<P> Polycyclic aromatic hydrocarbons (PAHs, also polyaromatic hydrocarbons or polynuclear aromatic 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 on these ring structures . The simplest such chemicals are naphthalene, having two aromatic rings, and the three - ring compounds anthracene and phenanthrene . </P> <P> PAHs are non-charged, non-polar molecules found in coal and in tar deposits . They are also produced by the incomplete combustion of organic matter (e.g., in engines and incinerators, when biomass burns in forest fires, etc .). </P>

List the elements that make up polycyclic aromatic hydrocarbons