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Không phải của các nhóm hợp chất polychlorinated tan trong mỡ xảy ra tự nhiên, và không phải của họ được tổng hợp cố ý. Cả hai xảy ra như các sản phẩm tổng hợp hóa học, quy trình công nghiệp, và đôi khi sự tương tác giữa các chất gây ô nhiễm hữu cơ trong môi trường. Bởi vì con người tiếp xúc với họ trong thời gian chiến tranh Việt Nam, tai nạn công nghiệp, và độc tính của động vật có vú cao của một số người trong số họ, họ đã nhận được nhiều sự chú. | 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 chemical 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 following 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 environment 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
