Polycyclic aromatic hydrocarbon: Abbreviated PAH. A hydrocarbon (a molecule made up of hydrogen and carbon) containing two or more closed rings of atoms. PAHs are persistent, bioaccumulative, and toxic (PBT) pollutant. Exposure usually occurs by breathing air contaminated by wild fires or coal tar, or by eating foods that have been grilled. Other sources include industrial processes, transportation, energy production and use, smoking tobacco, and disposal activities such as open trash burning. Some PAHs have been identified as carcinogen, mutagenic, and teratogenic.