U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2016–3017

April 2016

Coal-Tar-Based Pavement Sealcoat—Potential Concerns 

for Human Health and Aquatic Life

Sealcoat is the black, viscous liquid sprayed or painted on many asphalt parking lots, driveways, 

and playgrounds to protect and enhance the appearance of the underlying asphalt. Studies by the 

U.S. Geological Survey (USGS), academic institutions, and State and local agencies have identified 

coal-tar-based pavement sealcoat as a major source of polycyclic aromatic hydrocarbon (PAH) 

contamination in urban and suburban areas and a potential concern for human health and aquatic life.1

Key Findings:

Human Health Concerns—As coal-tar-based sealcoat ages, it wears into small particles with high levels of 

PAHs that can be tracked into homes and incorporated into house dust. For people who live adjacent to coaltar-sealcoated pavement, ingestion of PAH-contaminated house dust and soil results in an elevated potential 

cancer risk, particularly for young children. Exposure to PAHs, especially early in childhood, has been linked 

by health professionals to an increased risk of lung, skin, bladder, and respiratory cancers.2

Aquatic Life Concerns—Runoff from coal-tar-sealcoated pavement, even runoff collected more than 3 months 

after sealcoat application, is acutely toxic to fathead minnows and water fleas, two species commonly used to

assess toxicity to aquatic life. Exposure to even highly diluted runoff from coal-tar-sealcoated pavement can 

cause DNA damage and impair DNA repair. These findings demonstrate that coal-tar-sealcoat runoff can

remain a risk to aquatic life for months after application.

Coal-tar-sealcoat, which contains elevated levels of PAHs, is 

commonly applied to parking lots, driveways, and some recreational 

areas across the central and eastern parts of the United States. 

Friction from vehicle tires abrades sealcoat into small particles 

that can be tracked indoors or washed down storm drains and 

into streams, potentially harming human and aquatic life. 
Light gray patches of asphalt show where 

sealcoat has been worn from the pavement. 

Applicators recommend reapplication of 

sealcoat from every 1 to 5 years.1

As Sealcoat Wears Off, Where Does It Go?

Worn particles of coal-tar-based sealcoat containing high concentrations of PAHs and related chemicals 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). 

Associated PAH concentrations for these settings, from studies by the USGS, other government agencies, 

and academic institutions, are given below.

Asphalt pavement

D. Stormwatermanagement

device

E. Lake sediment

Wind

G. Soil

A. Sealcoat product

F. Tires

Abrasion

B. Pavement dust

Adhesion

H. House dust

Runoff

C. Runoff

Tracking

SEALANT

Sealcoat

NOT TO SCALE

Setting

PAH concentration*

(milligrams per kilogram)

Coal-tarsealcoat settingsNon-coal-tarsealcoat settings

(A) Sealcoat products 66,000 50

(B) Pavement dust 2,200 11

(C) Runoff, particles 

Runoff, unfiltered water

3,500

62

54

4

(D) Stormwater-management-

 device sediment

646 2

(E) Lake sediment 33 0.4

(F) Particles adhered to tires 1,380 3

(G) Soil 105 2

(H) House dust 129 5

*Concentrations are means or medians. References and additional information 

are provided in Mahler and others (2012).1

Write From Karen, CC BY-NC_ND 2.0

View the publication: 

http://dx.doi.org/10.1021/es203699x
PAH Levels in Asphalt-Based and Coal-Tar-Based Sealcoat

Pavement sealcoat is a commercial product that is applied to many asphalt 

parking lots, driveways, and playgrounds in North America in an effort to protect 

and beautify the underlying asphalt. It rarely is used on public roads.

Most sealcoat products are either coal-tar or asphalt emulsion, although some 

alternative products now are available.3 Coal tar and coal-tar pitch have extremely 

high concentrations of PAHs as do coal-tar-based sealcoat products, which 

typically are 20 –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, use of asphalt-based sealcoat in the 

United States is more common west of the Continental Divide and use of coal-tarbased sealcoat is more common east of the Continental Divide, except in States, 

counties, and municipalities where use of coal-tar-based sealcoat is prohibited.3

PAH levels in dust swept from sealed 

parking lots reflect the type of pavement 

sealcoat commonly used west and east of the 

Continental Divide.1 Concentrations, in units of milligrams per kilogram (mg/kg), also referred to as “parts per million” (ppm), shown 

here are for the sum of the 16 PAHs listed by the U.S. Environmental Protection Agency as Priority Pollutants. Concentrations are for 

composite samples from multiple parking lots or a median of several individual samples.5

Asphalt-based sealcoat, 

primarily used west of 

the Continental Divide, 

typically contains about 

50 mg/kg PAHs.4

Coal-tar-based sealcoat, primarily used 

east of the Continental Divide, typically 

contains 50,000 to 100,000 mg/kg PAHs.4

Seattle

Portland

Salt Lake City

Austin

Minneapolis

Chicago

Detroit

Washington, D.C.

New Haven

5.2

<13

2.1

570

3,200

3,400

3,200

1,300

2,000

Continental Divide

Polycyclic aromatic hydrocarbons (PAHs) are a 

group of chemicals created by heating or burning material 

that contains carbon. The many sources of PAHs to the 

urban environment span a wide range of PAH concentrations and include asphalt (2–9 mg/kg), tire particles 

(84 mg/kg), used motor oil (730 mg/kg), and coal-tar-based

sealcoat (34,000–202,000 mg/kg).6 PAHs are an environmental concern because many cause cancer, mutations, 

birth defects, or death in fish, wildlife, and invertebrates.7

Exposure to sunlight greatly intensifies the adverse effects

of several PAHs. The U.S. Environmental Protection Agency

(EPA) has classified seven PAHs as probable human

carcinogens (Class B2) and 16 PAHs as Priority Pollutants. 

Environmental and health effects depend on which PAHs 

are present and their concentrations.

Coal tar is a byproduct of the coking, liquefaction, or 

gasification of coal and is a complex mixture composed

primarily of aromatic hydrocarbons. Coal-tar pitch is the 

residue that remains after the distillation of coal tar; it is 

a complex mixture of high molecular weight aromatic 

hydrocarbons and black carbon solids. The primary use 

of coal-tar pitch is in electrode manufacturing for the 

aluminum industry.8 Coal-tar emulsion pavement sealants 

contain either crude coal tar (Chemical Abstracts Service 

[CAS] Registry Number 8007–45–2) or coal-tar pitch 

(CAS Registry Number 65996 –93–2). Coal tar and

coal-tar pitch are known human carcinogens.9

PAHs are made up of various arrangements of benzene rings. PAHs 

commonly occur in the environment as mixtures, which typically include at 

least some of the PAHs that are classified as probable human carcinogens.

Coal

Coke

Coke oven Coal tar Coal-tar

distillation

Coal-tar

pitch

Pavement

sealcoat
Dietary

ingestion Low house-dust 

ingestion rate 

(0.027 gram per day)

High house-dust 

ingestion rate 

(0.101 gram per day)

Non-dietary ingestion

0

50

100

150

250

200

Coal-tar sealcoat

Coal-tar sealcoat

No coal-tar sealcoat

No coal-tar sealcoat

Probable human carcinogen (Class B2)

intake, in nanograms per kilogram

body weight per day

5.1

Indoor

129

Indoor

No coal-tar

 sealcoat

Coal-tar

 sealcoat

9.0

Outdoor

4,760

Outdoor

Photograph by Jeffrey Melton, CC BY-NC 2.0

Children ingest 

house dust and soil when they put their hands or objects into

their mouth. Much of the estimated excess cancer risk associated 

with the ingestion of PAH-contaminated soil and house dust is 

incurred during early childhood.

The USGS partnered with a human-health-risk analyst to estimate the 

excess lifetime cancer risk associated with the ingestion of house dust and 

soil for people living adjacent to parking lots with and without coal-tarbased sealcoat. Excess cancer risk is the extra risk of developing cancer 

caused by exposure to a toxic substance. The excess cancer risk for people 

living adjacent to coal-tar-sealcoated pavement (1.1 cancer incidences 

for every 10,000 individuals exposed) was 38 times higher, on average 

(central tendency), than for people living adjacent to unsealed pavement. 

The central tendency excess cancer risk estimated for people living adjacent 

to coal-tar-sealcoated pavement exceeds the threshold generally considered 

by the EPA as making remediation advisable.

The assessment used measured concentrations of the B2 PAHs in 

house dust and soils adjacent to coal-tar-sealed pavement (adjusted for 

relative potency to the PAH benzo[a]pyrene), established house dust and 

soil ingestion rates, and the EPA-established slope factor to estimate the 

excess cancer risk. Much of the estimated excess risk comes from 

exposures to PAHs in early childhood (that is, 0−6 years of age). The

study did not consider the excess cancer risk associated with exposure 

to the sealcoated pavement itself, which has PAH concentrations 10 or 

more times greater than in adjacent residence house dust or soils.5, 10

Living adjacent to coal-tar-sealed pavement increases cancer risk12

View the publication:

http://pubs.acs.org/doi/pdf/10.1021/es303371t

PAHs from coal-tar-based sealcoat contaminate house dust10

In a study of 23 ground-floor apartments in Austin, Texas,

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.

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.

The preschooler 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.11

For the more active preschooler, 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.

PAH-contaminated dust on coal-tar-sealcoated pavement 

(right) is tracked indoors.10 Concentrations shown are 

median values for the sum of the 16 Priority Pollutant PAHs, 

in units of milligrams per kilogram, in house dust and 

parking lot dust.

View the publication:

http://pubs.acs.org/doi/pdf/10.1021/es902533r

Potential Risks to Human Health
Potential Risks to Aquatic Life

Runoff from coal-tar-sealcoated pavement is acutely toxic to aquatic biota13

View the publication:

http://pubs.acs.org/doi/abs/10.1021/acs.est.5b00933

Runoff from coal-tar-sealcoated pavement damages DNA and impairs DNA repair17

Types of DNA damage caused 

by exposure to runoff from 

coal-tar-sealed pavement 

include breaks in the 

DNA strands. 

(Image from Genetic Science Learning Center, http://learn.genetics.utah.edu.)

Simultaneous exposure to runoff from coal-tar-sealed pavement and simulated 

sunlight damaged DNA in rainbow trout liver cells, even when the runoff was 

diluted to 1 percent of its initial concentration. The cells were from a cell line 

developed to assess the effects of PAHs on DNA. The test assessed two types of 

DNA damage: strand breaks and alkylated bases.

Although cells can repair some DNA damage, a second experiment 

demonstrated that cells exposed to the coal-tar-sealcoat runoff had an impaired 

capacity to perform at least one type of DNA repair. The combination of DNA 

damage and impaired repair capacity intensifies the potential for long-term damage

to cell health. DNA damage has many possible consequences, including aging, cell 

death, and mutations. Mutations can affect the function of genes and can potentially 

lead to cancer.

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 experienced 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. 

The USGS collected samples of runoff from 5 hours to

111 days following sealcoat application to pavement by a 

professional applicator. Total PAH concentrations varied 

relatively little, as rapid decreases in concentrations of low 

molecular weight and nitrogen-substituted PAHs were offset 

by increases in high molecular weight PAHs.14 These results 

demonstrate that runoff from coal-tar-sealcoated pavement 

continues to contain elevated concentrations of PAHs and 

related compounds long after a 24-hour curing time.

A subsequent study by researchers at the National Oceanic 

and Atmospheric Administration (NOAA) and the U.S. Fish and 

Wildlife Service found that coal-tar-sealcoat runoff is acutely 

lethal to juvenile coho salmon (Oncorhynchus kisutch) and causes

a wide spectrum of abnormalities to zebrafish (Danio rerio) 

embryos.15 They also reported that filtration of the runoff

through a biorention system substantially reduced toxicity.

Runoff from coal-tar-sealcoated pavement is acutely toxic to fathead minnows 

(Pimephales promelas; left) and water fleas (Ceriodaphnia dubia; right).

Runoff from coal-tar-sealcoated pavement 

goes down storm drains to receiving 

water bodies. The runoff contains high 

concentrations of PAHs and related 

chemicals that can harm aquatic life.16
*bjmahler@usgs.gov

45,000 μg/m2-h

1.4 μg/m2-h

ISSN 2327-6916 (print)

ISSN 2327-6932 (online)

http://dx.doi.org/10.3133/fs20163017

Air-Quality Concerns18, 19

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. Although volatilization decreases rapidly over the weeks following 

application, it nonetheless 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. 

Nationwide, the combined PAH releases each year 

from newly applied coal-tar-based sealcoat are 

estimated to exceed annual vehicle emissions of 

PAHs.18 PAH releases shown here are in units of 

micrograms per meter squared per hour (μg/m2-h).

For more information

Access publications and learn more about PAHs and coal-tar-based 

pavement sealcoat at http://tx.usgs.gov/sealcoat.html.

References Cited

1. Mahler, B.J., Van Metre, P.C., Crane, J.L., Watts, A.W., Scoggins, M., 

and Williams, E.S., 2012, Coal-tar-based pavement sealcoat and PAHs—

Implications for the environment, human health, and stormwater management: Environmental Science and Technology, v. 56, p. 3039–3045.

2. Agency for Toxic Substances and Disease Registry, 1995, Toxicological 

profile for polycyclic aromatic hydrocarbons: Atlanta, Ga., U.S. Department

of Health and Human Services, Public Health Service, accessed 

November 16, 2015, at http://www.atsdr.cdc.gov/toxprofiles/

tp.asp?id=122&tid=25.

3. Minnesota Pollution Control Agency, 2014, Choosing alternatives to coal 

tar-based pavement sealcoats, accessed November 16, 2015, at https://www.

pca.state.mn.us/water/stormwater-great-lakes-coal-tar-sealcoat-pah-reduction.

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2011.12.072.

By Barbara J. Mahler,* Michael D. Woodside, and Peter C. Van Metre