able for quantitative exposure estimates that can be used to construct what are sometimes called exposure metrics. The metrics integrate quantitative estimates of exposure intensity (such as chemical concentration in air or extent of skin contact) with exposure duration to produce an estimate of cumulative exposure. Exposure also can be assessed by measuring chemicals and their metabolites in human tissues. Such biologic markers of exposure integrate absorption from all exposure routes, but their interpretation requires knowledge of pharmacokinetic processes. All those exposure-assessment approaches have been used in studies of Vietnam veterans.
Military use of herbicides in Vietnam took place from 1962 through 1971. Tests conducted in the United States and elsewhere designed to evaluate defoliation efficacy were used to select specific herbicides (IOM, 1994; Young and Newton, 2004). Four compounds were used in the herbicide formulations in Vietnam: 2,4-dichlorophenoxyacetic acid (2,4-D), 2,4,5-trichlorophenoxyacetic acid (2,4,5-T), 4-amino-3,5,6-trichloropicolinic acid (picloram), and dimethylarsinic acid (cacodylic acid). The chemical structures of those compounds are presented in Chapter 2 (Figure 2-1). The herbicides were used to defoliate inland hardwood forests, coastal mangrove forests, cultivated lands, and zones around military bases. In 1974, a National Resource Council committee estimated the amount of herbicides sprayed from helicopters and other aircraft by using records gathered from August 1965 through February 1971 (NRC, 1974). That committee calculated that about 18 million gallons (about 68 million liters) of herbicide was sprayed over about 3.6 million acres (about 1.5 million hectares) in Vietnam in that period. The amount of herbicides sprayed on the ground to defoliate the perimeters of base camps and fire bases and the amount sprayed by Navy boats along river banks were not estimated.
A revised analysis of spray activities and exposure potential of troops emerged from a study overseen by a committee of the Institute of Medicine (IOM, 1997, 2003a,b). That work yielded new estimates of the amounts of military herbicides used in Vietnam from 1961 through 1971 (Stellman et al., 2003a). The investigators reanalyzed the original data sources that were used to develop herbicide-use estimates in the 1970s and identified errors that inappropriately removed spraying missions from the dataset. They also added new data on spraying missions that took place before 1965. Finally, a comparison of procurement records with spraying records found errors that suggested that additional spraying had taken place but gone unrecorded at the time. The new analyses led to revision of estimates of the amounts of the agents applied, as indicated in Table 3-1. The new research effort estimated that about 77 million liters were applied, about 9 million liters more than the previous estimate.