Polycyclic aromatic hydrocarbons (PAHs) are chemical compounds that consist of fused aromatic rings and do not contain heteroatoms or carry substituents. Many of them are known or suspected carcinogens. They are formed by incomplete combustion of carbon-containing fuels such as wood, coal, diesel, fat, or tobacco. The simplest PAH is benzocyclobutene (C8H6).
PAHs composed only of six-membered rings are called benzenoid PAHs. The name comes from benzene, an aromatic hydrocarbon with a single, six-membered ring. The set of benzenoid PAHs is closely related to a set of mathematical entities called polyhexes, which are planar figures composed by conjoining regular hexagons of identical size.
PAHs containing upto 4 fused benzene rings are know as light PAHs and those containing more than four benzene rings are known as heavy PAHs. Heavy PAHs are more stable and more toxic than light ones.
PAHs of three rings or more have low solubilities in water and a low vapor pressure. As molecular weight increases, solubility and vapor pressure decrease. PAHs with two rings are more soluble in water and more volatile. Because of these properties, PAHs in the environment are found primarily in soil and sediment, as opposed to in water or air. However, PAHs are often found in particles suspended in water and air.
As molecular weight increases, the carcinogenicity of PAHs also increases, and acute toxicity decreases. One PAH compound, benzo[a]pyrene, is notable for being the first chemical carcinogen to be discovered.
Naphthalene (C10H8), consisting of two coplanar six-membered rings sharing an edge, is another aromatic hydrocarbon. By some conventions, it is not a true PAH, but is referred to as a bicyclic aromatic hydrocarbon. Its smell is familiar to those who have encountered mothballs.
PAHs are lipophilic in nature. Their presence has been reported in different edible oils from different countries.