Name given members of a family of closely-related chemicals. The term dioxin 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.
Figure 12.13.1: Structure of 2,3,7,8-tetrachlorodibenzo-p-dioxin.
When ingested or injected, TCDD is extremely poisonous to laboratory animals. At sub-lethal concentrations, it causes cancer and birth defects in them. Exposure to high levels of dioxins causes a severe skin disease (chloracne) in humans as well as damage to the liver and nervous system. While the evidence is still hotly debated, the U.S. Environmental Protection Agency (EPA) is convinced that dioxins cause cancer in humans. They base this conclusion on extrapolating from dose-response studies done in animals (rats) and following the health of industrial workers who were exposed to dioxins in the U.S., Germany, and the Netherlands.
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. 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. However, the amounts to which we are exposed have dropped some threefold since the mid-80s, and the cancer risk dioxins pose for most of us is probably close to zero.
Dioxin can prevent disease! (in mice). Experimental allergic encephalomyelitis (EAE) is a disease in experimental animals (e.g., mice, guinea pigs) that closely mimics multiple sclerosis, an autoimmune disease of humans in which the myelin sheaths of neurons are destroyed. In the 1 May 2008 issue of Nature, F. J. Quintana and colleagues reported that they could strongly suppress the induction of EAE in mice by pretreating them with 1 µg of TCDD. The protection appeared to be mediated by regulatory T cells (Treg) whose numbers rose sharply following TCDD treatment.