Mixture of compounds having from one to 10 chlorine atoms attached to a biphenyl ring structure. There are 209 possible structures theoretically; the manufacturing process results in approximately 120 different structures. PCBs resist biological and heat degradation and were used in numerous applications, including dielectric fluids in capacitors and transformers, heat transfer fluids, hydraulic fluids, plasticizers, dedusting agents, adhesives, dye carriers in carbonless copy paper, and pesticide extenders. The United States manufactured PCBs from 1929-1977, when they were banned due to adverse environmental effects and ubiquitous occurrence. They bioaccumulate in organisms and can cause skin disorders, liver dysfunction, reproductive disorders, and tumor formation. They are one of the most abundant organochlorine contaminants found throughout the world.