Veterans and Agent Orange Length of Presumptive Period for Association Between Exposure and Respiratory Cancer (2004) / Chapter Skim
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Executive Summary
Executive Summary
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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. In 1991, because of continuing uncertainty about the long-term health effects on Vietnam veterans of the herbicides sprayed, Congress passed Public Law 102-4, the Agent Orange Act of 1991.
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For respiratory cancer, the reports of some potentially informative studies did not include latency results, and no latent period could be estimated. On the basis of several subsequent studies that did address that issue, Update 1998 concluded that "the evidence suggests that if respiratory cancer does result from exposure to the herbicides used in Vietnam, the greatest relative risk for lung cancer may be in the first decade after exposure, but until further follow-up has been carried out for some of the cohorts, it will not be possible to put an upper limit on the length of time these herbicides could exert their effect." CHARGE TO THE COMMITTEE In response to the request from the VA, IOM extended the service of the Committee to Review the Health Effects in Vietnam Veterans of Exposure to
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On the basis of the available epidemiologic and toxicologic data, the committee considered TCDD to be the main chemical of concern for respiratory cancer and therefore focused on the toxicokinetic and mechanistic data related to TCDD. When discussing how long the effects of exposure in Vietnam might last, the committee found it important to differentiate between the actual latent period (time from disease induction to disease detection)
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A study of Australian Vietnam veterans showed increased lung cancer mortality in the first decade after the start of their military service, but that finding is based on a small number of deaths. The most recent study of the Ranch Hands (Air Force personnel responsible for aerial spraying of Agent Orange)
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Although exposure to herbicides in Vietnam was timelimited, potential elevated exposure of target organs could occur until serum TCDD in a Vietnam veteran reached the background value. Therefore, the persistence of an elevated body burden of TCDD could increase the duration of exposure of target organs, during which induction of disease may occur at any time, thereby lengthening the presumptive period (i.e., the interval between external exposure cessation and disease detection)
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Time courses cannot be directly extrapolated from other chemicals to TCDD, but the available data indicate that in many instances respiratory cancer can be associated with a chemical exposure that occurred many decades earlier. UNCERTAINTY To assess the presumptive period between cessation of exposure and a given health outcome in a particular group, such as Vietnam veterans, it must be established that the exposure in question is associated with the outcome, and then how long the risk of the health outcome remains increased after cessation of exposure must be evaluated.
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The main question for the committee is whether it is possible to put an upper limit on the length of time that TCDD can exert its effect and, if so, what that limit would be -- that is, the presumptive period. Because there are no epidemiologic data on the length of time after exposure to TCDD ceases during which an increase in respiratory cancer is associated with that exposure, the committee cannot determine a period beyond which occurrence of respiratory cancer could no longer be presumed to be related to exposure to TCDD (that is, no upper limits on the latency or presumptive period could be determined)
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