processes, physical transport and dispersal, and degradation processes generally predict a concentration gradient for environmental herbicide and TCDD, with greater attenuation at greater distances from the points of introduction (spraying), except for processes that result in increasing concentrations, such as bioconcentration and distillation of marine water to make potable water on blue-water ships. It is therefore generally reasonable to suppose that the greatest exposure opportunities would be related to proximity to herbicide use and to locations with higher herbicide and TCDD contamination and that personnel who are at a distance from these locations would have lower exposures. Applying that general expectation to the circumstances and populations of interest has been the major thrust of the committee’s efforts to characterize exposure. Specifically, the committee used fate and transport considerations combined with professional judgment to produce estimates of potential exposure opportunities in the three populations.

A schematic of the movement of Agent Orange and TCDD from the point of application (from spraying of Agent Orange from aircraft, from trucks, or from boats) to marine waters and air was shown in Figure 4-1. Working from that schematic, the committee sought to identify the exposure routes likely to result in Agent Orange and Agent Orange–associated TCDD exposure opportunities in each of the military populations. It should be noted that there are Blue Water Navy personnel who qualify as Brown Water Navy personnel as a result of their ships’ locations or activities but have not yet been so designated by the Department of Veterans Affairs (VA), although they may be in the future. The committee did not consider the possible reclassification of those Blue Water Navy sailors in its exposure evaluations.

PREVIOUS EXPOSURE MODELING EFFORTS

Given their inability to quantify exposure of US military personnel to Agent Orange–associated TCDD, several prior IOM committees identified various approaches to approximate it. The IOM report The Utility of Proximity-Based Herbicide Exposure Assessment in Epidemiologic Studies of Vietnam Veterans (IOM, 2008) defined the simplest approach to characterizing herbicide exposure as being “based on a veteran’s presence or absence in Vietnam during the period of herbicide spraying.” At the second, more complex level, “measures of exposure ... are based on information on the location, timing, and volume



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