PCB was the main component of transformer oil for many years, but its use has been banned for a good while now.
ETA, The ban was one of the main reasons the Pennsylvania railroad took those gorgeous GG-1 electric locomotives out of service.
Beautious engine!
They were used in lot's of things:
Adhesives
Asphalt Roofing Materials
Carbonless Copy Paper
Caulking
Compressor Oil
De-dusting Agents
Dielectric Fluid
Dyes
Electromagnets
Fluorescent Light Ballasts
Grout
Heat Transfer Fluid
Inks
Insulating Coatings - Military Applications - Mixed with Asbestos
Lubricants
Natural Gas Pipeline
Paints
Pesticides
Plasticizers
Rubberizers
Space Heaters
Submersible Well Pumps
Tar Paper
Wax Extenders
Whether China abides our dislike of pcb's...?