USGS scientist Peter Van Metre examines a parking lot where coal-tar sealcoat has been applied.A USGS scientist adjusts an air pump used to measure emission of polycyclic aromatic carbons (PAHs) into the air.

What is sealcoat?

Coal-tar-based sealant is the black liquid sprayed or painted on many parking lots, driveways, and playgrounds.

Several PAHs are probable human carcinogens, and many are toxic to fish and other aquatic life. Coal tar, which can cause cancer in humans, is made up of more than 50 percent PAHs. An estimated 85 million gallons of coal-tar-based sealant are used on parking lots and driveways each year, primarily in the central and eastern United States.

What are the rates of PAH emissions to the air from coal-tar-based sealcoat?

Coal-tar-based sealants are emitting PAHs into the air at rates that could be greater than annual emissions from vehicles in the United States based on a study in which USGS scientists tracked PAH levels in air and in dried sealcoat following sealcoat application to a parking lot. Two hours after sealcoat application, PAH emissions were 30,000 times higher than those from unsealed pavement. In a second study, USGS scientists measured PAHs in air above parking lots with and without sealcoat, in suburban Austin, Texas. Parking lots with three- to eight-year-old sealant still released 60 times more PAHs to the air than parking lots without sealant.

A USGS scientist adjusts an air pump used to measure emission of polycyclic aromatic carbons (PAHs) into the air.

Children living near sealed parking lots are exposed to PAHs

Children living near coal-tar-sealed pavement are exposed to twice as many PAHs from ingestion of contaminated house dust than from food, according to a separate new study by Baylor University and the USGS. Baylor University scientist Spencer Williams used USGS measurements of PAHs in house dust to estimate the potential ingestion of PAHs by young children living near coal-tar-sealed parking lots. Ingestion of PAHs from food has long been thought to be the primary route by which children are exposed to PAHs. PAH ingestion by children living near coal-tar-sealed parking lots  is estimated to be 14 times higher than by children in apartments adjacent to unsealed parking lots.

Sealcoat is a major source of PAHs to the environment

Past and current  research on environmental contamination and coal-tar-based pavement sealants and implications for human health and stormwater management are summarized in a new Feature Article in the journal Environmental Science and Technology. The article is jointly authored by researchers with the USGS, Minnesota Pollution Control Agency, University of New Hampshire, City of Austin, Texas, and Baylor University.

Bans on coal-tar sealcoat

Some governments have taken action on the use of coal-tar-based sealcoat.  Fifteen municipalities and two counties in four states (Minnesota, New York, Texas and Wisconsin), the District of Columbia and the state of Washington all have enacted some type of ban, affecting almost 10.4 million people. Several national and regional hardware and home-improvement retailers have voluntarily ceased selling coal-tar-based driveway-sealer products.

Coal-tar sealcoat compared to asphalt-based sealcoat

USGS scientists prepare a sampler used to measure emission of polycyclic aromatic carbons (PAHs) into the air.

Two kinds of sealcoat products are widely used: coal-tar-based and asphalt-based.  The coal-tar products have PAH levels about 1,000 times higher than the asphalt products. Coal-tar-based sealcoat is more commonly used in the Midwest, southern, and eastern United States. Asphalt-based sealcoat is more commonly used in the western United States.  Consumers can determine whether a product contains coal tar by reading the product label or the associated Materials Safety Data Sheet (MSDS), available from the applicator, retailer or on the Internet.

For more information, please visit the USGS website on PAHs and sealcoat, or contact Jennifer LaVista.