Polychlorinated dibenzo-dioxins (PCDDs) and polychlorinated dibenzo-furans (PCDFs) are commonly referred to together as "dioxins." Dioxins are a group of chlorinated organic compounds with similar chemical structures, called congeners. The most studied and most toxic dioxin congener is 2,3,7,8- tetrachlorodibenzo-p-dioxin (2,3,7,8-TCDD). 2,3,7,8-TCDD has been identified as a "known human carcinogen" (IARC 1997) and a probable human carcinogen by USEPA (Group B2 carcinogen). Other dioxins may cause cancer, disrupt the endocrine system, and cause reproduction and developmental effects (USEPA 2003; http://www.epa.gov/ncea/pdfs/dioxin/nas-review/). Dioxins are toxic to humans and other mammals at very low concentrations.
Dioxins are unintentionally produced by natural and industrial activities. Natural activities include forest fires or volcanic activity. Industrial processes include incomplete combustion of materials in the presence of chloride, such as burning of fuels, municipal and domestic waste incineration, as well as chlorine bleaching of pulp and paper, and chlorinated pesticide manufacturing.
Although dioxins are produced at very low levels (e.g., parts per trillion or parts per quadrillion), the compounds exist throughout the environment. Due to their chemical and physical properties, they persist and have the potential to bioaccumulate in the tissue of humans and wildlife.