"Polychlorinated biphenyls," or PCBs, have been used as coolants and lubricants in transformers, capacitors, and other electrical equipment because they don't burn easily and are good insulators.  The manufacture of PCBs ceased in the U.S. in 1977 because of evidence that PCBs build up in the environment and can cause harmful health effects.  In water, they bind with sediments and then bioconcentrate in fish and other seafood.  Because PCBs are soluble in fat, they are found at higher concentrations in fatty foods.