PCBs are industrial chemicals that were heavily used until they were banned in the US in 1977. Even though they have not been used in 30 years they continue to be measurable in US populations. They have a negative impact on thyroid levels.
PCBs build up in fish, similar to mercury. It is wise to avoid eating fish from contaminated waters such as the Great Lakes. It does appear that environmental PCBs are finally in measurable decline.