The term "dioxin" is used as shorthand for the family of 210 polychlorinated dibenzo dioxins (PCDDs) and dibenzo furans (PCDFs). Seventeen members of this family can be considered extremely toxic with the most toxic form having four chlorine atoms on the second, third, seventh and eighth position on a two benzene ring molecule. This is known as 2,3,7,8-tetrachlorodibenzo-p-dioxin, otherwise known as 2,3,7,8 TCDD. The sixteen 2,3,7,8-substituted PCDDs and PCDFs are given toxicity equivalent factors relative to 2,3,7,8 TCDD.

Very little of the dioxin present in the environment is either natural or inevitable. Most dioxin is the by-product of a specific set of industrial processes and products: those associated with chlorine chemistry.

Public concern about effects of dioxin on human health was heightened in 1976 following an accident at Seveso in Italy, when an explosion at a chemical factory caused the release of high levels of tetrachlorodibenzo-p-dioxin (TCDD). The most commonly reported effects on humans following this accident and other incidents of high dioxin exposure was a skin rash called chloracne.

Since then numerous animal experiments and several epidemiological studies in humans have shown that dioxin causes a wide range of health effects. New data from Seveso found increased deaths from digestive cancer among women in Seveso, who lived closest to the area that was highly contaminated with dioxin. In a follow-up study reported in the November, 1997 issue of the journal Epidemiology, Pier Alberto Bertazzi, University of Milan, and co-authors found that, fifteen years after the explosion and release of dioxins in Seveso, Italy, there were increased death rates from several types of cancer: digestive, rectal, hematologic, leukemia, multiple myeloma, hematologic, Hodgkin's disease, and soft tissue sarcoma. Rates were linked to gender as well as contamination zone. Another recent study of U.S. veterans exposed to dioxin from Agent Orange during the Vietnam War have higher rates of diabetes.

The United States Environmental Protection Agency (EPA) has conducted a reassessment of the environmental and human health risks from exposure to dioxin. It is expected to conclude that exposure to dioxin poses a large scale, long term threat to public health and the environment not only due to cancer risks but also because of possible birth defects and damage to immune systems.