Polychlorinated biphenyl molecule. Molecular model of the polychlorinated biphenyl (PCB) 3,4,5,3'4'5'-hexachlorobiphenyl. The atoms are represented as spheres and are colour-coded: carbon (blue), hydrogen (white) and chlorine (green). PCBs are organic compounds that are highly stable, good insulators and have low electrical conductivity. They were widely used in capacitors and transformers. The discovery, in the 1960s, that PCBs are toxic and carcinogenic, and accumulate in the food chain led to tight regulation of their use and disposal, and phasing out of their production.