dioxin contaminant are also addressed in this chapter. As a result of several major events relating to dioxin exposure, the public became aware of the potential health effects of exposure to dioxin in tandem with the increased concern over possible health effects of exposure to herbicides sprayed in Vietnam. Researchers studied populations (described in this chapter) that had potential health effects from exposure to herbicides and TCDD, including production workers in chemical plants, agricultural and forestry workers, pulp and paper mill workers, and residents environmentally exposed in specific areas, such as Times Beach, Missouri; Alsea, Oregon; and Seveso, Italy. For the studies introduced in this chapter, the methodological framework is described in Chapter 7, and the results are discussed in the health outcome chapters (8-11).

MILITARY USE OF HERBICIDES IN VIETNAM

The military use of herbicides in Vietnam began in 1962, was expanded during 1965 and 1966, and reached a peak from 1967 to 1969. Herbicides were used extensively in Vietnam by the U.S. Air Force's Operation Ranch Hand to defoliate inland hardwood forests, coastal mangrove forests, and, to a lesser extent, cultivated land, by aerial spraying from C-123 aircraft and helicopters. Soldiers also sprayed herbicides on the ground to defoliate the perimeters of base camps and fire bases; this spraying was executed from the rear of trucks and from spray units mounted on the backs of soldiers on foot. Navy riverboats also sprayed herbicides along riverbanks. The purpose of spraying herbicides was to improve the ability to detect enemy base camps and enemy forces along lines of communication and infiltration routes, and around U.S. base camps and fire bases. Spraying was also used to destroy the crops of the Vietcong and North Vietnamese (Dux and Young, 1980).

Herbicide Development and Testing

Experiments with chemicals for the control of vegetative growth were first conducted around the turn of this century. The practical purpose of these early compounds was to control weeds that competed with crops for available water, nutrients, and sunlight (NAS, 1974; Buckingham, 1982). It was not until the 1940s that agricultural chemical research led to the development of a number of synthetic compounds capable of regulating or suppressing plant growth. Some compounds, when applied at high doses, killed certain plants but did not harm others; these compounds were termed selective herbicides (NAS, 1974). Two of the most successful developments during that period were the discoveries of 2,4-D and 2,4,5-T. These chemicals were effective against broadleaf plants and several crops.



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