Polychlorinated Dibenzodioxins and Polychlorinated Dibenzofurans - Organic Pollutants: An Ecotoxicological Perspective - page 151

7 Polychlorinated

Dibenzodioxins
and Polychlorinated
Dibenzofurans

7.1 Background
Neither of these groups of polychlorinated lipophilic compounds occurs naturally,
and neither of them is synthesized intentionally. Both occur as by-products of chemi-
cal synthesis, industrial processes, and occasionally of interaction between other
organic contaminants in the environment. Because of human exposure to them
during the Vietnam War, industrial accidents, and the high mammalian toxicity
of some of them, they have received much attention as hazards to human health.
There has also been concern about some polychlorinated dibenzodioxins (PCDDs)
and polychlorinated dibenzofurans (PCDFs) from an ecotoxicological point of view
because they combine marked biological persistence with high toxicity. The follow-
ing account will deal mainly with PCDDs, which have been studied in some detail
and have been important in developing the concept of Ah receptor-mediated toxicity.
Less is known about PCDFs, which will be described only briefly.

7.2 orIgInS and cHemIcaL ProPertIeS
PCDDs are polychlorinated planar molecules with an underlying structure of two
benzene rings linked together by two bridging oxygens, thus creating a third “dioxin”
ring (Figure 7.1). They are sometimes simply termed dioxins (Environmental Health
Criteria 88). In theory, there are 75 different congeners, but only a few of them are
regarded as being important from an ecotoxicological point of view. 2,3,7,8-tetra-
chlorodibenzodioxin (TCDD) has received far more attention than the others due to
its high toxicity and persistence and the detection of significant levels in the environ-
ment (structure given in Figure 7.1). They are formed when o -chlorophenols, or their
alkali metal salts, are heated to a high temperature (see Crosby 1998). The formation
of 2,3,7,8-TCDD from an orthotrichlorophenol is shown in Figure 7.1.
PCDDs have been released into the environment in a number of different ways.
Sometimes this has been due to the use of a pesticide that is contaminated with
them. 2,4,5-T and related phenoxyalkanoic herbicides have been contaminated with
them as a consequence of the interaction of chlorophenols used in the manufacturing

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