Veterans and Agent Orange: Update 1996

Assessment of individual exposure to herbicides and dioxin is a key element in determining whether specific health outcomes are linked to these compounds. The committee has found, however, that the definition and quantification of exposure are the weakest methodologic aspects of the epidemiologic studies. Although different approaches have been used to estimate exposure among Vietnam veterans and among various occupationally and environmentally exposed groups, each approach is limited in its ability to determine precisely the intensity and duration of individual exposure.

Since the publication of VAO, there has been considerable progress in the use of serum TCDD levels and/or quantitative exposure indices, as summarized in Chapter 5. There also has been progress in characterizing the TCDD body burdens in several groups, including the Ranch Hand cohort, Seveso residents, German herbicide production employees, and Vietnamese civilians (Michalek, et al., 1996; Needham, et al., 1994; Flesh-Janys, et al., 1994; Ott, et al., 1993; and Verger, et al., 1994). The mean half-life of TCDD in humans has been calculated to be about 8.7 years in the Ranch Hand cohort (Michalek, et al., 1996). Serum TCDD measurements may provide valuable information about past herbicide exposure under some conditions, and they are best used to detect differences in exposure levels among large groups in epidemiologic studies. This additional information on TCDD body burdens in specific groups and information on half-lives allow more accurate comparisons of relative levels of exposure to TCDD among cohorts.

Although definitive data are lacking, the available evidence suggests that Vietnam veterans as a group had substantially lower exposure to herbicides and dioxin than did the subjects in many occupational studies. The participants in Operation Ranch Hand and the Army Chemical Corps are exceptions to this pattern, and it is likely that there are others who served in Vietnam who had exposures comparable in intensity to members of the occupationally exposed cohorts. It is currently not possible to identify this heavily exposed fraction of Vietnam veterans, although exposure reconstruction methods with this capability could perhaps be developed and validated.