PCBs, or polychlorinated biphenyls, are industrial products or chemicals. They have been banned worldwide since 2001 by the Stockholm Convention on Persistent Organic Pollutants because of studies suggesting that they could have unintended impacts on people and the environment. Before the ban, they were made extensively for products like microscope oils or electrical appliances, and were also used as sprays for roads to keep dust down. PCBs then entered the air, water and soil, which are still existing today.