Fifty years after the Vietnam War, researchers are still struggling to document the long-term health effects of the massive spraying of Agent Orange and other herbicides

The Vietnam War marked the biggest deployment of herbicidal warfare the world has ever seen. Between 1961 and 1971, lumbering aircraft sprayed an estimated 74 million liters of the chemicals over South Vietnam as well as border areas of Laos and Cambodia. The goal was to strip foliage from mangrove swamps and dense forests that were providing cover for North Vietnamese and Viet Cong troops, and to destroy crops they relied on for food.

The weapons of choice were called the rainbow herbicides—a half-dozen formulations identified by names such as Agent Pink and Agent White, after the colors used to mark barrels. Agent Orange, which became the most notorious, was a 50-50 blend of two commercially available herbicides, 2,4-dichlorophenoxyacetic acid and 2,4,5-trichlorophenoxyacetic acid (2,4,5-T). The defoliants were believed to be harmless to humans, and U.S. troops typically handled them without wearing protective gear.

By the late 1960s, however, lab experiments showed 2,4,5-T could cause abnormalities and stillbirths in mice, and there were reports of human birth defects in sprayed areas of Vietnam. Later it became clear that the herbicide manufacturing process introduced a particularly toxic dioxin, known as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), into Agent Orange and other herbicides. Four years before the war ended, the U.S. abandoned its decadelong spraying campaign amid mounting international condemnation and concerns about safety.