In 1991, because of continuing uncertainty about the long-term health effects on Vietnam veterans who where exposed to herbicides during their service in Vietnam (mixtures of 2,4-dichlorophenoxyacetic acid (2,4-D), 2,4,5-trichlorophenoxyacetic acid (2,4,5-T), picloram, and cacodylic acid), Congress passed legislation that directed the secretary of veterans affairs to ask the National Academy of Sciences (NAS) to perform a comprehensive evaluation of scientific and medical information regarding the health effects of exposure to Agent Orange, other herbicides used in Vietnam, and the various chemical components of those herbicides, including TCDD.
The resulting series of reports concluded that there is "limited/suggestive" evidence of an association between exposure to at least one of the chemicals of interest (2,4-D, 2,4,5-T and its contaminant 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), picloram, and cacodylic acid) and respiratory cancer.
In this report, the IOM was mandated to review "whether it is possible to identify a period of time after exposure to herbicides after which a presumption of service-connection" of respiratory cancer would not be warranted.
The committee concluded that there is no epidemiologic data on which to determine an upper limit on the length of time after cessation of exposure to TCDD during which an increase in respiratory cancer is associated with that exposure (i.e., the presumptive period). Given the latent period of up to 25 years seen in epidemiologic studies, the persistence of TCDD in the body, and that the risk of respiratory cancer posed by some other agents remains increased for 50 years or more following cessation of exposure, the committee further concluded that the effects of TCDD on respiratory cancer could last many decades.