2,4,5-Trichlorophenoxyacetic acid (2,4,5-T), a synthetic auxin, is a chlorophenoxy acetic acid herbicide used to defoliate broad-leafed plants. It was developed in the late 1940s and was widely used in the agricultural industry until being phased out, starting in the late 1970s due to toxicity concerns. Agent Orange, a defoliant used by the U.S. in the Vietnam War, was equal parts 2,4,5-T and 2,4-D (2,4-dichlorophenoxyacetic acid). "The U.S. military had tested Agent Orange in Thailand between April 1964 and June 1965 with the full knowledge of the Thai authorities."
2,4,5-T itself has low toxicity, with oral LD50 of 389 mg/kg in mice and 500 mg/kg in rats. However, the manufacturing process for 2,4,5-T contaminates this chemical with trace amounts of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). TCDD is a carcinogenic persistent organic pollutant with long-term effects on the environment. With proper temperature control during production of 2,4,5-T, TCDD levels can be held to about .005 ppm. Before the TCDD risk was well-understood, early production facilities lacked proper temperature controls and individual batches tested later were found to have as much as 60 ppm of TCDD.
In 1970, the United States Department of Agriculture halted the use of 2,4,5-T on all food crops except rice, and in 1985, the EPA terminated all remaining uses in the US of this herbicide. The international trade of 2,4,5-T is restricted by the Rotterdam Convention. 2,4,5-T has since largely been replaced by dicamba and triclopyr.
Apart from agricultural uses, 2,4,5-T was also a major ingredient in Agent Orange, a herbicide blend used by the U.S. military in Vietnam between January 1965 and April 1970 as a defoliant. Because of TCDD contamination in the 2,4,5-T component, it has been blamed for serious illnesses in many US veterans and Vietnamese civilians who were exposed to it. Agent Orange often had much higher levels of TCDD than 2,4,5-T used in the US.