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Polychlorinated biphenyls (PCB) are a class of organic compounds that are frequently highly toxic and persist in the environment, even though their production was banned in the United States in 1979 and internationally in 2001. They are a group of stable and non-flammable chemicals that were widely used both industrially and in commercial products. Despite their ban, they frequently remain in households, in items such as electrical equipment, thermal insulation, wood floor finishes, and caulk.
Most people are exposed only to low doses of PCBs, which the human body is capable of breaking down. These toxins are, however, cumulative. This means they do build up in the body over time.
The effects of exposure to large amounts of polychlorinated biphenyls can be quite profound. Acute toxicity can manifest as severe skin disorders. Animals that are exposed can develop liver cancer, and polychlorinated biphenyls are considered to be likely carcinogens in people. These substances have been found in human breast milk, and have been shown to cause developmental problems in babies that have been exposed to them. The thyroid gland and immune system can also be impacted by PCBs.
Structurally, PCBs are a diverse group of compounds. The basic polychlorinated biphenyl structure is a pair of benzene rings, which are comprised of a circle of six carbon atoms with hydrogen atoms attached. The rings have double bonds, and each ring has one or more chlorine atoms. The number of chlorine atoms per ring can range from one to ten, leading to 209 possible different compounds.
The chemical properties can vary, depending on the number and location of these chlorine atoms. For instance, with more chlorine atoms, it becomes more difficult to degrade the polychlorinated biphenyls. Because of their resistance to degradation, these PCBs remain in the environment for long periods of time, and are known as persistent organic pollutants.
PCBs can be found in the air, soil, and water, and have been found even in remote areas like the Arctic. Polychlorinated biphenyl molecules with few chlorine atoms are light and often found in the air. Molecules with more chlorine atoms are typically heavier, and more likely to be toxic and carcinogenic. These weighty compounds are frequently found in sediment in water.
The effects of polychlorinated biphenyls are partly due to their ability to undergo bioaccumulation. When attached to sediment in water, PCBs are usually eaten by small animals and fish. This can increase the concentration of polychlorinated biphenyl in these creatures. Any other organism that then eats the contaminated fish is subjected to a much higher dose of the chemical. People that eat a lot of fish from contaminated waters can be exposed to polychlorinated biphenyl in this way.
Despite an international ban on PCBs, these compounds still find their way into the environment. This can result from chemical leaks or fires. Inappropriately dumped industrial waste can also cause contamination. Some consumer products that contain polychlorinated biphenyls can also leach the chemical if they are disposed of in landfills that are not designed to deal with hazardous waste. Another source of contamination can be the incineration of wastes that contain PCBs.