Coal-Tar Pavement Sealcoat: Concerns for Human Health and Aquatic Life

Coal-tar sealcoat, high in PAHs, is widely applied to parking lots, driveways and some recreation areas across the central and eastern United States. Tyres grind it into particles that can be tracked indoors or washed down storm drains into streams. Image from the USGS fact sheet.

Sealcoat is the black, viscous liquid sprayed or painted onto many asphalt parking lots, driveways and playgrounds to protect the asphalt and make it look better. Studies by the U.S. Geological Survey, universities and state and local agencies have identified coal-tar-based sealcoat as a major source of polycyclic aromatic hydrocarbon (PAH) contamination in cities and suburbs, and a possible concern for people and aquatic life.

Key findings

• Human health. As coal-tar sealcoat ages it wears into small, PAH-rich particles that can be tracked into homes and end up in house dust. For people living next to coal-tar-sealed pavement, swallowing contaminated house dust and soil raises the potential risk of cancer, especially for young children. Health professionals have linked PAH exposure, particularly early in childhood, to higher risks of lung, skin, bladder and respiratory cancers.
• Aquatic life. Runoff from coal-tar-sealed pavement, even runoff collected more than 3 months after application, is acutely toxic to fathead minnows and water fleas, two species commonly used to test toxicity. Even highly diluted runoff can damage DNA and impair its repair.

What PAHs are

PAHs are chemicals formed when carbon-containing material is heated or burned. Urban sources vary enormously in concentration: asphalt (2–9 mg/kg), tyre particles (84 mg/kg), used motor oil (730 mg/kg) and coal-tar sealcoat (34,000–202,000 mg/kg). Many PAHs can cause cancer, mutations, birth defects or death in fish, wildlife and invertebrates, and sunlight greatly strengthens the harmful effects of several. The U.S. Environmental Protection Agency (EPA) classes seven PAHs as probable human carcinogens (Class B2) and 16 as Priority Pollutants.

Coal tar is a by-product of coking, liquefying or gasifying coal; coal-tar pitch is what remains after coal tar is distilled, and is used mainly to make electrodes for the aluminium industry. Both are known human carcinogens, and coal-tar sealants contain one or the other.

Two kinds of sealcoat

Sealcoat is rarely used on public roads. Most products are either coal-tar or asphalt emulsion, though alternatives now exist. Coal-tar products, typically 20–35 percent coal tar or coal-tar pitch, have extremely high PAH concentrations; asphalt-based products have much lower ones. For historical and economic reasons, asphalt-based sealcoat is more common west of the Continental Divide and coal-tar sealcoat east of it, except where states, counties and towns have banned coal tar.

PAHs in dust swept from sealed parking lots follow the kind of sealcoat used on each side of the Continental Divide. Values are the sum of the EPA's 16 Priority Pollutant PAHs, in milligrams per kilogram (parts per million), from composite samples or medians. USGS.

Where the worn sealcoat goes

Sealcoat product (A) dries and wears into pavement dust (B). Rain washes the dust (C) into stormwater devices (D) or streams and lakes (E); tyres (F) track it onto unsealed pavement; wind and runoff carry it to soils (G); and feet track it indoors into house dust (H). The photograph shows light patches where sealcoat has worn away; applicators recommend resealing every 1 to 5 years. USGS.

PAH concentrations in each setting, in milligrams per kilogram, from studies by the USGS, other agencies and universities (means or medians):

Risks to human health

House dust. In a study of 23 ground-floor apartments in Austin, Texas, PAH levels in house dust were 25 times higher where the parking lot was sealed with coal tar than where it was concrete, unsealed asphalt or asphalt-based sealcoat. House-dust PAHs showed no link to other indoor sources such as smoking or fireplaces. House dust is an important route of exposure, especially for small children, who spend time on the floor and put hands and objects in their mouths.

PAH-laden dust from coal-tar-sealed pavement (right) is tracked indoors. Median values for the 16 Priority Pollutant PAHs, in milligrams per kilogram, in house dust and parking-lot dust. USGS.

Cancer risk. The USGS and a human-health risk analyst estimated the excess lifetime cancer risk from swallowing house dust and soil for people living next to parking lots with and without coal-tar sealcoat, using measured concentrations of the B2 PAHs (weighted by potency relative to benzo[a]pyrene), established ingestion rates and the EPA's slope factor. Next to coal-tar-sealed pavement the risk was 1.1 cancers per 10,000 people exposed, on average 38 times higher than next to unsealed pavement, and above the level at which the EPA generally considers clean-up advisable. Much of the risk comes from exposure in early childhood (ages 0 to 6). The study did not count exposure to the pavement itself, whose PAH levels are 10 or more times those of nearby house dust or soil.

A preschooler living next to coal-tar-sealed pavement with little hand-to-mouth activity takes in about 2.5 times more PAHs from house dust than from food; a more active preschooler takes in nearly 10 times more. USGS.

Risks to aquatic life

Acute toxicity. Runoff collected up to 42 days after sealing killed all the day-old fathead minnows and water fleas exposed to it, two standard laboratory test species; runoff from unsealed pavement killed no more than 10 percent. Under simulated sunlight, which intensifies some PAHs' toxicity, runoff collected 111 days (more than 3 months) after sealing killed all of both species, and still killed all the water fleas when diluted to 10 percent strength. Samples taken from 5 hours to 111 days after a professional applied the sealcoat varied little in total PAHs, as losses of lighter compounds were offset by gains in heavier ones: the runoff stays contaminated long after the 24-hour curing time.

Runoff from coal-tar-sealed pavement is acutely toxic to fathead minnows (Pimephales promelas, left) and water fleas (Ceriodaphnia dubia, right). Image from the USGS fact sheet.

Researchers at the National Oceanic and Atmospheric Administration and the U.S. Fish and Wildlife Service later found the runoff acutely lethal to young coho salmon and the cause of many abnormalities in zebrafish embryos; filtering it through a bioretention system greatly reduced its toxicity.

DNA damage. Runoff combined with simulated sunlight damaged DNA in rainbow trout liver cells, from a cell line developed to test PAH effects on DNA, even at 1 percent of its original strength, causing strand breaks and alkylated bases. A second experiment showed that exposed cells were less able to perform at least one kind of DNA repair. Damage plus weakened repair raises the potential for long-term harm; DNA damage can lead to ageing, cell death and mutations, which can affect genes and potentially lead to cancer.

Air quality

PAH releases to the air from freshly coal-tar-sealed pavement are tens of thousands of times higher than from unsealed pavement. That matters because breathing is an important route of PAH exposure. Releases fall quickly in the weeks after application but go on for years: lots sealed 3 to 8 years before sampling released on average 60 times more than unsealed pavement.

Nationwide, PAHs released each year from newly applied coal-tar sealcoat are estimated to exceed annual PAH emissions from vehicles. Rates in micrograms per square metre per hour. Image from the USGS fact sheet.

Sources

• Barbara J. Mahler, Michael D. Woodside and Peter C. Van Metre, Coal-Tar-Based Pavement Sealcoat—Potential Concerns for Human Health and Aquatic Life, USGS Fact Sheet 2016–3017, April 2016. https://doi.org/10.3133/fs20163017
• The figures and table are taken from the fact sheet's PDF. Three photographs in it are not reproduced here: two under Creative Commons licences and a DNA illustration from the University of Utah's Genetic Science Learning Center.
• Corrected: "biorention" to "bioretention".