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Dioxin is the term used to describe a family of toxic chlorinated organic compounds that can travel long distances and bioaccumulate in humans and wildlife due to their fat solubility. The most notorious of those is 2,3,7,8-tetrachlorodibenzo-p-dioxin, often abbreviated as TCDD. The isomers containing chlorine in the 2,3,7, and/or 8 positions are the toxic isomers and the ones which bioaccumulate.

Dioxin builds up in living tissue over time, so even small exposures can accumulate to dangerous levels.

Dioxins cause a severe form of persistent acne (known as chloracne) in humans (only at very high dosages) and developmental abnormalities and cancers in animals. The U.S. government has recently listed dioxin as a known human carcinogen, but debate on human effects continues.

Both veterans groups and Vietnamese groups (including the Vietnamese government) have convened scientific studies to explore their belief that dioxins were responsible for a host of disorders, including tens of thousands of birth defects in children, amongst Vietnam veterans as well as an estimated one million Vietnamese, through their exposure to Agent Orange, which was found to be highly contaminated with TCDD.

The most recent study, paid for by the National Academy of Sciences, was released in an April 2003 report.

Large amounts of dioxin were released in an industrial accident at Seveso in 1976(no human fatalities or birth defects occured). In 1978, dioxin was one of the contaminants that forced the evacuation of the Love Canal neighborhood of Niagara Falls, New York. Dioxin also caused the 1983 evacuation of Times Beach, Missouri.

Dioxins are produced when organic material is burned in the presence of chlorine, whether the chlorine is present as chloride ions, or as organochlorine compounds, so they are widely produced in many contexts such as: