9.2. CHEMICAL STRUCTURE AND PROPERTIES

Polychlorinated dibenzodioxins (PCDDs), polychlorinated dibenzofurans (PCDFs), and polychlorinated biphenyls (PCBs) are chemically classified as halogenated aromatic hydrocarbons. The chlorinated and brominated dibenzodioxins and dibenzofurans are tricyclic aromatic compounds with similar physical and chemical properties, and both classes are similar structurally. Certain of the PCBs (the so-called coplanar or mono-ortho coplanar congeners) are also structurally and conformationally similar. The most widely studied of these compounds is 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). This compound, often called simply dioxin, represents the reference compound for this class of compounds. The structure of TCDD and several related compounds is shown in Figure 9-1.

For purposes of this document, dioxin-like compounds are defined to include the subset of this class of compounds, which are generally agreed to produce dioxin-like toxicity. These compounds are assigned individual toxicity equivalence factor (TEF) values as defined by international convention (U.S. EPA, 1989). Results of in vitro and in vivo laboratory studies contribute to the assignment of a relative toxicity value. TEFs are estimates of the toxicity of dioxin-like compounds relative to the toxicity of TCDD, which is assigned a TEF of 1.0. All chlorinated dibenzodioxins (CDDs) and chlorinated dibenzofurans (CDFs) with chlorines substituted in the 2,3,7, and 8 positions are assigned TEF values. Additionally, the analogous brominated dioxins and furans (BDDs and BDFs) and certain polychlorinated biphenyls have recently been identified as having dioxin-like toxicity and thus are also included in the definition of dioxin-like compounds. Generally accepted TEF values for chlorinated dibenzodioxins and dibenzofurans are shown in Table 9-1 A recent World Health Organization/International Program on Chemical Safety meeting held in The Netherlands in December 1993 considered the need to derive internationally acceptable interim TEFs for the dioxin-like PCBs. Recommendations arising from that meeting of experts (Ahlborg et al., 1994) suggest that in general only a few of the dioxin-like PCBs are likely to be significant contributors to general population exposures to dioxin-like

compounds. Dioxin-like PCBs may be responsible for approximately one-fourth to one-half of the total toxicity equivalence associated with general population environmental exposures to this class of related compounds. Both the refinement of the toxicity equivalence factors for dioxin-like PCB congeners (DeVito et al., 1993) as well as a compilation and analysis of all available data on relative toxicities of dioxin-like PCBs with respect to a number of end points (Ahlborg et al., 1994) support these findings. Although these findings have been published recently, additional review and data collection will be needed. In addition, this panel urged investigation of companion TEFs for ecotoxicological use, based on data from ecotoxicological studies.

Throughout this document, concentrations of dioxin and related compounds will be presented as TEQs. TEQs are determined by summing the products of multiplying concentrations of individual dioxin-like compounds times the corresponding TEF for that compound. At times, levels will be presented as concentrations of TCDD because many past studies monitored this congener alone. At most times, TEQs for CDDs and CDFs will be discussed. When TEQ values include the dioxin-like PCBs as well, this will be specifically mentioned. Readers of this chapter are encouraged to review previous chapters in the Health Assessment Document and the Exposure Document for more details on estimates of TEQ presented in this chapter. The strengths and weaknesses as well as the uncertainties associated with the TEF/TEQ approach are discussed later in this chapter.

There are 75 individual compounds comprising the CDDs, depending on the positioning of the chlorine(s), and 135 different CDFs. These are called individual congeners. Likewise, there are 75 different positional congeners of BDDs and 135 different congeners of BDFs (see Exposure Document, Table 2-1). Only 7 of the 75 congeners of CDDs or of BDDs are thought to have dioxin-like toxicity; these are ones with chlorine/bromine substitutions in, at least, the 2, 3, 7, and 8 positions. Only 10 of the 135 possible congeners of CDFs or of BDFs are thought to have dioxin-like toxicity; these also are ones with substitutions in the 2, 3, 7, and 8 positions. While this suggests 34 individual CDDs, CDFs, BDDs, or BDFs with dioxin-like toxicity, inclusion of the mixed chloro/bromo congeners substantially increases the number of possible congeners with dioxin-like activity. There are 209 PCB congeners. Only 13 of the 209 congeners are thought to have dioxin-like toxicity; these are PCBs with 4 or more chlorines with just 1 or no substitution in the ortho position. These compounds are sometimes referred to as coplanar, meaning that they can assume a flat configuration with rings in the same plane. Similarly configured polybrominated biphenyls are likely to have similar properties; however, the data base on these compounds with regard to dioxin-like activity has been less extensively evaluated. Mixed chlorinated and brominated congeners also exist, increasing the number of compounds considered dioxin-like. The physical/chemical properties of each congener vary according to the degree and position of chlorine and/or bromine substitution. Very little is known about occurrence and toxicity of the mixed (chlorinated and brominated) dioxin, furan, and biphenyl congeners.

In general, these compounds have very low water solubility, high octanol-water partition coefficients, and low vapor pressure and tend to bioaccumulate. Volume II of the Exposure Document presents congener-specific values for water solubility, vapor pressure, partition coefficients, and photo quantum yields and discusses other physicochemical characteristics of the chlorinated dioxins and dibenzofurans. These physicochemical properties result in the environmental fate and transport discussed below. Expanded discussions will be required in future documents to account for dioxin-like PCBs and for brominated or mixed halogenated congeners.