PCBs - PolyChlorinated Biphenyls - were manufactured by a one company - Monsanto Chemical - starting in the 1930s. They were promoted as a nearly indestructible replacement for hydraulic oil, pump oil, and the oil bath for electrical transformers and capacitors. It was known even in the 1930s, when production began, that they were extremely toxic. Exposed workers began showing signs of toxic exposure almost immediately.
The two largest users of PCBs were General Electric (Pittsfield, MA and Corning, NY) and Westinghouse Electric (Bloomington, Indiana) in their transformer manufacturing operations. Air cooling of transformers was difficult, expensive, and unreliable, so PCB oil was used to transfer heat Transformers used for transmission of electricity are ubiquitous, and millions of gallons of PCB oil were used to transfer heat from the transformer coils to the heat sink - the metal can surrounding the coils and holding the oil. Inevitably, both during manufacture of the transformers, in auto and weather accidents, and in the disposal of defective or damaged transformers, millions of gallons of PCB oil has leaked out. Westinghouse apparently dumped huge amounts of PCB-contaminated trash into unlined landfills in Bloomington, Indiana. General Electric's Pittsfield site has been declared a Superfund site, and GE has agreed to clean up its now-abandoned plant and several miles of the Housatonic River in western Massachusetts. The GE plant in Corning, NY contaminated 100 miles of the Hudson River3