phenyls (three chlorines) and tetrachlorobiphenyls (four chlorines)). The PCBs within a series of structures of specific chlorine content are known as homologues (i.e., the mono-, di-, tri-, tetra-, penta-, hexa-, hepta-, octa-, nona-, and decachlorobiphenyl homologues). Within a homologue group (e.g., the trichlorobiphenyls), the individual chlorobiphenyl molecules are isomers of each other, meaning that they each have the same number of chlorine atoms, but these chlorine atoms are arranged at different positions on the biphenyl rings. Examples of chemical structures of PCBs are provided in Figures 2–2 and 2–3. A complete list of congeners is in Appendix H. Since the chlorine atom is part of the group of elements known as halogens (others are fluorine, bromine, and iodine), polychlorinated biphenyls are part of a larger group of chemicals known as halogenated aromatic compounds.
Industrial PCBs were complex mixtures composed of up to 50 or 60 congeners (or individual chlorobiphenyls). The composition of the PCB mixture was governed by the reaction conditions and the reaction properties by which they were manufactured. These conditions and properties favor the production of specific congeners; thus, there are different relative proportions of congeners within given industrial mixtures. These mixtures exist as liquids to viscous solids. Between 1930 and 1977, when their industrial manufacture was banned in the United States, these mixtures were produced almost exclusively by Monsanto under the commercial name of Aroclors. Each Aroclor has a code number (e.g., Aroclor 1242, Aroclor 1248, and Aroclor 1254), the