Polychlorinated Dibenzodioxins and Polychlorinated Dibenzofurans - PSL1

Toxicity Equivalency Factors

The toxicity of dioxin and furan mixtures in the environment can be assessed by using an internationally accepted system of comparison known as "toxicity equivalency factors." The scientific and regulatory communities of eight countries⁶ have agreed upon a standard set of International Toxicity Equivalency Factors that compare the toxicity of the 17 most toxic dioxins and furans found in complex mixtures to that of 2,3,7,8-tetrachlorodibenzodioxin. This system has been broadly corroborated in laboratory studies.

Dioxins and furans usually enter and are present in the environment as complex mixtures. While it is possible to separate and study each type of dioxin and furan individually, it would be time-consuming and very expensive (if not impossible) to test each of the 75 dioxins and 135 furans as well as all conceivable combinations.

Fortunately, individual testing can be avoided. Many dioxin and furan compounds have been tested for their toxic effects on living animals and in cell cultures, as well as for their ability to induce one or more of a number of liver enzymes. Researchers have developed a method for expressing the toxicity of different dioxins and furans on a common basis; International Toxicity Equivalency Factors recognize and compare the similarities and differences between the toxic action of the dioxins and furans.⁷

International Toxicity Equivalency Factors are assigned to individual dioxins and furans on the basis of how toxic they are in comparison with the toxicity of 2,3,7,8-tetrachlorodibenzodioxin, the most potent dioxin. This contaminant has been assigned the value of 1.0.⁸ By comparison, animal and cell tests show that 2,3,7,8-tetrachlorodibenzofuran is approximately one-tenth as toxic as 2,3,7,8-tetrachlorodibenzodioxin. Consequently, its toxic equivalent value is 0.1. International Toxicity Equivalency Factors have been developed for those dioxins and furans that contribute most to the toxicity of a complex mixture, which are those that have chlorines in at least the 2, 3, 7 and 8 positions. The toxicity is decreased from that of 2,3,7,8-tetrachlorodibenzodioxin in a predictable manner for those dioxins or furans which have either fewer or more chlorines than this compound, and/or which have chlorines in positions 1, 4, 6 or 9.

Table 4 International Toxicity Equivalency Factors (after NATO, 1988a)

Dioxin/Furan	Equivalency Factor
2,3,7,8-tetrachlorodibenzodioxin	1
1,2,3,7,8-pentachlorodibenzodioxin	0.5
1,2,3,4,7,8-hexachlorodibenzodioxin	0.1
1,2,3,7,8,9-hexachlorodibenzodioxin
1,2,3,6,7,8-hexachlorodibenzodioxin
1,2,4,6,7,8-heptachlorodibenzodioxin octachlorodibenzodioxin	0.01
2,3,7,8-tetrachlorodibenzofuran	0.001
2,3,4,7,8-pentachlorodibenzofuran	0.5
1,2,3,7,8-pentachlorodibenzofuran	0.05
1,2,3,4,7,8-hexachlorodibenzofuran	0.1
1,2,3,7,8,9-hexachlorodibenzofuran
1,2,3,6,7,8-hexachlorodibenzofuran
2,3,4,6,7,8-hexachlorodibenzofuran
1,2,3,4,6,7,8-heptachlorodibenzofuran	0.01
1,2,3,4,7,8,9-heptachlorodibenzofuran
octachlorodibenzofuran	0.001

Of the 210 dioxins and furans, 17 contribute most to the toxicity of a complex mixture and are of the greatest concern. This does not mean that the remaining 193 dioxins and furans are not toxic, but merely that they contribute comparatively little to the toxicity of a complex mixture.

⁶ Canada, Denmark, the Federal Republic of Germany, Italy, the Netherlands, Norway, the United Kingdom and the United States.

⁷ For a detailed review of this method, see NATO, 1988a, b.

⁸ Table 4; NATO, 1988a, b