Polycyclic aromatic hydrocarbons (PAHs) are organic compounds that contain two or more aromatic rings. PAHs are known for mutagen and carcinogen in terms of ecotoxicology. 17 PAHs including benzo(e)pyrene are regulated as priority pollutants by the U.S. Environmental Protection Agency (EPA), 33 PAHs (31 PAHs and 2 alkylated PAHs) are regulated by WHO.
Some low molecular weight PAHs can be biodegraded in natural environment, but most of the PAHs are stable and insoluble compounds having high melting and boiling point. The larger the number of aromatic rings, the lower its volatility. Therefore, they are found as a solid in soils or an adsorbate onto dust.
They are mainly produced from by-products of fuel combustion, industrial process, automobile exhausts and vehicle fuels, also can be generated naturally from volcanos, forest fires, and crude oils.