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Polychlorinated biphenyls (PCBs) are one of the most widely studied environmental contaminants, and many studies in animals and human populations have been performed to assess the potential carcinogenicity of PCBs. PCBs are mixtures of up to 209 individual chlorinated compounds (known as congeners or chlorinated hydrocarbons), which are no longer produced in the United States, but are still found in the environment. Exposure to PCBs includes acne-like skin conditions in adults and neurobehavioral and immune alterations in children. PCBs are carcinogenic to animals, and they have been found in at least 500 of the 1598 National Priorities List sites identified by the US Environmental Protection Agency (EPA). PCBs (US trade name Aroclor) have no known natural sources, are either oily liquids or solids that are colorless to light yellow, and can exist as a vapor in air. PCBs typically do not exhibit any smell or taste.
The US Toxic Substances Control Act (TSCA) was enacted by the US Congress to control the distribution, use, and disposal of harmful chemicals, including PCBs. PCBs have been demonstrated to cause a variety of adverse health effects. Studies in humans provide supportive evidence for potential carcinogenic and noncarcinogenic effects of PCBs, and based on the weight of the data, it is suggested that PCBs are probable human carcinogens.

PCBs are a class of biphenyl compounds with one to ten hydrogen atoms replaced by chlorine. At room temperature, they range in physical state from light- to dark-yellow oily liquids to white crystalline solids and hard noncrystalline resins (IPCS 1992, HSDB 2009). PCBs are produced commercially by chlorination of biphenyl, resulting in 209 possible PCB congeners (Silberhorn et al. 1990). However, McFarland and Clarke (1989) reported that about half of these molecules accounted for nearly all environmental contamination by PCBs, and they considered only 36 to be environmentally relevant, because of their potential toxicity, environmental prevalence, and relative abundance in animal tissues. Commercial PCB formulations are complex mixtures of chlorinated biphenyls that vary in the degree of chlorination, and similar mixtures can show significant lotto-lot variation in composition (ATSDR 2000). Of the 209 possible PCB congeners, about 100 are present in commercial PCB mixtures, and about 70 have been detected in human adipose tissue (Mühlebach et al. 1991). At least 20 of the 209 possible congeners have not been identified in commercial mixtures of PCBs (Kimbrough 1987).
Physical and chemical properties of PCBs are affected by the numbers and positions of chlorine atoms (Carpenter 2006). PCBs with fewer chlorine atoms tend to be more soluble in water, more volatile, and more easily metabolized. Larger numbers of chlorine atoms are associated with increased resistance to biodegradation, which can increase bioaccumulation in the environment. PCBs are practically insoluble in water, but soluble in organic solvents and fats (IPCS 1992). They are very stable and persistent in the environment. Physical and chemical properties representative of PCBs are listed in the following table.
PCBs have been categorized as “dioxinlike” or “non-dioxinlike,” based on their ability to exert biochemical and toxic effects similar to those of TCDD through activation of the aryl hydrocarbon receptor (Carpenter 2006, Knerr and Schrenk 2006). Dioxin-like activity is seen for PCB congeners with chlorine atoms occupying meta (carbon atoms 3, 3′, 5, or 5′) and para (carbon atoms 4 or 4′) positions, with no more than one ortho (carbon atoms 2, 2′,6,or 6′) chlorine; these molecules are likely to exist with a planar conformation. Twelve tetra-, penta-, hexa-, or hepta-chlorobiphenyls meet these criteria and have been assigned toxic equivalency factors (TEQs) of 0.0001 to 0.1 by the World Health Organization based on their toxicity relative to that of TCDD, which has the highest toxic potency for activation of the aryl hydrocarbon receptor (TEQ = 1.0). PCB 126 has a TEQ of 0.1, which is the highest value for this class of molecules.

Before 1974, PCBs were used in the United States for both enclosed applications, such as transformers, capacitors, and heat transfer and hydraulic fluids, and open applications, such as inks, flame retardants, adhesives, carbonless duplicating paper, paints, plasticizers, wire insulators, metal coatings, and pesticide extenders (IARC 1978, ATSDR 2000). After 1974, all uses of PCBs were limited to enclosed applications (transformers and capacitors), and after January 1979, no PCBs were used in the manufacturing of transformers or capacitors. The only remaining permitted uses of PCBs are as a mounting medium in microscopy, as an immersion oil in low-fluorescence microscopy, as an optical liquid, and for other research and development purposes (ATSDR 2000).

PCBs were used as coolants, lubricants, and insulators in electrical capacitors, electrical transformers, vacuum pumps, and gas transmission turbines. They have also been used as hydraulic fluids, plasticizers, adhesives, fire retardants in plastics, wax extenders, lubricants and cutting oils, inks, and cleaning agents. PCBs were banned from production internationally in the late 1970s, but are still widespread as contaminants in the environment. They are also produced unintentionally in the production of organic chlorinated herbicides, bleaching of papers, in the incineration of wastes, and in industrial and thermal processes involving chlorine and chlorinated chemicals.

ChEBI: 3,4,5,3',4',5'-Hexachlorobiphenyl is a trichlorobenzene and a hexachlorobiphenyl.

PCBs are synthesized by the chlorination of biphenyl and the resulting products are designated according to their percent (by weight) chlorine content (2). For example, Aroclors 1221, 1242, and 1260 contain 21, 42, and 60 wt% chlorine. The commercial Aroclors were produced by the Monsanto Chemical Corp. and similar PCB mixtures were manufactured worldwide by other chemical companies. Over 600 million kg of commercial PCBs were produced in the United States and the estimated worldwide production is approximately double this quantity (Table 1). Properties of the commercial PCBs varied from highly fluid liquids (Aroclor 1221) to viscous liquids or solids. All of these preparations contained a complex mixture of isomers and congeners and as the degree of chlorination increased there was a corresponding increase in the relative concentrations of the more highly chlorinated congeners. There are 209 possible PCBs and the properties of these commercial mixtures and the individual PCBs have been extensively investigated. More recent studies indicate that the commercial PCBs contained 132 different compounds (18).

Several studies have reported relatively high levels of PCBs in the serum or adipose tissues of occupationallyexposed individuals, e.g.,>3000 ppb in the serum (57 58). Not surprisingly, after these exposures were terminated, the PCB serum concentrations tended to decrease (59 61). Chloracne and related skin problems have been observed in several groups of workers and it was suggested that the air concentrations of commercial PCBs > 0.2 mg/m3 were associated with this effect (62). It was also reported that after occupational exposure to PCBs was terminated there was a gradual decrease in the severity and number of dermatological problems in the exposed workers, and this paralleled a decrease in their serum levels of PCBs (61). The effects of occupational exposure to PCBs on the concentrations of several serum clinical, chemical, and hematological parameters have been reported (58). Mildly elevated SGOT and γ -glutamyl transpeptidase (GGTP) suggest some liver damage and induction of hepatic monooxygenase enzymes; these results are similar to those observed in animal studies. In one study, it was reported that as PCB serum levels decreased over time the GGTP serum levels also decreased to normal values.

Polychlorinated biphenyls (PCBs) are reasonably anticipated to be human carcinogens based on sufficient evidence of carcinogenicity from studies in experimental animals. Not all PCB mixtures caused tumors in experimental animals.

Before being banned and before the US Clean Water Act regulated wastewater discharges, PCBs could be found, often at high levels, in wastewaters from industries handling PCB equipment. These wastewaters either were discharged directly to surface waters or sent to municipal sewage treatment plants. Urban industrial areas are more likely to have higher PCB contamination than rural areas. While not highly volatile, PCBs, especially the less chlorinated ones, will partition into the air. Atmospheric transport is the most important mechanism for dispersion of PCBs.
Those PCBs with a high degree of chlorination are much more persistent in the environment than those with lower degrees of chlorination because they are more resistant to metabolism. Microbial metabolism is the most important mechanism for the removal of persistent organic pollutants as the PCBs from the environment. Anaerobic dehalogenation of the highly chlorinated PCBs in aquatic sediments is a major mechanism for their elimination by generating lower chlorinated congeners that are more readily metabolized by aerobic enzymes. As a consequence, the environmental levels of PCBs are slowly decreasing with time.

PCBs and related halogenated aromatic hydrocarbons elicit a diverse spectrum of toxic and biochemical responses in laboratory animals dependent on a number of factors including age, sex, species, and strain of the test animal and the dosing regimen (single or multiple) (27 32). The toxic responses elicited by most PCB preparations are also observed for other classes of HAHs (27–32) and include a progressive weight loss not simply related to decreased food consumption and accompanied by weakness, debilitation, and ultimately death, i.e., a wasting syndrome; lymphoid involution, thymic and splenic atrophy with associated humoral and/or cellmediated immunosuppression and/or associated bone marrow and hematologic dyscrasia; a skin disorder called chloracne accompanied by acneform eruptions, alopecia, edema, hyperkeratosis, and blepharitis resulting from hypertrophy of the Meibomian glands; hyperplasia of the epithelial lining of the extrahepatic bile duct, the gall bladder, and urinary tract; hepatomegaly and liver damage accompanied by necrosis, hemorrhage, and intrahepatic bile duct hyperplasia; hepatotoxicity also manifested by the development of porphyria and altered metabolism of porphyrins; teratogenesis, developmental and reproductive toxicity observed in several animal species: Carcinogenesis caused by PCBs in laboratory animals is primarily associated with their effects as promoters. Endocrine and reproductive dysfunction, i.e.,altered plasma levels of steroid and thyroid hormones with menstrual irregularities, reduced conception rate, early abortion, excessive menstrual and postconceptional hemorrhage, and anovulation in females, and testicular atrophy and decreased spermatogenesis in males have also been reported in some species.