Food can be a major source of human PCB exposure, usually from fish and animal fat.
Bottom feeders and other aquatic organisms then ingest and accumulate PCBs, resulting in bioconcentration upward in the food chain. The toxicity of PCBs was dramatically illustrated in 1968 when over 1600 people in Japan were poisoned by cooking oil contaminated with PCBs from a heat transfer unit. The contaminating oil likely contained furans and dioxins, compounds generally more toxic than PCBs themselves. Another important reason for their persistence is their resistance to biodegradation. Low levels of PCBs can be found throughout the world.