Four decades after the U.S. started using Agent Orange in Vietnam, the two countries will begin working together to assess the effects of the toxic chemical on human health and the environment. Agent Orange is a defoliant that contains TCDD, the most dangerous form of dioxin, which causes cancer, immune system malfunction, and birth defects. The U.S. sprayed millions of gallons of Agent Orange on Vietnam from 1962 to 1971 in a frustrated effort to expose the enemy and win the war by denying forest cover to jungle fighters. The practice created one of the most enduring, damaging, and controversial legacies of the conflict; in the first decade after the war, about 50,000 children were born with deformities or paralyses attributed to Agent Orange. Delegates from the two countries will begin to discuss joint research into the effects at a four-day conference in Hanoi starting in early March.