Coal-tar-based sealcoat -- the black, shiny substance sprayed or painted on many parking lots, driveways, and playgrounds -- has been linked to elevated concentrations of the contaminants polycyclic aromatic hydrocarbons (PAHs) in house dust. Apartments with adjacent parking lots treated with the coal-tar based sealcoat contained house dust with much higher concentrations of PAHs than apartments next to other types of parking lots, according to new research published online in Environmental Science and Technology (ES&T).
The study was conducted in Austin, Texas, by scientists at the U.S. Geological Survey (USGS).
Coal tar is a byproduct of the coking of coal, and can contain 50 percent or more PAHs by weight. Coal-tar-based pavement sealants therefore have very high levels of PAHs compared to other PAH sources (e.g., soot, vehicle emissions, used motor oil). PAHs are an environmental health issue because several are probable human carcinogens and they are toxic to fish and other aquatic life.
Small particles of sealcoat, which contains extremely high concentrations of PAHs, likely are tracked indoors by residents after they walk across the parking lot. The study found that apartments adjacent to coal-tar-sealcoated parking lots contained concentrations of PAHs in house dust with that were 25 times higher than in house dust from apartments with concrete, asphalt, or asphalt-based sealcoat parking lot surfaces. The study also found that dust directly on the coal-tar-sealcoated parking lots had PAH concentrations that were 530 times higher than in dust on the parking lots without coal-tar sealcoat.
"These findings represent a breakthrough in our understanding of one of the important sources of these contaminants in house dust and how these contaminants can move from outdoors to indoors. The study provides evidence that will be potentially useful for policy makers," said Bob Joseph, Director of the USGS Texas Water Science Center.
In the past, several factors have been thought to affect PAH concentrations in house dust, including tobacco smoking and frequency of vacuuming. Researchers have had little success, however, demonstrating a relation between any of those factors and PAH concentrations.
Sealcoat products are widely used in the U.S., both commercially and by homeowners on their driveways. The products are commonly applied to parking lots of commercial businesses (including strip malls and shopping centers); apartment and condominium complexes; churches, schools, and business parks; residential driveways; and playgrounds. The City of Austin, Texas, estimates that before a ban on use of coal-tar-based sealcoat in 2006, about 660,000 gallons of sealcoat was applied every year in the city. The sealcoat wears off of the surface relatively rapidly, especially in areas of high traffic, and manufacturers recommend resealing every three to five years.
Two kinds of sealcoat products are widely used: coal-tar-emulsion based products and asphalt-emulsion based products. National use numbers are not available; however, previous research suggests that asphalt-based sealcoat is more commonly used on the West Coast, and coal-tar based sealcoat is more commonly used in the Midwest, the South, and on the East Coast.
Previous research by the same group of USGS scientists, published earlier in 2009, demonstrated that dust from sealcoated parking lots in cities east of the Continental Divide had concentrations of PAHs that were about 1,000 times higher than in dust from sealcoated parking lots in cities west of the Continental Divide.
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