 The electric industry used PCBs as a non-flammable replacement for mineral oil to cool and insulate industrial transformers and capacitors.PCBs were also commonly used as heat stabilizer in cables and electronic components to enhance the heat and fire resistance of PVC.In the 1930s, the toxicity associated with PCBs and other chlorinated hydrocarbons, including polychlorinated naphthalenes, was recognized because of a variety of industrial incidents.