Dioxin

Name given any member of a family of closely-related chemicals. The term is often used for one of these: 2,3,7,8-tetrachlorodibenzo-p-dioxin or TCDD. This substance was present as a contaminant in the herbicide agent orange, which was so widely used during the Vietnam war.

When ingested or injected, dioxin is extremely poisonous to laboratory animals. At sub-lethal concentrations, it causes cancer and birth defects in them. Exposure to high levels of dioxin causes a severe skin disease (chloracne) in humans as well as damage to the liver and nervous system. There is as yet no good evidence that dioxin causes cancer in humans, although people exposed in recent years (chiefly through industrial accidents) will have to be followed for a long time before we can know for certain.

TCDD (and other dioxins) are produced when organic matter is burned. Measurable levels are found in soot from wood-burning stoves and the ash of municipal incinerators. Thanks to the development of delicate analytical techniques, it is possible to detect trace amounts in everyone's blood. Most of us have a few parts per trillion (ppt) of TCDD in our serum. Anything under 20 ppt is not thought to be harmful.