Fluoranthene is a polycyclic aromatic hydrocarbon (PAH) consisting of a naphthalene and a benzene unit connected by a five-membered ring. It is a member of the class of PAHs known as non-alternant PAHs because it has rings other than those with six carbon atoms. It is a structural isomer of the alternant PAH pyrene. It is not as thermodynamically stable as pyrene because its electrons cannot resonate throughout the complete structure as the corresponding ones in pyrene.
Fluoranthene is found in many combustion products, along with other PAHs. Its presence is an indicator of less efficient or lower-temperature combustion, since non-alternant PAHs are less preferred in formation than alternant PAHs. Fluoranthene is one of the U.S. Environmental Protection Agency's 16 priority pollutant PAHs and is a carcinogen.