Polychlorinated dibenzodioxins
[1] They are commonly but inaccurately referred to as dioxins for simplicity, because every PCDD molecule contains a dibenzo-1,4-dioxin skeletal structure, with 1,4-dioxin as the central ring.
Members of the PCDD family bioaccumulate in humans and wildlife because of their lipophilic properties, and may cause developmental disturbances and cancer.
Because dioxins can persist in the environment for more than 100 years, the majority of PCDD pollution today is not the result of recent emissions, but the cumulative result of synthetic processes undertaken since the beginning of the 20th century, including organochloride-related manufacturing, incineration of chlorine-containing substances such as polyvinyl chloride (PVC), and chlorine bleaching of paper.
[2][3] Forest fires and volcanic eruptions have also been cited as an airborne source, although their contribution to the current levels of PCDD accumulation are minor in comparison.
The most toxic dioxin, 2,3,7,8-tetrachlorodibenzodioxin (TCDD), became well known as a contaminant of Agent Orange, a herbicide used in the Malayan Emergency and the Vietnam War.
Since 1987, however, backyard barrel burning has showed almost no decrease, and is now the largest source of dioxin emissions, producing about one third of the total output.
[18] In incineration, dioxins can also reform or form de novo in the atmosphere above the stack as the exhaust gases cool through a temperature window of 600 to 200 °C.
The most common method of reducing the quantity of dioxins reforming or forming de novo is through rapid (30 millisecond) quenching of the exhaust gases through that 400 °C window.
[21] Tolerable daily, monthly or annual intakes have been set by the World Health Organization and a number of governments.
This is why the WHO consultation group assessed the tolerable intake so as to prevent a woman from accumulating harmful body burdens before her first pregnancy.
[citation needed] 2,3,7,8-Tetrachlorodibenzodioxin (TCDD) is considered the most toxic of the congeners (for the mechanism of action, see 2,3,7,8-Tetrachlorodibenzodioxin and Aryl hydrocarbon receptor).
TEFs are consensus values and, because of the strong species dependence for toxicity, are listed separately for mammals, fish, and birds.
The TEQ approach and current TEFs have been adopted internationally as the most appropriate way to estimate the potential health risks of mixture of dioxins.
Recent data suggest that this type of simple scaling factor may not be the most appropriate treatment for complex mixtures of dioxins; both transfer from the source and absorption and elimination vary among different congeners, and the TEF value is not able to accurately reflect this.
Dioxins build up primarily in fatty tissues over time (bioaccumulation), so even small exposures may eventually reach dangerous levels.
In 1994, the US EPA reported that dioxins are a probable carcinogen, but noted that non-cancer effects (reproduction and sexual development, immune system) may pose a greater threat to human health.
TCDD, the most toxic of the dibenzodioxins, is classified as a Group 1 carcinogen by the International Agency for Research on Cancer (IARC).
[34] High occupational or accidental levels of exposures to dioxins have been shown by epidemiological studies to lead to an increased risk of tumors at all sites.
[58] By 1971, 12% of the total area of South Vietnam had been sprayed with defoliating chemicals, which were often applied at rates that were 13 times as high as the legal USDA limit.
[60] In some areas, TCDD concentrations in soil and water were hundreds of times greater than the levels considered safe by the U.S. Environmental Protection Agency.
[63] The Red Cross of Vietnam estimates that up to 1 million people are disabled or have health problems due to Agent Orange contamination.
