Any of a class of highly stable organic compounds prepared by the reaction of chlorine with biphenyl, a two-ring compound. The commercial product, a mix of several PCB isomers, is a colourless, viscous liquid that is almost insoluble in water, does not degrade under high temperatures, and is a good electrical insulator (seedielectric). PCBs became widely used as lubricants, heat-transfer fluids, and fire-resistant dielectric fluids in transformers and capacitors in the 1930s and '40s. In the mid 1970s they were found to cause liver dysfunction in humans and came under suspicion as carcinogens; their manufacture and use were consequently restricted in the U.S. and many other countries, though illegal dumping by manufacturers continued. They persist in the environment and have entered the food chain, causing great harm especially to invertebrates and fish.