Halogenated Biphenyls

Abstract

Polyhalogenated biphenyls are a family of compounds based on biphenyl as the parent compound. Chlorinated and brominated biphenyls have been manufactured for commercial purposes. There are 209 possible structural congeners of polychlorinated biphenyl (PCB), and for polybrominated biphenyls (PBBs) the same number of congeners can exist. A listing of all PCB congeners is given along with their Chemical Abstract Services (CAS) Registry numbers—an identifying numbering system. PCBs used commercially are complex mixtures consisting of various PCB congeners and isomers. It has been estimated that in the United States a total of 1400 million lb of PCBs was manufactured of which 150 million lb was exported and 3 million lb was imported. Because of their nonflammability, thermal stability, outstanding dielectric properties, and resistance to chemical reactivity, PCBs have been used extensively as dielectric fluids in liquid‐cooled electric motors, transformers, and capacitors; electromagnets, vacuum pumps, gas turbines, and hydraulic fluids; heat‐transfer systems; voltage regulators, liquid‐filled cable, fluorescent light ballasts, and electronic equipment; as plasticizers in synthetic resins, adhesives, surface coatings, sealants, printing and carbonless copying papers, pesticide extenders, inks, lubricants, and cutting oils. PCBs have also been used in slide mounting medium for oil immersion lens microscopy. Although PCBs are no longer produced and their use has been discontinued in most countries, many old equipments filled with PCBs are still in service today. Worldwide approximately 800,000 tons of PCBs is still in use. These equipments are the new sources of environmental contamination when PCBs start leaking or when the equipments are disposed of. Commercial production of polybrominated biphenyls in the United States began in 1970 and was discontinued in early 1977. The general population may be exposed to PCBs from a variety of sources, including food, ambient air, occupational settings, and consumer products. The major exposure routes to humans are through food. Accidental oral exposures to PCBs have been reported. In all cases, total PCB levels must be based on specific congener analysis or direct perchlorination rather than in terms of Aroclors because the congener patterns in environmental media and biological tissues usually do not match those in Aroclor fluids unless massive contamination has occurred (typical of spills and some occupational situations). Thus, predictive models based on specific congener data must also be utilized. The less chlorinated congeners predominate in air samples from known contaminated areas and in water and wet deposition samples with temperature and the amount of sediment in river and water samples being important covariables. In contrast, the more highly chlorinated isomers with substituents at the 2,4,5 or 2′,4′,5′ positions tend to bioaccumulate in some crop vegetables, game animals, fish, and human tissue samples. PCBs in contaminated soils can be absorbed by plants and vegetables with shallow root systems to PCB contamination, although volatilization in this situation is also favored; erosion of such soils will also cause contamination of sediments. The more chlorinated congeners dominate in soils and sediments and the resident biota (cash crops, vegetables, fish, aquatic life). The absolute levels in any situation depend on which of the competing processes dominates. PCB mixtures produce low to moderate acute toxicity in mammalian species, but produce pronounced subacute and chronic toxicity. In contrast, invertebrates exhibit greater acute toxicity to PCBs (LC 50 < 1 mg/L). In addition, as reported for other halogenated aromatic hydrocarbons, PCBs exhibit significant interspecies variability in toxicity. In considering the health effects of PCBs in animals, it is important to consider the isomer and congener composition of the PCBs, potential impurities, the length of exposure, and the species under investigation.