Dioxin

Dioxins are a class of organic compounds, with a basic structure that includes two oxygen atoms joining a pair of benzene rings. Chlorinated dioxins have some amount of substitution with chlorine for hydrogen atoms in the benzene rings. A particular chemical, 2,3,7,8-tetrachlorodibenzo-p-dioxin (abbreviated as TCDD, or as 2,3,7,8-TCDD), is one of 75 chlorinated derivatives of dibenzo-p-dioxin. There is a very wide range of toxicity within the larger group of dioxins and chlorinated dioxins, but TCDD is acknowledged as being the most poisonous dioxin compound, at least to certain species of animals.

Dioxins have no particular uses. They are not manufactured intentionally, but are synthesized incidentally during some industrial processes. For example, under certain conditions relatively large concentrations of dioxins are inadvertently synthesized during industrial reactions involving 2,4,5-trichlorophenol. A well known case of this phenomenon is in the manufacturing of the phenoxy herbicide, 2,4,5-T (2,4,5-trichlorophenoxy acetic acid). This chemical (which is no longer used) was once manufactured in large amounts, and much of the material was badly contaminated by TCDD. Concentrations in the range of 10-50 parts per million (ppm, or mg per liter) occurred in 2,4,5-T manufactured for use during the Vietnam War. However, there was a much smaller contamination (less than 0.1 ppm) in 2,4,5-T manufactured after 1972, in accordance with regulations enacted by the U.S. Environmental Protection Agency.

TCDD is also a trace contaminant of other products manufactured from trichlorophenol, including hexachlorophene, once commonly used as a topical antibacterial treatment. TCDD is incidentally synthesized when wood pulp is bleached using chlorine-based oxidants. The low-temperature combustion of chlorine-containing organic materials (for example, in cigarettes, burning garbage dumps, and barbecues) also produce dioxins, including TCDD. The incineration of municipal waste synthesizes small quantities of TCDD, although the relatively high temperatures reduce the yield of dioxins compared with the smoldering kinds of combustion just mentioned. Dioxins are also synthesized naturally in trace quantities, mostly during forest fires.

TCDD is a persistent chemical in the environment, and because it is virtually insoluble in water, but highly soluble in fats and oils, it strongly biomagnifies and occurs in especially large concentrations in predators at the top of the ecological food web. Moreover, TCDD is globally distributed, meaning that any chemical analysis of a biological tissue, especially of the fat of an animal, will detect residues of this dioxin (assuming that the analytical chemistry is sensitive enough).