Coal Tar-Based Sealcoat (PAH)

Coal-tar 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 polycyclic aromatic hydrocarbons (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 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.

USGS PAH Illustration

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.

Resources & Information

  1. What can I do?
  2. Identifying Coal-Tar Sealant or High PAH Products
  3. Coal-tar scientific studies

Homeowners and those who manage grounds for private entities such as churches, daycares or businesses: Consider eliminating sealcoating as a maintenance practice for asphalt surfaces.

If sealcoating cannot be eliminated, use asphalt-based sealcoat rather than coal tar sealcoat. There are contractors who will apply asphalt-based sealcoat, and this alternative is also available at most hardware stores. (Most major hardware store chains have already removed coal tar sealcoat from their shelves.)

OSU - PAH Infographic