Polychlorinated Dibenzodioxins and Polychlorinated Dibenzofurans - PSL1

Identity and Properties

Dioxins and furans are two large groups of chlorinated organic chemicals with properties that indicate a strong affinity for sediments and a high potential for accumulating in fish, birds, animals and humans.

Dioxins and furans are groups of chemicals whose structural formulae are shown in Figure 1. A dioxin or furan molecule can have as few as one, or as many as eight chlorine atoms attached to the molecule at any of eight locations.

Figure 1 Dioxin, Furan

Dioxin

Furan

It is both the number of chlorine atoms, and their position in the molecule, that determine the physical and chemical properties, as well as the toxic potency, of a given dioxin or furan. The most hazardous dioxin has chlorine atoms attached at positions 2, 3, 7 and 8, and is therefore referred to as "2,3,7,8-tetrachlorodibenzodioxin" (Figure 2).

Figure 2

2,3,7,8-tetrachlorodibenzodioxin

In total, there are 210 dioxins and furans: 75 from the dioxin group (Table 1), and 135 from the furan group (Table 2). The most studied of these is 2,3,7,8-tetrachlorodibenzodioxin, but information regarding selected characteristics of some of the others is also available.³

Table 1 Dioxins and the Numbering System used to name them (after NRCC, 1981a)

MONOCHLORO-(2)1		PENTACHLORO-(14)
1-	2-	1,2,3,4,6-	1,2,3,4,7-
		1,2,3,6,7-	1,2,3,6,8-
		1,2,3,6,9-	1,2,3,7,8-
DICHLORO-(10)		1,2,3,7,9-	1,2,3,8,9-
1,2- 1,3- 1,4-		1,2,4,6,7-	1,2,4,6,8-
1,6- 1,7- 1,8-		1,2,4,6,9-	1,2,4,7,8-
1,9- 2,3- 2,7-		1,2,4,7,9-	1,2,4,8,9-
2,8-
TRICHLORO-(14)		HEXACHLORO-(10)
1,2,3-	1,2,4-	1,2,3,4,6,7-
1,2,6-	1,2,7-	1,2,3,4,6,8-
1,2,8-	1,2,9-	1,2,3,4,6,9-
1,3,6-	1,3,7-	1,2,3,4,7,8-
1,3,8-	1,3,9-	1,2,3,6,7,8-
1,4,6-	1,4,7-	1,2,3,6,7,9-
1,7,8-	2,3,7-	1,2,3,6,8,9-
TETRACHLORO-(22)		1,2,3,7,8,9-
1,2,3,4-	1,2,3,6-	1,2,4,6,7,9-
1,2,3,7-	1,2,3,8-	1,2,4,6,8,9-
1,2,3,9-	1,2,4,6-	HEPTACHLORO-(2)
1,2,4,7-	1,2,4,8-	1,2,3,4,6,7,8-
1,2,4,9-	1,2,6,8-	1,2,3,4,6,7,9-
1,2,6,7-	1,2,7,8-	OCTACHLORO-(1)
1,2,6,9-	1,2,8,9-	1,2,3,4,6,7,8,9-
1,2,7,9-	1,3,6,9-
1,3,6,8-	1,3,7,9-
1,3,7,8-	1,4,7,8-
1,4,6,9-	2,3,7,8-

1 Number of Isomers in that group

Table 2 Furans and the Numbering System used to name them (after OME, 1985)

MONOCHLORO-(4)1				HEXACHLORO-(16)
1-	2-	3-	4-	1,2,3,4,6,7-	1,2,3,4,6,8-
				1,2,3,4,6,9-	1,2,3,4,7,8-
				1,2,3,4,7,9-	1,2,3,4,8,9-
DICHLORO-(16)				1,2,3,6,7,8-	1,2,3,6,7,9-
1,2-	1,3-	1,4-	1,6-	1,2,3,6,8,9-	1,2,3,7,8,9-
1,7-	1,8-	1,9-	2,3-	1,2,4,6,7,8-	1,2,4,6,7,9-
2,4-	2,6-	2,7-	2,8-	1,2,4,6,8,9-	1,3,4,6,7,8-
3,4-	3,6-	3,7-	4,6-	1,3,4,6,7,9-	2,3,4,6,7,8-
TRICHLORO-(28)				HEPTACHLORO-(4)
1,2,3-	1,2,4-	1,2,6-	1,2,7-	1,2,3,4,6,7,8-
1,2,8-	1,2,9-	1,3,4-	1,3,6-	1,2,3,4,6,7,9-
1,3,7-	1,3,8-	1,3,9-	1,4,6-	1,2,3,4,6,8,9-
1,4,7-	1,4,8-	1,4,9-	1,6,7-	1,2,3,4,7,8,9-
1,6,8-	1,7,8-	2,3,4-	2,3,6-	OCTACHLORO-(1)
2,3,7-	2,3,8-	2,4,6-	2,4,7-	1,2,3,4,6,7,8,9-
2,4,8-	2,6,8-	3,4,6-	3,4,7-
TETRACHLORO-(38)
1,2,3,4-	1,2,3,6-	1,2,3,7-
1,2,3,8-	1,2,3,9-	1,2,4,6-
1,2,4,7-	1,2,4,8-	1,2,4,9-
1,2,6,7-	1,2,6,8-	1,2,6,9-
1,2,7,8-	1,2,7,9-	1,2,8,9-
1,3,4,6-	1,3,4,7-	1,3,4,8-
1,3,4,9-	1,3,6,7-	1,3,6,8-
1,3,6,9-	1,3,7,8-	1,3,7,9-
1,4,6,7-	1,4,6,8-	1,4,6,9-
1,4,7,8-	1,6,7,8-	2,3,4,6-
2,3,4,7-	2,3,4,8-	2,3,6,7-
2,3,7,8-	2,3,8,9-	2,4,6,7-
2,4,6,8-	3,4,6,7-
PENTACHLORO-(28)
1,2,3,4,6-	1,2,3,4,7-	1,2,3,4,8-	1,2,3,4,9-
1,2,3,6,7-	1,2,3,6,8-	1,2,3,6,9-	1,2,3,7,8-
1,2,3,7,9-	1,2,3,8,9-	1,2,4,6,7-	1,2,4,6,8-
1,2,6,7,8-	1,2,6,7,9-	1,3,4,6,7-	1,3,4,6,8-
1,2,4,6,9-	1,2,4,7,8-	1,3,4,7,9-	1,2,4,8,9-
1,3,4,6,9-	1,3,4,7,8-	1,3,4,7,9-	1,3,6,7,8-
1,4,6,7,8-	2,3,4,6,7-	2,3,4,6,8-	2,3,4,7,8-

¹ Number of Isomers in that group.

The fate of contaminants in the environment depends on both the properties of the chemical and the properties of the ecosystem itself.⁴ One can make predictions about how dioxins and furans partition among water, soil, sediments, plants and animals by examining such properties as the contaminants' molecular weights, vapour pressures, and solubilities in water. One measure that is particularly relevant for predicting the environmental fate of dioxins and furans is the octanol-water partition coefficient. A high octanol-water value means that a chemical is more attracted to animal or human fat tissues; compounds with a high octanol-water partition coefficient have a low water solubility and tend to accumulate in soils, aquatic sediments and animals. The octanol-water partition coefficient of dioxins and furans increases as the number of chlorine atoms in the molecule increases (see Table 3). Therefore, the higher the chlorine content of the dioxins and furans, the greater their potential to accumulate in sediments, soils or living organisms. However, after the log octanol-water coefficient reaches a value of about 6, increased molecular size and decreased solubility usually result in decreased bioconcentration. Substances with high octanol-water coefficients also tend to be singly bound to sediments and therefore generally unavailable to animals.⁵

Table 3 Log Octanol-Water Partition Coefficients for selected dioxins (after SHIU et al., 1988)

1-chlorodibenzodioxin	4.75
2-chlorodibenzodioxin	5.00
2,3-dich1orodibenzodioxin	5.60
2,7-dich1orodibenzodioxin	5.75
2,8-dich1orodibenzodioxin	5.60
1,2,4-trich1orodibenzodioxin	6.35
1,2,3,4-tetrach1orodibenzodioxin	6.60
1,2,3,7-tetrachlorodibenzodioxin	6.90
1,3,6,8-tetrach1orodibenzodioxin	7.10
1,3,7,8-tetrach1orodibenzodioxin	6.80
1,2,3,4,7-pentachlorodibenzodioxin	7.40
1,2,3,4,7,8-hexachlorodibenzodioxin	7.80
1 ,2,3,4,6,7,8-heptachlorodibenzodioxin	8.00
octachlorodibenzodioxin	8.20

³ Recent estimates of the physical and chemical characteristics of 2,3,7,8-tetrachlorodibenzodioxin are given in U.S.EPA, 1988. Information on selected properties of some other dioxins and furans is also available (Shiu et al., 1988; U.S.EPA, 1988).

⁴ Roberts and Boddington, 1983.

⁵ Connell and Hawker, 1988.