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USGS Research: PAHs and Coal-Tar-Based Pavement Sealcoat
Polycyclic aromatic hydrocarbons, or PAHs, are a group of organic compounds composed of fused benzene rings. PAHs occur naturally in crude oil, coal, and tar, and are produced by the incomplete combustion of fossil fuels and biomass (wood, grass, tobacco). They are widely distributed in the environment and are of concern because they are toxic, mutagenic, teratogenic, and several are probable human carcinogens.
Pavement sealcoat (also called sealant) is a black liquid that is sprayed or painted on asphalt parking lots and driveways. It is marketed as protecting and beautifying the asphalt pavement. Sealcoat is used commercially and by homeowners across the Nation. It is applied to residential driveways, playgrounds, 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 commonly used in the central, southern, and eastern U.S., and asphalt-based sealcoat is commonly used in the western U.S.
Coal-tar can contain 50 percent or more PAHs by weight and 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 50,000 mg/kg PAH - about 100 times more PAHs than in used motor oil and about 1,000 times more PAHs than sealcoat products with an asphalt base (City of Austin, 2005).
Sealcoat contributes PAHs to many different parts of our environment1. PAHs are released from sealcoated pavement into the air. Dried sealcoat is abraded from pavement by vehicle tires and snow plows into fine dust containing high concentrations of PAHs. The dust can wash off in stormwater runoff and be carried to stormwater ponds, streams, and lakes. It can be blown onto adjacent pavement or soils, or be tracked into homes. Living adjacent to coal-tar-sealed pavement is associated with a significanty increased risk of cancer2.
Learn more about PAHs and coal-tar-based sealcoat from the USGS factsheet and the Feature Article in Environmental Science and Technology.
Key Findings from USGS Studies
- PAH concentrations in Lady Bird Lake sediment decline following a ban on coal-tar-based pavement sealants in Austin, Texas.13
- 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.2
- Emissions of PAHs to air 2 hours after application of coal-tar sealant are 30,000 times greater than from unsealed pavement, and total annual PAH emissions from newly applied sealcoat likely exceed those from vehicles in the U.S.3
- Emissions of PAHs to air from parking lots with older (3-8 years) coal-tar sealant are 60 times greater than from unsealed pavement.4
- Residences adjacent to parking lots with coal-tar-based sealcoat have housedust with elevated concentrations of PAHs.5
- Doses of carcinogenic PAHs through incidental 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 exposure exceeds non-dietary exposure (first author E. Spencer Williams, Baylor Univ.).6
- 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.5,9,10
- Coal-tar pitch from sealcoat reaches streams and lakes in runoff and soils and unsealed pavement near sealed parking lots.7,8
- Coal-tar-based sealcoat was determined to be the largest source of PAH contamination to urban lakes.7
- Use of coal-tar-based sealcoat is the primary cause of upward trends in PAHs in urban lake sediment.7
- Concentrations of polycyclic aromatic hydrocarbons (PAHs) in runoff from pavement with coal-tar-based sealcoat remain elevated for months following sealcoat application.12
Publications in Peer-Reviewed Scientific Journals
- Mahler, B.J.; Van Metre, P.C.; Crane, J.L.; Watts, A.W.; Scoggins, M.; Williams, E.S., Coal-tar-based pavement sealcoat and PAHs: Implications for the environment, human health, and stormwater management. Environ. Sci. Technol., 2012. [download PDF]
- Williams, E.S.; Mahler, B.J.; Van Metre, P.C., Cancer risk from incidental ingestion exposures to PAHs
associated with coal-tar-sealed pavement. Environ. Sci. Technol. 2012, v. 47 (2), 1101-1109.
- Van Metre, P. C.; Majewski, M. S.; Mahler, B. J.; Foreman, W. T.; Braun, C. L.; Wilson, J. T.; Burbank, T. PAH volatilization following application of coal-tar-based pavement sealant. Atmos. Environ. 2012. [download PDF]
- Van Metre, P. C.; Majewski, M. S.; Mahler, B. J.; Foreman, W. T.; Braun, C. L.; Wilson, J. T.; Burbank, T. Volatilization of polycyclic aromatic hydrocarbons from coal-tar-sealed pavement. Chemosphere, 2012. [download PDF]
- Mahler, B. J.; Van Metre, P. C.; Wilson, J. T.; Musgrove, M.; Burbank, T. L.; Ennis, T.; Bashara, T. J., Coal-tar-based parking lot sealcoat: An unrecognized source of PAH to settled house dust. Environ. Sci. Technol. 2010, 44, 894-900.
- Williams, E. S.; Mahler, B. J.; Van Metre, P. C. Coal-tar pavement sealants might substantially increase children's PAH exposures. Environ. Pollut. 2012.
- Van Metre, P. C.; Mahler, B. J., Contribution of PAHs from Coal-Tar Pavement Sealcoat and Other Sources to 40 U.S. Lakes. Sci. of the Total Environ., 2010, v.409, 334-344.
- Yang, Y., Van Metre, P.C., Mahler, B.J., Wilson, J.T., Ligouis, B., Razzaque, M.M., Schaeffer, D.J., and Werth, C.J., 2010, Influence of coal-tar sealcoat and other carbonaceous materials on polycyclic aromatic hydrocarbon loading in an urban watershed: Environ. Sci. Technol., v. 44, p. 1217-1223.
- Van Metre, P. C.; Mahler, B. J.; Wilson, J., PAHs underfoot: Contaminated dust from coal-tar sealcoated pavement is widespread in the United States. Environ. Sci. Technol. 2009, 43, (1), 20-25. Environ. Sci. Technol. 2009, 43, (1), 20-25.
- Mahler, B. J.; Van Metre, P. C.; Bashara, T. J.; Wilson, J. T.; Johns, D. A., Parking lot sealcoat: An unrecognized source of urban PAHs. Environ. Sci. Technol. 2005, 39, (15), 5560-5566.
- Van Metre, P.C., and Mahler, B.J., 2005, Trends in Hydrophobic Organic Contaminants in Lake Sediments Across the United States, 1970-2001: Environ. Sci. Technol., v. 39, no. 15, p. 5567-5574.
- Mahler, B.J., Van Metre, P.C., and Foreman, W.T. Concentrations of polycyclic aromatic hydrocarbons (PAHs) and azaarenes in runoff from coal-tar- and asphalt-sealcoated pavement. Environ. Pollut., v. 88, p. 81-87.
- Van Metre, P.C. and Mahler, B.J., PAH Concentrations in Lake Sediment Decline Following Ban on Coal-Tar-Based Pavement Sealants in Austin, Texas. Environ. Sci. Technol., Article ASAP.
Article Supporting Information: text | tables
- Wilson, J.T., Van Metre, P.C., Werth, C.J., and Yang, Yanning, 2006 Particle-associated contaminants in street dust, parking lot dust, soil, lake-bottom sediment, and suspended and streambed sediment, Lake Como and Fosdic Lake watersheds, Fort Worth, Texas, 2004: U.S. Geological Survey Data Series 211, 24 p.—ONLINE ONLY
- Van Metre, P. C.; Mahler, B. J.; Wilson, J. T.; Burbank, T. L. 2008. Collection and Analysis of Samples for Polycyclic Aromatic Hydrocarbons in Dust and Other Solids Related to Sealed and Unsealed Pavement From 10 Cities Across the United States, 2005-07. USGS Data Series 361.
- Mahler, Barbara J.; Van Metre, Peter C.; Wilson, Jennifer T. 2004. Concentrations of Polycyclic Aromatic Hydrocarbons (PAHs) and Major and Trace Elements in Simulated Rainfall Runoff from Parking Lots, Austin, Texas, 2003. USGS OFR 2004-1208. This report was subject to an "Information Quality Act" challenge from the sealcoat industry, to which the USGS responded. A press release summarized the USGS response.
- Van Metre, P.C.; Mahler, B.J.; Scoggins, M.; and Hamilton, P.A., 2005. Parking Lot Sealcoat: A Major Source of Polycyclic Aromatic Hydrocarbons (PAHs) in Urban and Suburban Environments: U.S. Geological Survey Fact Sheet 2005-3147, 4 p (.pdf)
- Mahler, B.J., and Van Metre, P.C., 2011, Coal-tar-based pavement sealcoat, polycyclic aromatic hydrocarbons (PAHs), and environmental health: U.S. Geological Survey Fact Sheet 2011–3010, 6 p.
- Coal-tar-based pavement sealcoat: Health and environmental overview: presented by Barbara Mahler to the CDC and ATSDR, June 20, 2014
- Coal-tar-based pavement sealcoat: Health and environmental overview: presented by Barbara Mahler at a Texas Water Science Center brownbag, December 5, 2013
- PAHs, coal-tar sealants, and environmental health: presented by Barbara Mahler in a Congressional Briefing, Washington, D.C., April 14, 2011
- PAH trends and the link to coal-tar sealants: presented by Peter Van Metre in the USGS Water Mission Area Lecture Series, 2011
- Summary of USGS research on sealcoat: presented by Barbara Mahler to Society of Environmental Toxicology and Chemistry (SETAC), Portland, OR, 2010
- Volatilization of PAHs from sealcoated pavement: presented by Peter Van Metre to Society of Environmental Toxicology and Chemistry (SETAC), Portland, OR, 2010
- Sources of PAHs to urban lakes: presented by Peter Van Metre to Geological Society of America (GSA), Denver, CO, 2010
- Relation of sealcoated parking lots and PAHs in settled house dust: presented by Barbara Mahler to Society of Environmental Toxicology and Chemistry (SETAC), New Orleans, LA, 2009
USGS Information Quality Act
Some USGS publications on PAHs and coal-tar-based pavement sealant have been subject to Information Quality Act correction requests. USGS Information Quality Guidelines, the correction requests, and USGS responses can be found at: http://www.usgs.gov/info_qual/
Contacts for further information
- Peter Van Metre, USGS Research Hydrologist (512) 927-3506
- Barbara Mahler, USGS Research Hydrologist (512) 927-3566
- Mailing Address:
U.S. Geological Survey
1505 Ferguson Lane
Austin, TX 78754
Frequently Asked Questions about sealcoat, coal tar, and PAHs
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.
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.
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 sed 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).
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.