly ones that are very uncommon, any association with the chemicals of interest has remained unaddressed in the medical research literature; for these, the committee remains neutral based on the understanding that "absence of evidence is not evidence of absence." To obtain additional information potentially relevant to the evaluation of health effects related to herbicide exposure, in addition to that available in studies of Vietnam veterans, the committee reviewed studies of other groups potentially exposed to the constituents of the herbicide mixtures used in Vietnam (2,4-D, 2,4,5-T, TCDD, cacodylic acid, and picloram)
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From page 4... ...
It has been the practice of all VAO committees to evaluate all studies according to the same criteria and then to weight findings of similar strength and validity equivalently, whether or not the study subjects are Vietnam veterans, when drawing conclusions. The committee recognizes that an absolute conclusion about the absence of association might never be attained, because, as is generally the case in science, studies of health outcomes after herbicide exposure cannot demonstrate that a purported effect is impossible, only that it is statistically improbable.
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From page 5... ...
However, the exact mechanisms by which those molecular events cause the various health outcomes seen in animals and in humans remain unknown. Studies of effects of 2,4-D and 2,4,5-T published since Update 2004 are consistent with the earlier conclusion that these chemicals are not acutely toxic and have only weak carcinogenic potential.
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From page 6... ...
Veterans who reported spraying herbicides had significantly higher serum TCDD concentrations than did Vietnam veterans and other veterans who did not report herbicide spraying. The final analysis of the self-reported occurrence of health outcomes compared herbicide sprayers with non-sprayers among those ACC subjects who had served in Vietnam.
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From page 7... ...
Analytic approaches based on serum TCDD concentrations that had been applied to the Ranch Hand subjects were extended to cancer incidence in the comparison subjects, Vietnam-era veterans deployed elsewhere in Southeast Asia (SEA) ; the findings in the SEA group were consistent with results reported previously for respiratory and prostate cancers in the Ranch Hand subjects and appeared to strengthen the evidence with respect to melanoma.
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From page 8... ...
The concern is fueled in part by persisting reports of congenital defects in the present Vietnamese population, whose mode of continuing exposure is quite different from that experienced by predominantly male US soldiers. The limited number of offspring among studied populations of US and allied Vietnam veterans has seriously constrained the ability to detect associations between herbicide exposures and specific birth defects, so meta-analyses of all the available data on such effects among the children of veterans would be valuable.
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From page 9... ...
In two new studies, Vietnam veterans with the highest exposure to herbicides exhibited distinct increases in the prevalence of hypertension; the prevalence of heart disease was also increased, with ischemic heart disease representing nearly 70 percent of cardiac conditions reported. Those studies have the strengths of controlling for the major circulatory-disease risk factors, confirming diagnoses by medical-record review, and relating the health outcome to the degree of herbicide or dioxin exposure.
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From page 10... ...
Epidemiologic methods and analytic capabilities have improved, but many of the recent studies were also particularly useful for this committee's purpose because they produced results in terms of serum TCDD concentrations or because their findings consisted of observations on the aging population of primary concern, Vietnam veterans. The committee assigned each health outcome to one of four categories on the basis of the evidence.
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From page 11... ...
For example, a well-conducted study with strong findings in accord with less compelling results from studies of populations with similar exposures could constitute such evidence. There is limited or suggestive evidence of an association between exposure to the chemicals of interest and the following health outcomes: Laryngeal cancer Cancer of the lung, bronchus, or trachea Prostate cancer Multiple myeloma AL amyloidosis (category change from Update 2004)
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From page 12... ...
By default, any health outcome on which no epidemiologic information has been found falls into this category. Limited or Suggestive Evidence of No Association Several adequate studies, which cover the full range of human exposure, are consistent in not showing a positive association between any magnitude of exposure to the herbicides of interest and the outcome.
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From page 13... ...
For the first time, a VAO committee found itself deadlocked with respect to classifying the evidence for several health outcomes. The committee could not reach consensus about the strength of the evidence concerning association of herbicide exposure with three health outcomes: two cancers, breast cancer and melanoma, and the specific cardiovascular condition, ischemic heart disease.
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From page 14... ...
The requisite information to assign risk estimates continues to be absent despite concerted efforts to model the exposure of the troops in Vietnam, to measure the serum TCDD concentrations of individual veterans, and to model the dynamics of retention and clearance of TCDD in the human body. Accordingly, this committee has deleted the repetitious statements about the inability to calculate risk for Vietnam veterans that had appeared with each health outcome in prior updates.
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From page 15... ...
Consideration should also be given to restarting the National Vietnam Veterans Longitudinal Study. New epidemiologic studies, such as a case–control study of tonsil cancer developed from VA's existing files or a study of reproductive effects in the Vietnamese population, could enable the recovery of valuable information.
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