Polychlorinated biphenyls - PCBs - were used as plasticizers in paints, sealants or insulating agents. PCBs have been banned in Germany since 1989, and in the USA for a decade longer.
The chemicals accumulate in the environment over many years and are considered harmful to health. People ingest PCBs, for example, by eating contaminated fish. The chemical can also be problematic for animals - it has already been found in high concentrations in marine mammals, such as killer whales.