USGS Research: PAHs and Coal-Tar-Based Pavement Sealcoat
Questions and Answers
What are sealcoat, coal tar, and PAHs?
Pavement sealcoat, or sealant, is a black liquid that is sprayed or painted on asphalt pavement. It is marketed as protecting and beautifying the asphalt surface. Sealcoat is used commercially and by homeowners across the Nation. It is applied to residential driveways, playground, and parking lots associated with commercial businesses, apartment and condominium complexes, churches, schools, and business parks. Most sealcoat products have a coal-tar-pitch or asphalt base. Coal-tar-based sealcoat is most commonly used in the central, southern, and eastern U.S., and asphalt-based sealcoat is used predominantly in the western U.S.
Coal tar is a byproduct of the coking of coal, and can contain 50 percent or more PAHs by weight. Coal-tar pitch is known to cause cancer in humans (International Agency for Research on Cancer, 1985). Coal-tar-based sealcoat products typically are 20 to 35 percent coal-tar pitch. Product analyses indicate that coal-tar-based sealcoat products contain about 1,000 times more PAHs than sealcoat products with an asphalt base (City of Austin, 2005).
Polycyclic aromatic hydrocarbons (PAHs) are a group of chemical compounds that form during the combustion of anything with a carbon base, from wood and gasoline to cigarettes and meat. PAHs also are found in products whose production involves the heating of hydrocarbons, such as automobile tires and coal-tar pitch. PAHs are of environmental concern because several are toxic, carcinogenic, mutagenic, and(or) teratogenic (causing birth defects) to aquatic life and because several are probable human carcinogens.
What are the concerns for our environment and for us?
PAHs are toxic to mammals (including humans), birds, fish, amphibians, invertebrates, and plants. Aquatic invertebrates, the insects and other small animals that live in streams and lakes, are particularly susceptible to PAH contamination, especially the bottom dwellers (benthic invertebrates) that live in the mud where PAHs tend to accumulate. They are an important part of the food chain and are often monitored as indicators of stream quality (analogous to the "canary in the coal mine" concept). Possible effects of PAHs on aquatic invertebrates include inhibited reproduction, delayed emergence, sediment avoidance, and mortality, and possible adverse effects on fish include fin erosion, liver abnormalities, cataracts, and immune system impairments. PAHs tend to attach to sediment; the Probable Effect Concentration (PEC)—a widely used sediment quality guideline that is the concentration of a contaminant in bed sediment expected to adversely affect benthic (or bottom-dwelling) biota—is 22.8 mg/kg (milligrams per kilogram) for total PAHs.
Human health risk from environmental contaminants usually is evaluated in terms of exposure pathways. For example, people could potentially be exposed to PAHs in sealcoat through skin contact with abraded particles from driveways or parking lots either by direct touching of touching of toys or other items that have been in contact with the pavement, inhalation of wind-blown particles, and inhalation of fumes that volatilize from sealed parking lots. PAHs in streams and lakes rarely pose a human health risk via drinking water because of their tendency to attach to sediment rather than dissolve in water.
Where is coal-tar-based sealcoat used?
In the U.S., coal-tar-based sealcoat is used primarily east of the Continental Divide, and asphalt-based sealcoat is used primarily west of the Continental Divide. Coal-tar-based sealcoat also is used in Canada. PAH concentrations in the products themselves are about 90,000 mg/kg in coal-tar-based products and 50 mg/kg in asphalt-based products (City of Austin, 2005).
In one USGS study, dust was swept from sealcoated and unsealcoated parking lots in nine cities across the U.S. and analyzed for PAHs. For six cities in the central and eastern U.S., the median PAH concentration in dust from sealcoated parking lots was 2,200 milligrams per kilogram (mg/kg), about 1,000 times higher than in dust from sealcoated parking lots in the western U.S. (median concentration 0.8 mg/kg).
How does sealcoat get from driveways and parking lots into streams and lakes?
Friction from vehicle tires abrades pavement sealcoat into small particles. These small particles are washed off pavement by precipitation and into storm drains and streams. Wear and tear of sealcoat is visible in high traffic areas within a few months after application. It has been estimated that about 5% of sealcoat wears off the driving areas of parking lots each year (Scoggins, 2009). Sealcoat manufacturers recommend reapplication every 2 to 4 years.