<unk> s <unk> s) are chemical compounds that consist of fused<unk>  rings and do not contain heteroatoms or carry substituents . These compounds can be point source pollutants (e.g. oil spill) or non-point source (e.g. atmospheric deposition) and are one of the most widespread organic pollutants. Some of them are known or suspected carcinogens, and are linked to other health problems.  They are primarily formed by incomplete combustion of carbon-containing fuels such as wood, coal, diesel, fat, or tobacco. Tar also  contains<unk> s. Since human civilization relies so heavily on combustion,<unk> s are inevitably linked to our energy production. In this sense,<unk>  can be thought of as marker molecules as their abundance can be directly proportional to combustion processes in the region and therefore directly related to air quality. Different types of combustion yield different distributions of<unk> s in both relative amounts of individual<unk> s and in which isomers are produced. Thus, those produced from coal burning are different from those produced by motor-fuel combustion, which differ from those produced by forest fires. Some<unk> s occur within crude oil, arising from chemical conversion of natural product molecules, such as steroids, to<unk><unk> s.
They are also found in the interstellar medium, in comets, and in meteorites and are a candidate molecule to act as a basis for the earliest forms of life. In graphene the<unk>  motive is extended to large 2D sheets.
Polycyclic aromatic hydrocarbon
Terpene
Polycyclic compound
Indole alkaloid
Organic compound
Haloalkane
Cycloalkane
Heterocyclic compound