Dioxins form whenever chlorine-containing compounds are burned or treated with catalysts in the presence of organic material. The most toxic form of dioxin is TCDD (2,3,7,8-tetrachlorodibenzo-p-dioxin). It is a known carcinogen.
Prior to industrialization and the introduction of chlorine, dioxin existed naturally only in very small amounts. Although dioxin was never intentionally produced, it enters the air from thousands of sources including incinerators that burn medical, municipal, and hazardous waste, chemical processing facilities that use chlorine to make products such as pesticides and PVC plastic, and metal refining and smelting operations.
Airborne dioxin can travel great distances, eventually settling onto soil, plants, and water. Dioxin dissolves readily in oils, fats, and organic solvents but poorly in water and it does not evaporate readily. Since dioxin does not react with oxygen or water and is not broken down by bacteria, it persists in the environment for long periods of time, making it a ‘persistent organic pollutant.’
Dioxin is found everywhere in the world, even in places where dioxin or dioxin-containing products have never been used. When dairy cows and beef cattle eat feed crops containing settled airborne dioxin or are given feed that contains dioxin from other animal fats, it becomes concentrated in their fat.
Humans ingest dioxin primarily in fatty foods such as beef, dairy products, and fish, but also in lesser concentrations in other foods. Dioxin accumulates in human fatty tissue (with a half-life of 7-12 years). It is metabolized (mainly by the liver) and eliminated much more slowly than in other animals. TCDD is excreted mainly through feces, and also through breast milk.
Children, nursing infants, some workers, people who eat fish as a main staple of their diet, and people who live near dioxin-release-sites are potential high-exposure groups. Children and fetuses are the most susceptible to the health effects of dioxin exposure.