Polycyclic Aromatic Hydrocarbons

Over 30 Polycyclic Aromatic Hydrocarbons (PAH's) have been classified as “priority pollutants” by the EPA, 7 of which are known to cause cancer. Prenatal exposure to PAH's has also been associated with birth defects, asthma, low IQ, and behavioral problems. PAH's are chemicals that contain multiple rings of carbon atoms, usually 6 atoms per ring. The simplest of these compounds, which contains only two of these “benzene rings”, naphthalene, has been used in mothballs, and has been shown to cause cancer in animals.

The largest source of PAH's in the environment appears to be due to the sealcoating of parking lots and driveways with sealants containing coal tar which contains about 30% PAH compounds. The rainwater runoff from these parking lots drain into storm sewers, and then directly into local streams or rivers, is toxic to marine organisms in streams and lakes, especially aquatic invertebrates that live in the mud and are the beginning of the food chain for many fish.

When coal is heated to produce coke for making steel, the vapors that are driven off are condensed to produce many valuable chemicals. The leftover “gunk” that remains is coal tar. It is somewhat similar to the residue from oil refining, which is called asphalt and is used in paving highways and parking lots. Asphalt also contains some PAH's, but at very much lower concentrations than coal tar. Asphalt based sealcoating for parking lots is produced and is used mostly west of the Rocky Mountains, whereas coal tar based sealcoating is mostly used east of the continental divide.

Run-off containing PAH’s washes into sewers, eventually polluting our streams, and it contaminates the soil surrounding parking lots and roadways. Also, bits can be carried into homes on the soles of shoes and as resurfaced areas deteriorate, particles containing PAH’s can become airborne, settling in the dust of area homes.

Two major studies, the first in Austin, Texas, showed a link between coal tar sealants and PAH’s in streams and the atmosphere, and a second study at the University of New Hampshire confirmed those results. In Lake of the Hills, a suburb of Chicago, the PAH concentration increased by a factor of 10 during 10 years of urbanization. The levels of PAH's in household dust was measured in houses and apartments, some of which were adjacent to parking lots that had been coated with coal tar products, while others were not. The levels were 25 times higher inside the residences adjacent to the coal tar coated lots than those adjacent to parking lots that were not coated with coal tar products. Much more information is available from the United States Geological Survey (USGS).

While the EPA has not taken action, some cities and counties have placed restrictions or even banned the use of coal tar sealants. Austin, Texas, has been a pioneer in this area. They banned the use of coal tar sealants in 2006, after conducting many experimental studies in conjunction with the USGS. They measured the runoff from parking lots, concentrations in streams and lakes, and set up aquaria for testing the effects of PAH's. Coal tar sealants have also been banned in Washington, D. C., in Dane County Wisconsin, and in several Minneapolis suburbs. The success of such bans to lower PAHs concentrations in receiving stream sediments remains to be determined.

Thank you to Earl Gose, an involved SCWN member, who researched and contributed this article