PCBs, known to cause cancer in animals and believed to cause cancer in humans, are among the most widespread and hazardous synthetic pollutants. They comprise a group of 209 structurally similar compounds, so-called congeners. The individual congeners differ in the degree of chlorination and the positions of the chlorine atoms in the molecule. They are numbered from one to 209 according to a scheme proposed by Ballschmiter and Zell (hence, the term BZ numbers).

PCBs are obtained by the controlled reaction of biphenyl with elemental chlorine under the catalytic influence of iron or iron chloride. The commercial formulations were mixtures of various congeners. They were marketed under trade names such as Aroclor, Pyranol, and Clophen and were characterized by their average chlorine content, ranging from 21 to 68 percent. The pattern of congener composition is characteristic for each product and may serve to identify the source of a...