Baylor University and U.S. Geological Survey (USGS) scientists discovered that people living near playgrounds, driveways and parking lots coated with sealants containing coal tar—a potent carcinogen—could be more likely to develop cancer over the course of a lifetime. That's because the toxic compounds in the sealant readily break down and wind up in the soil and air, finding their way into houses. In the scientific risk assessment model, children faced the greatest risk.
"The increased cancer risk associated with coal-tar-sealed asphalt likely affects a large number of people in the U.S.," says lead study author E. Spencer Williams, PhD, assistant research scientist at Baylor University's Center for Reservoir and Aquatic Systems Research. "Exposure to these compounds in settled house dust is a particularly important source of risk for children younger than six years of age, as they are expected to ingest this material at higher rates."
While generally not used on public roads, up to 85 million gallons of coal tar sealant is applied to pavement each year, mostly in areas east of the Continental Divide in the U.S. and parts of Canada.
Labeled a human carcinogen by the International Agency for Research on Cancer, up to one-third of the contents of coal-tar sealants is cancer-causing polycyclic aromatic hydrocarbons. (Safer asphalt sealants are on the market and readily available—more on that later.) "It's a huge concentration," says Barbara Mahler, PhD, research hydrologist with USGS's Texas Water Science Center.
To put that in perspective, used motor oil, a product that's illegal to pour down storm drains, contains about 500 milligrams per kilogram of PAH chemicals. "Coal tar in the bucket is 50,000," Mahler explains. "But we're still spreading it on our parking lots, driveways, and sometimes even our playgrounds."
The trouble is, PAHs don't stay put. The constant wear and tear from tires and sneakers on pavement wears down the dried sealant, sending off tiny PAH particles that are tracked into homes or blown through open windows. Research shows that concentrations of PAHs are 25 times higher in household dust in homes adjacent to coal-tar-sealed pavement compared to other types of sealants or unsealed concrete.
This study, published in the journal Environmental Science and Technology , is one of the first steps in understanding how this widely used carcinogen is impacting human health. Researchers say the further research will help them better understand whether the risk assessment model is translated to real-life exposures.
Here's how to protect your family from this potent carcinogen.
• Create a no-shoes policy. PAHs are readily tracked into the home, so making family members and guests shed their shoes before entering the home can cut back on exposure.
• Close your windows. While coal-tar-treated surfaces continually shed dangerous PAH chemicals, the levels in the air are extremely high in the hours and days following a fresh coal-tar application, explains Peter Van Metre, USGS research hydrologist and author of two studies on volatilization of PAHs from sealcoat. If there's a potential seal coat application in your neighborhood, consider closing your windows on those days.
If you're doing DIY driveway sealing…
• Don't trust labels. If you don't see the word "coal tar" on the sealant bucket, that doesn't mean it's necessarily coal-tar free, Mahler warns. There are dozens of names for coal tar, including RT12, distilled tar, or refined tar. "Tar," is this instance, is a word you'll want to avoid.
• Do your homework. If you find a product that appears to be coal-tar free, do a quick online search of the product name plus "Material Safety Data Sheet." That sheet will contain a CAS number, digits totally unique to a specific chemical or compound. The number unique to coal tar is 65996-93-2. If you see those digits, don't buy it.
• Shop where it's not. If you don't feel like doing the extra work, head to a home improvement chains like Lowes, Home Depot, Ace, or Menards—they've all banned coal tar sealants from store shelves nationwide.
If you're hiring someone to do the job…
• Know the product. Find out the exact name of the sealing product the company uses. Warn neighbors, too. Applicators typically try to sell their services to an entire neighborhood. Once you know the product…
• Call up the CAS. Once you have the exact sealant name in hand, do an online search of the product name and "MSDS" or "Material Safety Data Sheet." On that document, look for the required CAS number; in the case of coal tar it's 65996-93-2.
If you live near a playground or parking lot…
• Alert store managers and playground officials of the dangers of carcinogenic coal-tar sealants, and let them know that alternatives containing thousands of times fewer PAHs are readily available.
• Speak up. For broad-sweeping protection in your city, borough, or township, consider joining forces with concerned neighbors and lobby your local and state governments to ban the sale and application of coal-tar sealants. These bans are popping up all over the country, from Washington, DC, to Washington state.
If you're installing a brand new driveway…
• Go for gravel. Consider building a blacktop-free driveway. It will alleviate the headache of trying to dodge toxic coal tar. And, healthier driveways made of gravel or permeable pavers helps reduce harmful motor oil runoff from your property. That helps keep pressure off of water treatment plants and helps reduce flooding in your community.
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