Nomenclature and structure of dioxins

The term dioxin is used as a shortening of 2,3,7,8-tetrachlorodibenzo-p-dioxin and substances with similar modes of action, whereas the term TCDD (or better, 2,3,7,8-TCDD) is used for 2,3,7,8-tetrachlorodibenzo-p-dioxin only. Unsubstituted dibenzodioxin or chlorosubstituted dibenzodioxin with substitution patterns other than 2,3,7,8-TCDD may have low toxicity. Dioxin without dibenzo is very unstable. 

Note The term TEQ is explained on p. 232. The values are taken from Dybing (2001). See also Lindstrom et al. (2000). 

Many other substances have similar effects and modes of action. Common to them is a flat rectangular shape 3 x 10 A in size and a charge distribution similar to that of TCDD. The most important ones are some polychlorinated biphenyls and chlorinated dibenzofurans. 

The PCBs and dibenzofurans have less relevance for pesticide science. 

The term TEQ is an abbreviation for TCDD equivalents. If the toxicity of 2,3,7,8-TCDD is 1, the toxicity of other congeners is reduced by a weighting factor. The extreme variation of potency among the TCDDs, TCDBs, and PCBs makes this necessary when risk assessments of residues are carried out. The residues are multiplied by these factors. The residue of octachlo-rodibenzodioxin is regarded as less potent and is multiplied by 0.001, whereas the concentration of 2,3,7,8-tetrachlorodibenzofuran is multiplied by 0.1, and so forth. 

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