Polychlorinated biphenyls (PCBs) come in various forms - typically, as a thick oily liquid, used to cool and insulate industrial electrical generators and transformers. PCBs do not conduct electricity and or vaporise at ordinary temperatures, they are fire-resistant and have been produced in large quantities since the 1920s. In the 1960s, there were incidents of poisoning; in Japan, rice oil contaminated by a PCB spill poisoned a large number of people. The risks, combined with this chemical's remarkable persistence (it's really difficult to get rid of, even as waste) caused it to be banned in most applications through the 1970s and 1980s.
But it's still around, circling the planet in the eco-system (PCBs have been found in polar bears), and will take decades to disappear.