2,4,5-Trichlorophenoxyacetic acid

2,4,5-Trichlorophenoxyacetic acid (2,4,5-T), a synthetic auxin, is a 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.
2,4,5-T itself is of only moderate 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 reported to be extremely toxic to humans, however this is disputed. 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. In 1985, the EPA terminated all uses in the US of this herbicide on rice fields as well as all non-crop sites. 2,4,5-T has since largely been replaced by dicamba and triclopyr.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. Agent Orange was a mixture of 2,4,5-T, 2,4-dichlorophenoxyacetic acid (2,4-D) and picloram. Because of TCDD contamination in the 2,4,5-T component, it has been blamed for serious illnesses in many veterans who were exposed to it. However, other research on populations exposed to its dioxin contaminant have been inconsistent and inconclusive. Agent Orange often had much higher levels of TCDD than 2,4,5-T used in the US.

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