From 1962 to 1971, the United States Air Force (USAF) sprayed various herbicide mixtures (chemicals that kill plants) in South Vietnam. The purpose of the spraying was to defoliate jungle growth to deprive the Communist forces of ground cover, and to destroy enemy crops to restrict food supplies. The most extensively used of these herbicide mixtures was known as Agent Orange, a 50:50 mix of two common herbicides called 1,4,5-T and 2,4-D (2,4,5-trichlorophenoxyacetic acid and 2,4-dichlorophenoxyacetic acid). The third chemical present in the mixture in small amounts was TCDD, an inevitable by-product of the manufacture of 2,4,5-T. This chemical, called tetrachlorodibenzo-para-dioxin or simply "dioxin," is highly toxic to laboratory animals when administered in its pure form. CRS has been unable to locate any report of a human death from exposure to pure TCDD. This report discusses the human health effects that have occurred from exposure to TCDD, as well as related Congressional concerns.