Polycyclic Aromatic Hydrocarbons

Polycyclic Aromatic Hydrocarbons

Polycyclic aromatic hydrocarbons (PAHs) are byproducts of incomplete burning, usually found in a mixture such as soot. They can also have industrial sources, whether as contaminants or in certain limited product uses. As their name indicates, they are not products of chlorine chemistry like most PBTs, but have more than one carbon ring. There are more than a hundred PAHs. Because they have so many sources, PAHs are found widely throughout the environment.

Benzo(a)pyrene is the most well-known PAH, and is sometimes on regulatory lists as a PBT in its own right, instead of being included in a PAH category. It is the most studied of this group of compounds, both for health effects and for levels in the environment.

Health Effects

A large dose of a PAH such as benzo(a)pyrene can cause red blood cell damage and suppress the immune system. Smaller chronic doses can cause developmental and reproductive effects, as well as also showing an immune suppressive effect in animals. Mice that were fed one PAH had difficulty reproducing, and so did their offspring. Many of the PAHs are known animal carcinogens. Reports in humans indicate that people exposed by breathing and skin contact to PAHs for a long time can develop cancer at a higher rate than normal.

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Human exposure to PAHs can occur by breathing air contaminated by wildfires or coal tar, eating foods that have been grilled or smoked, or by smoking tobacco products. Cigarette use is a large source of exposure. PAHs can also occur in drinking water, whether through contamination of water supplies, or through leaching from coal tar and asphalt linings in water storage tanks. Because they occur in so many industrial processes, occupational exposure to PAHs can often occur.

In the Environment

The presence of PAHs in areas far from sources indicate that they are reasonably stable in the atmosphere and capable of long distance transport. Most PAHs do not dissolve readily in water, but some will evaporate into the air. They will stick strongly to soil or sediment. There are higher concentrations of PAHs near urban sources; the background level in air in urban areas is about 10 times that in rural areas. PAH levels in soil in normal urban environments may be well above the risk-based level calculated as potentially dangerous to human health.

Tests of PAH biodegradation have resulted in a wide range of soil half-lives: from 2 days to 1.9 years. Benzo(a)pyrene, the most well-studied PAH, will bioaccumulate in aquatic organisms that do not have a metabolic detoxification enzyme system to get rid of it. Thus some organisms, like mudsuckers, sculpins, and sand dabs, have bioconcentration factors for benzo(a)pyrine that are very low (<1), which others have higher bioconcentration factors (3000 for oysters, 2,660 for bluegills, and 1000 to 13,000 for zooplankton).

Sources

Unlike many PBTs, PAHs have large natural as well as man-made (anthropogenic) sources. Wildfires and volcanoes are major sources of PAHs. An EPA estimate of air releases includes wildfires and prescribed burnings as the third largest source, although it doesn't estimate the amount produced by volcanoes. Human sources of PAHs include most fires, whether small-scale or industrial in nature, and a wide range of industrial products and wastes such as coal tar, creosote, waste oils, and wood-treating residues. Most PAHs have no known use, but a few are produced deliberately and used in medicines and to make dyes, plastics, and pesticides.

EPA estimates that total emissions of 16 specific PAHs to the air in 1990 were 26,500 tons. The largest sources of air releases from the EPA estimate are residential wood combustion (33 percent), consumer and commercial solvent use (22 percent), wildfires and prescribed burning (9.6 percent), aerospace industry surface coating (6.2 percent), and coke ovens (4.3 percent). Consumer and commercial solvent use involves sources that are personal care items, household care items (cleaning, polishing, etc.), automobile aftermarket products (detailing, maintenance, and repair), pesticides, adhesives, and coatings. Petroleum refining, pulp and paper production, gasoline distribution, primary aluminum production, and blast furnaces and steels mills are also large sources. The lesser sources span the gamut of industrial activity.

As is usual for PBTs, water and land releases are not as well characterized as air releases. Some indication can be obtained from the Toxic Release Inventory (TRI), which added PAHs in 1995. TRI only covers a limited set of manufacturing and Federal government entities. For that year, air releases of PAHs from TRI were 340 tons, water releases were 2.3 tons, land releases were 16 tons, and 4,900 tons were transferred off-site.

References

* Environmental Protection Agency. 1990 Emissions Inventory of Section 112(c)(6) Pollutants: Polycyclic Organic Matter (POM), TCDD, TCDF, PCBs, Hexachlorobenzene, Mercury, and Alkylated Lead: Final Report. Office of Air and Radiation. April 1998.

* Journal of Soil Contamination, 3(4): (1994). Background Levels of Polycyclic Aromatic Hydrocarbons (PAH) and Selected Metals in New England Urban Soils. Bradley, L.J.N.; Magee, B.H.; Allen, S.L.

* Environmental Protection Agency. National Primary Drinking Water Regulations: Technical Factsheet on: Polycyclic Aromatic Hydrocarbons (PAHs). Office of Ground Water and Drinking Water. January 1998.

* Emission Inventory Improvement Program (STAPPA/ALAPCO/EPA). EIIP Preferred and Alternative Methods for Estimating Air Emissions: Volume III: Chapter I: Comsumer and Commercial Product Use: Final Report. 454/R-97-004c. August 1996.

* Agency for Toxic Substances and Disease Registry. ToxFAQ for Polycyclic Aromatic Hydrocarbons. September 1996.

* Environmental Protection Agency. Toxic Release Inventory Database (1996 "frozen" version).

Detailed Sources of PAH Emissions

The following table shows sources of air emissions of PAHs. The table shows quantities of 16 specific PAHs that are tested for in commonly used analytical methods. They are: acenaphthene, acenaphthylene, anthracene, benz(a)anthracene, benzo(a)pyrene, benzo(b)fluoranthene, benzo(k)fluoranthene, benzo(ghi)perylene, chrysene, dibenz(a,h)anthracene, fluoranthene, fluorene, indeno(1,2,3-cd)pyrene, naphthalene, phenanthrene, and pyrene. The source of this table is EPA's 122(c)(6) inventory, Table 3-1. Numbers are an estimate for 1990. Sources producing less than 10 tons per year have been removed from the table, but kept in the total. For a better description of "Consumer and commercial solvent usage", see the sources section above.