Strictly defined, dioxins are a class of compounds consisting of 75 PCDD (dioxins) and 135 PCDF (fUran) compounds (collectively referred to as dioxin congeners). The PCDD molecule consists of two phenyl rings joined by two oxygen bridges. The PCDF molecule comprises two phenyl rings joined by one oxygen bridge and one single bond (Figure 1). The individual dioxin congeners differ in their patterns of chlorine substitution. The degree of chlorination and the pattern of substitution on the two phenyl rings affect the stereochemistry of the congener, and are responsible for inter-congener differences in environmental behavior and toxicity. The 17 dioxin and furan compounds substituted only at the
2-, 3-, 7-, or 8-positions are widely considered the most toxic to humans and biota.
Increasingly, the term 'dioxin-like' is used to describe compounds that share structural similarities to PCDDs and PCDFs and share a common mechanism of toxic action. At present, the term is most often used to describe four non-ortho- and eight mono-ortho-substituted polychlorinated biphenyls (PCBs; see Figure 1). For simplicity, in this article, the 29 dioxin-like compounds (17 dioxin and furan congeners plus 12 PCB congeners) will be referred to as 'dioxins', unless a distinction is necessary to distinguish between individual compounds, homolog groups (i.e., mono- to octachlorinated congeners), or classes of congeners (i.e., PCDDs, PCDFs, and PCBs). Table 1 provides information on the physical and chemical properties of dioxins and dioxin-like compounds.
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