Long-term studies of human workers exposed to high amounts of PCBs have not consistently demonstrated that PCBs cause cancer in people. PCBs have been designated as probable human carcinogens (cancer-causing chemicals) by the USEPA, however there is no proof that PCBs cause cancer at the low levels present in the environment. Studies have shown that animals fed diets containing large amounts of PCBs develop tumors in various parts of their bodies, but it is not clear whether these findings can be applied to humans. PCBs are known human carcinogens. They have been classified as such by the IARC and the EPA. The WHO also lists them as possible human carcinogens. PCBs are a group of more than 100 chemicals that were originally used in electrical equipment because they are non-conducting. People become exposed to PCBs through food, water, air, and most commonly through eating fish who have eaten contaminated seafood. High levels of exposure to certain types of PCBs may cause health problems such as kidney damage, hormone disruption, immune system dysfunction, nervous system disorders, reproductive problems, and cancer. The study of PCBs was popular in the 1940s, '50s, and '60s, when they were used in many products including plastic materials, lubricants, heat-transfer fluids, dielectrics, and insulators. Because of this use, workers in industries using lots of PCBs experience higher rates of cancer.