Polycyclic Aromatic Hydrocarbons

Polycyclic aromatic hydrocarbons, or polynuclear aromatic hydrocarbons, are a family of hydrocarbons containing two or more closed aromatic ring structures, each based on the structure of benzene. The simplest of these chemicals is naphthalene, consisting of two fused benzene rings. Sometimes there is limited substitution of halogens for the hydrogen of polycyclic aromatic hydrocarbons, in which case the larger category of chemicals is known as polycyclic aromatic compounds. Some of the better known polycyclic aromatic compounds in environmental chemistry include anthracene, benzopyrene, benzofluoranthene, benzanthracene, dibenzanthracene, phenanthrene, pyrene, and perylene.

Benzopyrene, for example, is an organic chemical with the general formula C₂₀H₁₂, containing a five-ring structure. Benzopyrene is extremely insoluble in water but very soluble in certain organic solvents such as benzene. There are various isomers, or structural variants of benzopyrene which differ greatly in their toxicological properties. The most poisonous form is benzo(a)pyrene, which is believed to be highly carcinogenic. In contrast, benzo(e)pyrene is not known to be carcinogenic. Similarly, benzo(b)fluoranthene demonstrates carcinogenicity in laboratory assays, but benzo(k)fluoranthene does not.

Benzo(a)pyrene and other polycyclic aromatic compounds are among the diverse products of the incomplete oxidation of organic fuels, such as coal, oil, wood, and organic wastes. Consequently, polycyclic aromatic compounds can be found in the waste gases of coal- and oil-fired generating stations, steam plants, petroleum refineries, incinerators, and coking ovens. Polycyclic aromatic compounds are also present in the exhaust gases emitted from diesel and internal combustion engines of vehicles, in fumes from barbecues, in smoke from wood stoves and fireplaces, and in cigarette, cigar, and pipe smoke. Residues of polycyclic aromatic compounds are also found in burnt toast, barbecued meat, smoked fish, and other foods prepared by charring. Forest fires are an important natural source of emission of polycyclic aromatic compounds to the atmospheric environment.

Many human cancers, probably more than half, are believed to result from some environmental influence. Because some polycyclic aromatic compounds are strongly suspected as being carcinogens, and are commonly encountered in the environment, they are considered to be an important problem in terms of toxicity potentially caused to humans. The most important human exposures to polycyclic aromatic compounds are voluntary and are associated, for example, with cigarette smoking and eating barbecued foods. However, there is also a more pervasive contamination of the atmospheric environment with polycyclic aromatic compounds, resulting from emissions from power plants, refineries, automobiles, and other sources. This chronic contamination largely occurs in the form of tiny particulates that are within the size range that is retained by the lungs upon inhalation (that is, smaller than about 3 æm in diameter).

Both voluntary and non-voluntary exposures to polycyclic aromatic compounds are considered to be important environmental problems. However, the most intense exposures are caused by cigarette smoking. These are also among the most easily prevented sources of emission of these (and other) toxic chemicals.