Polycyclic Aromatic Hydrocarbons

Group of organic compounds with at least three condensed six-membered rings

PAH (Polycyclic aromatic hydrocarbons) denominates a class of several hundreds of compounds which share a common structure of at least three condensed aromatic rings. The number of rings strongly influences the physico-chemical properties of the respective PAH. Increasing ring numbers are associated with decreasing volatility and solubility whereas toxicity mostly increases.
PAH almost always occur as mixtures. The composition of these mixtures depends on the respective generation process.
PAH occur naturally in crude oil, peat, lignite, and coal. Moreover, they are formed by heating or combustion of organic material under anoxic conditions. They can be found in fried or grilled meat and tobacco smoke as well as in tar- and pitch-containing adhesives and coatings, in bitumen compounds and asphalt.

PAH are ubiquitous in the environment. In the atmosphere, compounds with 3 aromatic rings are mostly found in the gaseous phase whereas PAH with 4 and more rings adhere to particles. They enter waters and soils by wet and dry deposition.
In air and water, PAH can undergo photodegradation when exposed to ultraviolet light. Microbial degradation in soil and sediment is much slower.

They are of environmental concern due to their persistence and bioaccumulation potential. Many of the compounds are toxic, carcinogenic, mutagenic, teratogenic and reproductive toxic.

The U.S. EPA included 16 PAH in the so called EPA-list of Priority Pollutants. These 16 ‘EPA-PAH’ are frequently used as representatives of the whole class of PAH. The EU has included 16 partly different compounds in its list of PAH recommended for analysis (EU-list).