The Toxicity of PCBs

The acute toxicity of PCB mixtures to vertebrates tends to be low, typically, 1-10 g/ kg to rats. The concern is about sublethal and chronic toxicity. Different PCB congeners show different modes of toxic action, so it is not surprising that their mixtures can produce a disconcerting range of toxic effects (Robertson and Hansen 2001). 

It has come to be suggested that coplanar congeners as a group express toxicity through a common mechanism interaction with the cytosolic Ah receptor (Safe 1990 Ahlborg et al. 1994) (Environmental Health Criteria 140). Although the full 

Organic Pollutants An Ecotoxicological Perspective, Second Edition 

Ah-receptor-mediated toxicity is particularly associated with the highly toxic compound 2,3,7,8-tetrachloro-dibenzo-p-dioxin (TCDD), commonly referred to as dioxin. TCDD, and the concept of toxicity equivalency factors (TEFs) based on TCDDs, will be dealt with in Chapter 7. The main point to make at this juncture is that the toxicity of each individual coplanar congener in a mixture can be expressed in terms of a toxic equivalent calculated relative to the toxicity of dioxin. Summation of the toxic equivalents of the individual coplanar PCBs gives a measure of the toxicity of the whole mixture, as expressed through the Ah receptor mechanism. 

In mammals, there is evidence that hydroxy PCBs are transferred across the placenta into the fetus, where they accumulate (Morse et al. 1995). In an experiment with pregnant rats exposed to 3,3, 4,4 -TCB, substantial levels of 4 OH,3,3, 4,5 -TCB accumulated in the fetus, with concomitant reduction in levels of T4. This was due to competitive binding of the PCB metabolite to TTR, thereby excluding T4. Similar 

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The toxicity of PCBs is determined by the number and position of chlorine atoms on the biphenyl molecule. When the ortho-position contains no or 1 to 2 chlorine atoms, the molecule can assume a planar position and is associated with a higher degree of toxicity. This coplanar subgroup of PCBs is therefore a higher concern and provides a synthetic chemist with guidance on avoiding similar structural features in a new or redesigned molecule. 

The toxicity of PCBs and PCDDs/DFs is complicated by the presence of large numbers of congeners, each with its own toxicity. Commercial PCB mixtiues ehcit a broad spectrum of toxic responses that are dependent on several... 

The nos. of single components containing are chosen according to the classification of Ballschmiter and Zell [7]. Their results of former investigations showed that for judgement of the toxicity of PCB it is necessary to know their components. PCB in the environment is a mixture of those isomeres found in the region of with main contents of the technical products known as chlophen A30 (30% of chlorine), chlophen A60 (60% of chlorine) and aro-chlor 1256 (56% of chlorine). 

The toxicity of PCB mixtures i,n vivo and In vitro is largely attributable to tetra-, penta- and hexachloro congeners that are capable of assuming a planar conformation (57,58,59). Mono ortho-chloro, coplanar congeners mimic the toxicological effects of... 

Each PCB congener is named according to the positions of chlorine substitution on the two phenyl rings of biphenyl, and the toxicity of PCBs correlates strongly with their structures. The toxicity of PCB mixtures is due principally to a small group of... 

In static renewal tests with embryos and larvae of seven species of fish and amphibians, the toxicity of PCB compounds (i.e.. Capacitor 21, Aroclor 1016, i roclor 1242, Aroclor 1254) was found to increase with percent chlorination 20). The more chlorinated PCBs also exhibit longer environmental half-life (757). The combination of high toxicity and persistence in aquatic ecosystems renders these compounds especially hazardous. Despite curtailment of production in the United States, PCBs still in use or disposed of in recent years continue to pose a problem to water quality. These compounds not only exhibit high toxicity to eggs, embryos, and larvae of aquatic species, but also affect sex hormones and the endocrine mechanisms which regulate reproduction in fish 82). 

Polychlorinated biphenyls are highly persistent in the environment. and. being fat soluble, tend to accumulate in the food chain. PCBs have been found in rainwater, in many species of fish, birds, and other animals (including polar bears) all over the globe, and in human tissue. Fish that feed in PCB-contaminated waters, for example, have PCB levels 1000-100,000 times the level of the surrounding water, and this amount is further magnified in birds that feed on the fish. The toxicity of PCBs depends on the composition of the individual mixture. 

The toxicity of PCBs depends not only on the degree of chlorination but also on the types of isomer. For example, PCBs devoid of orthosubstitution but heavily substituted at the meta and para positions can assume a planar conformation that can interact with the same receptor as TCDD. Examples of these isomers are 3,3, 4,4 -tetrachlorobiphenyl 3,3, 4,4, 5-pentachlorobiphenyl and 3,3, 4,4, 5,5 -hexachlorobiphenyl (NIOSH, 1977). 

Polychlorinated napthalenes (PCNs) have been characterized in commercial PCBs and probably resulted from the presence of napthalene as an impurity in the biphenyl raw material used for the manufacturing process. Typical concentrations in PCB fluids are less than 600 ppm. Results obtained by Kimbrough (1981) and Goldstein (1980) indicated that it is unlikely that PCNs would add significantly to the toxicity of PCBs at the concentration levels typically observed. 

Polychlorinated biphenyls (PCBs) are compounds that were once used in a variety of electrical devices, industrial applications, and polymers. Their use and production were banned in 1979, however, owing to the toxicity of PCBs and their tendency to accumulate in the food chain. 

As experimental toxicity data for most PCBs are not known, the toxicities of PCBs as determined by theoretical calculation carried out by Kafafi et al. [39] have been considered. In Table 2, the results of Kafafi et al. [39] are reviewed as a function of the total number of chlorine atoms substituted in the PCB mole cule and the number of chlorine atoms substituted in ortho positions only. The values given in Table 2 are those suggested by Safe [38] and correspond to a... 

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