PAHs, short for polycyclic aromatic hydrocarbons, are a group of chemicals consisting of numerous carbon atoms joined together to form multiple rings. There are at least 10,000 different PAH compounds. Most are formed from the incomplete combustion of plant or animal matter, or carbon fuels, such as coal or petroleum.
PAHs in the environment will be familiar to most as the sooty part of smoke or ash. PAHs can be the result of natural (forest fires) or man-made practices or industrial processes. Many useful products contain PAHs, such as mothballs, blacktop, and creosote wood preservatives. They are also found at low concentrations in some special-purpose skin creams and anti-dandruff shampoos that contain coal tars.
Automobile exhaust, industrial emissions and smoke from burning wood, charcoal and tobacco contain high levels of PAHs. In general, more PAHs form when organic materials burn at low temperatures, such as in wood fires or cigarettes. High-temperature furnaces produce fewer PAHs. Smoke from fires contains tiny particles of PAHs and other chemicals. These are known as particulate matter, or PM. When PM is microscopic in size, it remains suspended in air and can move long distances.
Most PAHs do not dissolve in water but, instead, bind to sediments. When sediments become suspended in water, PAHs can be transported with the sediment. PAHs can enter groundwater from ash, tar, or creosote that is improperly disposed in landfills.
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