Polycyclic aromatic hydrocarbon
![Standard line-angle schematic representation of an important PAH, benzo[a]pyrene, where carbon atoms are represented by the vertices of the hexagons, and hydrogens are inferred as projecting out at 120° angles to fill the fourth carbon valence (as necessary)](/uploads/202502/03/Benzo-a-pyrene.svg1505.png)
Polycyclic aromatic hydrocarbons (PAHs, also polyaromatic hydrocarbons) are hydrocarbons—organic compounds containing only carbon and hydrogen—that are composed of multiple aromatic rings (organic rings in which the electrons are delocalized). Formally, the class is further defined as lacking further branching substituents off of these ring structures. Polynuclear aromatic hydrocarbons (PNAs) are a subset of PAHs that have fused aromatic rings, that is, rings that share one or more sides. The simplest such chemicals are naphthalene, having two aromatic rings, and the three-ring compounds anthracene and phenanthrene.