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Environmental Issues

Contaminated House Dust Linked to Parking Lots with Coal Tar Sealant

Seal Coat Picture

by Ranger Kidwell-Ross

A recent U.S. Geological Survey study found that apartments adjacent to coal-tar-sealcoated parking lots contained concentrations of the contaminants polycyclic aromatic hydrocarbons (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.

In the past decade an enormous interest has sprung up concerning how effective sweepers are at picking up pollution from streets. Until now, that has not carried over into the parking lot arena. As a result of this study, which was released January 12, 2010 by Environmental Science and Technology (ES&T), that may well change.

Coal-tar-based sealcoat—the black, shiny substance sprayed or painted onto pavement by the employees of many of the people reading these words—has been linked to elevated concentrations of the contaminants polycyclic aromatic hydrocarbons (PAHs) in house dust. A new research study on the topic was released on-line by Environmental Science and Technology on January 12, 2010.

Apartments where adjacent parking lots had been 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 (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.

"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," reports 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.

Ultimately, these research findings are likely to deal a severe blow to the use of coal tar-based sealcoating industry. Those in that industry would be well advised to cite this research and switch to asphalt-based sealcoating products. With proper presentation of the research findings, the sweeping industry should be able to benefit from the study results.

For example, sweeping contractors will be well served by citing/furnishing this article information to property managers of apartment complexes. Unlike retail property, apartments tend to be swept much less often since there is much less frequency of littering. Since most managers only recognize the need for "cosmetic sweeping," the degree to which pavement becomes unsitely is directly linked to typical sweeping frequency.

However, this research suggests that a more regular frequency of sweeping, on seal-coated pavement around apartment and other housing complexes, would likely reduce the amount of cancer-causing PAH track-in to the housing units. As far as I can determine to date, though, there has been no research done that might prove or disprove the ability of air sweeping to handle the relatively small PAH particles.

I spoke by phone to one of the principal researchers of the study, Barbara Mahler. She told me that to her knowledge the role of pavement sweeping in reducing PAH track-in or other transport had not been studied or considered. She did note that they had seen a higher PAH concentration on unsealed parking lots that were next to sealed parking lots than there was on isolated unsealed pavement.

Because parking lot air sweepers are largely used at night and there are no standardized pickup regulatory guidelines, most give off a substantial dust cloud. Since the ability of air sweepers to handle PAH has not been tested to date, it may even be that sweepers contribute to the increased PAH on adjacent, unsealed lots, due to the particle-laden air blown into the atmosphere as a component of the sweeping process.

I believe this study poses both a challenge and an opportunity for the power sweeping industry. If it can be shown that current or technologically-feasible air sweeper technology can impact PAH transport, it would provide a clear incentive for many property mangers to increase their frequency of sweeping. This would be especially true of seal-coated lots with a relatively low incidence of littering. If air sweepers can, indeed, make an impact on PAH track-in, it would clearly be another reason to sweep many types of indoor and outdoor pavement.

On the other hand, if current parking lot class air sweepers are contributing to the problem, the issue is one that may need to be addressed across the industry before regulatory agencies provide the mandate for change.

An interview with USGS hydrologist Barbara Mahler can be heard in episode 116 of the USGS CoreCast. In it, Mahler explains how she and her team identified the link between polycyclic aromatic hydrocarbons in coal-tar-based pavement sealcoat and house dust.

The abstract of the ES&T article is available here; look under the "Research ASAP" tab.

Comprehensive coverage of this story was done on MSNBC. That information is available at this link.

If you have any questions about any of the above ideas, feel free to contact the Team for a further explanation.

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