Second, an additional task brought to this committee through its legislative charge was the determination of whether additional studies of Vietnam veterans are feasible. Drawing upon the committee's evaluation of the available literature and upon information on the military use of herbicides (see Chapter 3), this chapter summarizes what is known about exposure to herbicides in Vietnam in comparison to other populations with widely different types of exposure (e.g., in factories, of professional herbicide sprayers, from environmental accidents). Valid measures of exposure are critical to further epidemiologic studies, and this chapter proposes a method for developing such a measure for future studies of Vietnam veterans.
A third and related reason for the committee's concern about exposure assessment in epidemiologic studies is that these data are needed to draw inferences on the health effects of exposure in Vietnam veterans from studies of those occupationally and environmentally exposed. Studies of these other groups address the issue of whether herbicides are associated with particular health outcomes, but they have only an indirect bearing on the question of associations in veterans themselves. Exposure data in all groups are needed to translate the results of occupational and environmental studies to estimates of increased risk for Vietnam veterans.
When epidemiologists assess the potential health risks of exposure to a toxic chemical, they compare the disease experience of groups of people with different levels of exposure to the substance of interest. Accurate risk estimates depend on the ability to accurately identify those who are "exposed" and those who are not. When the concern is with low-level, possibly intermittent exposure to a chemical such as an herbicide, it becomes important not simply to assess exposure as its presence or absence, but to characterize the degree of exposure—its intensity and duration. At root there are three essential steps in epidemiology:
assembling a cohort of people with similar, well-defined exposures to some agent, and another cohort identical to the first but with members who lack exposure to the agent;
measuring and then comparing the disease experience of each of these cohorts; and,
drawing inferences from these comparisons about the risks that may derive from the exposure differences between cohorts.
Exposure assessment contributes to the epidemiologic study process in several ways. First, accurate measures of exposure are essential to a study's validity because if there is not a well-defined contrast in exposure among