Coal-Tar-Based Pavement Sealcoat, PAHs, and Environmental Health

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Sealcoat is the black, viscous liquid applied to many asphalt parking lots, driveways, and playgrounds in North America to protect and enhance the appearance of the underlying asphalt. Coal-tar-based pavement sealcoat is a potent source of polycyclic aromatic hydrocarbon (PAH) contamination in urban and suburban areas and a potential concern for human health and aquatic life.

BACKGROUND

Pavement sealcoat is a commercial product marked for use primarily on parking lots and driveways and is rarely used on public roads. Most sealcoat products are either coal-tar or asphalt emulsion. Coal tar and coal-tar pitch, both used in sealcoat products, have extremely high concentrations of PAHs, and both are classified as known human carcinogens. Coal-tar-based sealcoat products typically are 20 to 35 percent coal tar or coal-tar pitch.  Asphalt and asphalt-based sealcoat products have much lower concentrations of PAHs.

For historical and economic reasons, coal-tar-based sealcoat is more common east of the Continental Divide (except in those states, counties, and municipalities where its use is prohibited), and asphalt-based sealcoat is more west of the Continental Divide. 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 (oil) base. Levels of PAHs in dust swept from sealed parking lots to the east and west of the Continental Divide reflect this difference in use, with PAH concentrations in sealed-pavement dust the East about 1,000 times higher than concentrations in the West.

What are coal tar and coal-tar pitch?

Diagram describing coal tar process

Coal tar is a byproduct of the coking of coal, and coal-tar pitch is the residue that remains after the distillation of coal tar. Coal tar and coal-tar pitch are used in coal-tar-based sealcoat products, although use of coal-tar pitch is more common than use of coal tar. Both coal tar and coal-tar pitch are known human carcinogens. Learn more about coal-tar-based sealcoat here.

Coal tar is a byproduct of the coking, liquefaction or gasification of coal. Coal-tar pitch is the residue that remains after the distillation of coal tar, that is, the thick black liquid remaining after various oils are removed from coal tar for use in the manufacture of industrial and consumer products. Coal-tar pitch is separated, or "refined", into 12 grades of viscosity. The most viscous grade, RT-12, is used in coal-tar-based sealcoat. The primary use of coal-tar pitch is in electrode manufacturing for the aluminum industry and for steel arc furnaces. 

What are PAHs?

PAHs are a group of chemicals created by heating or burning material that contains carbon. There are many sources of PAHs to the environment, representing a wide range of PAHs concentrations, including asphalt (2-9 milligrams per kilogram, or mg/kg), tire particles (84 mg/kg), used motor oil (730 mg/kg), and coal-tar-based sealcoat (34,000-202,000 mg/kg). PAHs cause cancer, mutations, birth defects, and/or death in fish, wildlife, and invertebrates. Several PAHs are photoactivated, meaning that their toxic effects are greatly intensified when exposed to sunlight. The EPA has classified seven PAHs as probable human carcinogens, and 16 PAHs as Priority Pollutants.

AS SEALCOAT WEARS OFF, WHERE DOES IT GO?

Abraded dried sealcoat particles containing high concentrations of PAHs and related chemicals can be transported by rain, wind, car tires, and even our feet to surrounding areas, including our homes. Concentrations of PAHs in runoff, sediment, soils, and dust near coal-tar-sealcoated pavement are substantially higher than concentrations in those media near concrete pavement, unsealed asphalt pavement, and asphalt pavement with asphalt-based sealcoat. 

Diagram indicating pathways for transport of PAHs from coal-tar sealcoat

Coal-tar-based sealcoat is a potent source of PAHs and related chemicals. Worn particles of coal-tar-based sealcoat are transported by rain, wind, tires, and even our feet from pavement to other environmental settings. Sealcoat product (A), after it dries, gradually abrades to a powder and becomes part of the dust on the pavement (B). Pavement dust is transported by rainfall runoff (C) to stormwater-management devices (D) or to receiving streams and lakes (E). Pavement dust also adheres to tires (F) that track it onto unsealed pavement, and wind and runoff transport the dust to nearby soils (G). Sealcoat particles tracked into residences can become incorporated into the house dust (H). Graphic from USGS Fact Sheet 2016-3017.

 

Table of PAH concentrations near coal-tar-sealcoated pavement

Concentrations of polycyclic aromatic hydrocarbons (PAHs) in settings near pavement sealed with coal-tar-based sealants were substantially higher than concentrations in settings where coal-tar sealant was not used. Letters refer to lettered settings in figure above. Graphic from USGS Fact Sheet 2016-3017.

POTENTIAL RISKS TO HUMAN HEALTH

PAHs from coal-tar-based sealcoat contaminate house dust. House dust is an important pathway for human exposure to many contaminants, including PAHs. This is particularly true for small children, who spend time on the floor and put their hands and objects into their mouths. In a study of 23 ground-floor apartments, PAH levels in house dust in apartments with parking lots sealed with a coal-tar-based product were 25 times higher than in house dust in apartments with parking lots with other surface types (concrete, unsealed asphalt, and asphalt-based sealcoat). No relation was found between PAHs in house dust and other possible indoor PAH sources such as tobacco smoking and fireplace use.

The pre-schooler living in a residence adjacent to coal-tar-sealed pavement who has relatively low hand-to-mouth activity consumes about 2.5 times more PAHs from house dust than from their diet. For the more active pre-schooler, whose hand-to-mouth activity is higher, the PAH intake from house dust is nearly 10 times more than the PAH intake from their diet. These findings upset the paradigm that diet is the greatest source of PAHs for small children.

 

POTENTIAL RISKS TO AQUATIC LIFE

Runoff from coal-tar-sealcoated pavement is acutely toxic to aquatic biota. Exposure to runoff from coal-tar-sealed pavement collected as much as 42 days after sealcoat application resulted in 100 percent mortality to two commonly tested laboratory organisms: day-old fathead minnows (Pimephales promelas) and water fleas (Ceriodaphnia dubia). In contrast, minnows and water fleas exposed to runoff from unsealed pavement experience no more than 10 percent mortality. When the minnows and water fleas were also exposed to simulated sunlight, which intensifies the toxicity of some PAHs, runoff collected 111 days (more than 3 months) after sealcoat application caused 100 percent mortality to both species, and caused 100 percent mortality to water fleas even when diluted to 10 percent of its original strength. These results demonstrate that runoff from coal-tar-sealcoated pavement continues to be toxic from aquatic organisms long after the 24- to 48-hour curing time.

A subsequent collaborative study by researchers at the National Oceanic and Atmospheric Adminstration (NOAA), U.S. Fish and Wildlife Service, and University of Washington reported that coal-tar-sealcoat runoff is acutely lethal to juvenile coho salmon (Oncorhynchus kisutch) and causes a wide spectrum of abnormalities to zebrafish (Danio rario) embryos. The study also reported that filtration of the runoff through a bioretention system substantially reduced toxicity.

 

COAL-TAR-BASED SEALCOAT, PAHS, AND STREAM AND LAKE SEDIMENT

Slicing sample of lake bed sediment

Sediment cores from lakes and reservoirs can be used to reconstruct contaminant histories in watersheds. Sediment-core analyses indicate that concentrations of PAHs in many U.S. urban lakes are increasing, and multiple lines of evidence indicate that the source of the PAHs is coal-tar-based pavement sealants. (Credit: Pete Van Metre, USGS)

Dust on coal-tar-sealed parking lots contains bits of abraded sealcoat particles, and measured concentrations of PAHs in the dust swept from coal-tar-sealed pavement typically are in the thousands of milligrams per kilogram. In some cases, stormwater runoff washes high-PAH pavement dust to a stormwater management device, such as a retention pond, leading to high costs of disposal. PAH-contaminated dust that is not trapped by management devices can be transported to streams and lakes, where it settles in the stream or lake bed. Application of a wide range of "forensic" methods has indicated that, in areas where it's used, coal-tar-based sealcoat is the primary source of PAHs to stream and lake sediment.

Analysis of lake sediment cores demonstrates that coal-tar-based sealcoat is a major contributor to upward trends in PAHs in urban lakes across the U.S. In Austin, Texas, coal-tar-sealcoat was banned in 2006—sediment cores collected in 2012 and 2014 from Lady Bird Lake, the primary receiving water body for the Austin area, showed a 58 percent decrease in PAH concentrations since the peak prior to the ban.

     ► Read more about use of sediment cores to reconstruct contaminant histories here.

Other studies have used different approaches to determine the source of PAHs to urban stream and lake sediment and come to a similar conclusion regarding source. Those approaches include a land-use-based analysis and organic petroscopy. Those studies concluded that coal-tar-based sealcoat was the source of 70 to 80 percent of the PAHs in the sediment of the water bodies studied.

 

AIR QUALITY CONCERNS

Although unseen, releases of PAHs to the atmosphere (volatilization) from freshly coal-tar-sealed pavement are tens of thousands of times higher than from unsealed pavement. Volatilization is a potential human-health concern because inhalation is an important pathway for human exposure to PAHs. Volatilization of sealed surfaces is highest just after application and decreases rapidly over the following weeks. Nonetheless, volatilization continues long after application—PAH releases to the atmosphere from parking lots sealed from 3 to 8 years prior to sampling were on average 60 times higher than PAH releases from unsealed pavement. The results suggest that PAH emissions from new coal-tar-based sealcoat applications each year nationwide (~1000 Mg) are larger than annual vehicle emissions of PAHs.

 

ADDITIONAL RESOURCES