states has accrued. 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 (unless the condition is logically subsumed under a broader disease category that has been evaluated), the committee remains neutral, abiding by the maxim that “absence of evidence is not evidence of absence.”
In accord with Congress’s mandated presumption of herbicide exposure for 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. To obtain information potentially relevant to the evaluation of health effects related to herbicide exposure in addition to that available from 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). In addition to retrieving articles identified on the basis of keywords specifying the compounds and chemical classes of interest, literature searches for the earliest reports in the VAO series had been structured to retrieve all studies of several occupational groups, including chemical, agricultural, pulp and paper, sawmill, and forestry workers. To the extent that studies of those workforces were recovered in new searches directed at particular agents of exposure, they were incorporated into the database. Some occupational and environmental cohorts that received exceptionally high exposures (such as the International Agency for Research on Cancer [IARC] and Seveso cohorts discussed in this report) are now well characterized and are producing a stream of informative results. A continuing prospective cohort study of agricultural populations with specific information on the chemicals of interest is also steadily contributing new findings to the database. Most important, the Vietnam veterans themselves are advancing in age and, when studied, are capable of providing substantial information on chronic health conditions directly. As the information in the database on populations with established exposures to the chemicals of interest has grown, the committee has come to depend less on data from studies with nonspecific exposure information and has been able to focus more on findings of studies with refined exposure specificity.
In this update, the committee endeavored to emphasize and clarify the relationship among the succession of publications that have provided ever increasing insight into the health responses of particular exposed populations that have been studied for many years. The information in the results tables for individual health outcomes has grown over eight cycles of revision, but this committee found that the presentation of new findings in update-specific clusters obscured the interdependent nature of many of the studies on a given cohort. Therefore, the findings in the results tables have been rearranged and grouped by study population. In addition, the cohorts themselves have been ordered on these tables to reflect the hierarchical nature of many of these study populations (for example, workers at the Dow plant in Midland, Michigan, are one of several cohorts composing the