USGS Research: PAHs and Coal-Tar-Based Pavement Sealcoat

Science Center Objects

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.

Key Findings
  • Human health
    • Residences adjacent to parking lots with coal-tar-based sealcoat have concentrations of PAHs in house dust 25 times higher than residences adjacent to unsealed pavement or asphalt-sealed pavement.

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    • Doses of carcinogenic PAHs through non-dietary ingestion of house dust in residences with coal-tar sealant on the parking lot are 14 times greater than in residences with unsealed pavement, and are more than twice the dose from dietary ingestion, reversing a long-held assumption that dietary PAH exposure exceeds non-dietary exposure.

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    • Living adjacent to coal-tar-sealed pavement (a parking lot or driveway, for example) is estimated to increase excess lifetime cancer risk 38 times, and much of the increased risk occurs during early childhood.

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  • Environmental health
    • Runoff from freshly applied coal-tar sealcoat is acutely toxic to two test organisms (fathead minnows (Pimephales promelas) and a water flea (Ceriodaphnia dubia)).

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    • Toxic effects to test organisms continue for samples collected as long as 111 (3+ months) days following application if organisms also are exposed to ultra-violet light mimicking sunlight.

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    • Runoff from coal-tar-sealed pavement, diluted 1:100, causes DNA damage when cells also are exposed to ultra-violet radiation that mimics sunlight.

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    • Runoff from coal-tar-sealed pavement, diluted 1:10, impairs the ability of cells to repair DNA damage.

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  • Air
    • Emissions of PAHs to air 2 hours after application of coal-tar sealant are 30,000 times greater than from unsealed pavement.

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    • Emissions of PAHs to air from parking lots with older (3-8 years) coal-tar sealant are 60 times greater than from unsealed pavement.

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    • Total annual PAH emissions from newly applied sealcoat are estimated to be larger than those from vehicles in the U.S.

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  • Streams and Lakes
    • PAH concentrations in Lady Bird Lake sediment decline following a ban on coal-tar-based pavement sealants in Austin, Texas.

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    • Coal-tar pitch from sealcoat reaches streams and lakes in runoff and soils and unsealed pavement near sealed parking lots.

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    • Coal-tar-based sealcoat was determined to be the largest source of PAH contamination to urban lakes sampled by the USGS.

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    • Use of coal-tar-based sealcoat is the primary cause of upward trends in PAHs in urban lake sediment.

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  • Stormwater runoff
    • Concentrations of polycyclic aromatic hydrocarbons (PAHs) in runoff from pavement with coal-tar-based sealcoat remain elevated for months to years following sealcoat application.

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    • Particles in runoff from coal-tar-sealed pavement contained PAHs at concentrations that are 65 times higher than particles in runoff from unsealed asphalt pavement.

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    • Runoff from freshly applied coal-tar sealcoat contained elevated concentrations of acridine and carbazole in addition to PAHs.

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  • What's on coal-tar-sealed pavement?
    • Scrapings of dried coal-tar sealant contain 9,400-93,300 mg/kg PAHs.

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    • Dust from pavement with coal-tar-based sealcoat has PAH concentrations hundreds to thousands of times higher than dust from pavement with no sealcoat or with asphalt-based sealcoat.

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