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Dioxin (2,3,7,8-Tetrachlorodibenzo-p-dioxin)
Dioxins differ in the number and location of chlorine atoms on a basic three-ringed structure of carbon, hydrogen, and oxygen atoms. The most toxic forms of dioxin are those that contain four to eight chlorine atoms with four of the atoms in the lateral position (2,3,7,8).
Dioxins usually exist as complex mixtures of various chemical structures and the toxicity of such compounds depends on the number and location of chlorine atoms.[1]
2,3,7,8- tetrachloriodibenzo-p-dioxin (2,3,7,8 TCDD) is the most researched and most toxic dioxin. It is a colorless and odorless solid that occurs only as a byproduct of incineration and the manufacturing of herbicides and germicides. TCDD accounts for about 10% of background dioxin risk. [2]
TCDD is used as a reference for the toxicity levels of all other dioxins. The 2,3,7,8 congener is given a toxicity equivalence factor (TEF) of one and other dioxins are given TEFs as fractions of one. The total toxic equivalent quantity (TEQ) is the sum of all the concentrations in a substance multiplied by their specific TEFs. The TEF and TEQ are useful, since dioxins are usually found in complex mixtures and determining the toxicity of these compounds would be a difficult process without the equivalency scale.[3]
Dioxins have a low vapor pressure (slow to evaporate), do not dissolve readily in water, but are lipophilic (soluble in fat and inorganic matter with fat-like properties). Dioxins are not highly reactive with other chemicals.[4]
[1] Ontario Green Facts http://www.ene.gov.on.ca/cons/681e01.htm
[2] Eco-USA website http://www.eco-usa.net/toxics/dioxin.shtml
[3] EPA dioxin facts sheet http://www.epa.gov/ncea/pdfs/dioxin/factsheets/dioxin_short2.pdf
[4] Chlorine Online Information http://www.eurochlor.org/chlorine/issues/dioxins.htm
[5] Furan and PCBs- www.epa.gov/oppfead1/international/pops.htm “Persistent OrganicPollutants”
