Polychlorinated Biphenyls (PCBs) are a group of synthetic chlorinated organic compounds, which were commonly used as dielectric fluids in electrical equipment such as transformers and capacitors. PCBs include known carcinogens and can bioaccumulate.
They have also been used as heat exchange fluids and hydraulic fluids. PCBs are very stable chemicals that resist change due to fire or mixture with other chemicals. This characteristic of PCBs can cause problems for disposal as they are not easily broken down in the environment.