Polycyclic aromatic hydrocarbons

Polycyclic aromatic hydrocarbons are a class of organic compounds having two or more fused benzene rings. While usually referring to compounds made of carbon and hydrogen , PAH also may include fused aromatic compounds containing nitrogen , sulfur, or cyclopentene rings. Some of the more common PAH include naphthalene (2 rings), anthracene (3 rings), phenanthrene (3 rings), pyrene (4 rings), chrysene (4 rings), fluoranthene (4 rings), benzo(a)pyrene (5 rings), benzo(e)pyrene (5 rings), perylene (5 rings), benzo(g,h,i)perylene (6 rings), and coronene (7 rings).

PAH are formed by a variety of human activities including incomplete combustion of fossil fuels , wood, and tobacco ; the incineration of garbage ; coal gasification and liquefaction processes; smelting operations; and coke, asphalt, and petroleum cracking; they are also formed naturally during forest fires and volcanic eruptions. Low molecular-weight PAH (those with four or fewer rings) are generally vapors while heavier molecules condense on submicron, breathable particles. It is estimated that more than 800 tons of PAH are emitted annually in the United States. PAH are found worldwide and are present in elevated concentrations in urban aerosols, and in lake sediments in industrialized countries. They also are found in developing countries due to coal and wood heating, open burning, coke production, and vehicle exhaust.

The association of PAH with small particles gives them atmospheric residence times of days to weeks, and allows them to be transported long distances. They are removed from the atmosphere by gravitational settling and are washed out during precipitation to the earth's surface, where they accumulate in soils and surface waters. They also directly enter water in discharges, runoff , and oil spills . They associate with water particulates due to their low water solubility, and eventually accumulate in sediments. They do not bioaccumulate in biota to any appreciable extent, as they are largely metabolized.

Many but not all PAH have carcinogenic and mutagenic activity; the most notorious is benzo(a)pyrene, which has been shown to be a potent carcinogen . Coal tar and soot were implicated in the elevated skin cancer incidence found in the refining, shale oil, and coal tar industries in the late nineteenth century. Subsequent research led to the isolation and identification of several carcinogens in the early part of this century, including benzo(a)pyrene. More recent research into the carcinogenicity of PAH has revealed that there is significant additional biological activity in urban aerosols and soot beyond that explained by known carcinogens such as benzo(a)pyrene. While benzo(a)pyrene must be activated metabolically, these other components have direct biological activity as demonstrated by the Ames test . They are polar compounds, thought to be mixtures of monoand dinitro-PAH and hydroxy-nitro derivatives. Tobacco smoking exposes more humans to PAH than any other source.

[Deborah L. Swackhammer ]

RESOURCES

BOOKS

Dias, J. R. Handbook of Polycyclic Aromatic Hydrocarbons. New York: Elsevier, 1987.

Harvey, R. G. Polycyclic Aromatic Hydrocarbons: Chemistry and Carcinogenicity. New York: Cambridge University Press, 1991.