Texas Water Science Center
Coal-tar-based sealcoat being applied to a test plot.
Pavement sealcoat, or sealant, is a black liquid that is sprayed or painted on asphalt pavement. It is marketed as protecting and beautifying the asphalt surface. Sealcoat is used commercially and by homeowners across the Nation.
Most sealcoat products have a coal-tar-pitch or asphalt base. Coal-tar-based sealcoat is most commonly used in the central, southern, and eastern U.S., and asphalt-based sealcoat is used predominantly in the western U.S.
Polycyclic aromatic hydrocarbons (PAHs) are a group of chemical compounds that form during the combustion of anything with a carbon base, from wood and gasoline to cigarettes and meat. PAHs also are found in products whose production involves the heating of hydrocarbons, such as automobile tires and coal-tar pitch.
PAHs are of environmental concern because several are toxic, carcinogenic, mutagenic, and(or) teratogenic (causing birth defects) to aquatic life and because several are probable human carcinogens.
Coal tar is a byproduct of the coking of coal, and can contain 50 percent or more PAHs by weight. Coal-tar pitch is known to cause cancer in humans (International Agency for Research on Cancer, 1985).
Coal-tar-based sealcoat products typically are 20 to 35 percent coal-tar pitch. Product analyses indicate that coal-tar-based sealcoat products contain about 1,000 times more PAHs than sealcoat products with an asphalt base (City of Austin, 2005).
PAH and Sealcoat Contacts
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PAHs and Coal-Tar-Based Pavement Sealcoat
Coal-tar-based pavement sealant is a potent source of polycyclic aromatic hydrocarbons (PAHs), as documented by the USGS and other researchers.
Coal-tar-based sealcoat products typically are 20 to 35% coal tar or coal-tar pitch—these materials are known human carcinogens that contain high concentrations of PAHs and related chemicals. Coal-tar-based pavement sealcoat typically contains 35,000 to 200,000 mg/kg (parts per million, or ppm) PAHs, about 100 times more PAHs than in used motor oil and about 1,000 times more PAHs than in sealcoat products with an asphalt base.
Coal-tar sealcoat is abraded to a fine dust by car tires and snow plows, requiring reapplication every 2–5 years. The mobile high-PAH dust is blown, washed, or tracked into nearby soil, stormwater ponds, streams, lakes, and house dust. Many of the lighter-weight chemicals in coal-tar sealcoat volatilize (evaporate) into the air, primarily during the 2 weeks following application, but continuing for years after application.
Learn more about PAHs and coal-tar-based sealcoat
Related USGS Research
Some USGS publications on PAHs and coal-tar-based pavement sealant have been subject to Information Quality Act correction requests. USGS Information Quality Guidelines, the correction requests, and USGS responses can be found at: http://www.usgs.gov/info_qual/