Polycyclic aromatic hydrocarbons are a class of organic compounds having two or more fused benzene rings. While usually referring to compounds made of carbon and hydrogen, PAH also may include fused aromatic compounds containing nitrogen, sulfur, or cyclopentene rings. Some of the more common PAH include naphthalene (2 rings), anthracene (3 rings), phenanthrene (3 rings), pyrene (4 rings), chrysene (4 rings), fluoranthene (4 rings), benzo(a)pyrene (5 rings), benzo(e)pyrene (5 rings), perylene (5 rings), benzo(g,h,i)perylene (6 rings), and coronene (7 rings).

PAH are formed by a variety of human activities including incomplete combustion of fossil fuels, wood, and tobacco; the incineration of garbage; coal gasification and liquefaction processes; smelting operations; and coke, asphalt, and petroleum cracking; they are also formed naturally during forest fires and volcanic eruptions. Low molecular-weight PAH (those with four or fewer rings) are generally vapors while heavier molecules condense on submicron, breathable particles. It...