A family of chemicals banned in the United States in 1977, but once used in electrical equipment. PCBs can still be detected in fish, breast milk, and most people’s bodies because of environmental exposures. According to the U.S. Environmental Protection Agency and the International Agency for Research on Cancer, PCBs are probably carcinogenic to humans. Four studies have shown higher breast cancer risk in women with higher PCB exposure in conjunction with a genetic variation that affects metabolism.