Is your driveway toxic? (© Stephen Mallon/Getty Images)

Protecting your pavement from the harsh effects of winter ice and summer heat can be smart home maintenance — if you're not coating your driveway with carcinogens. Scientists at the U.S. Geological Survey found that coal-tar pavement sealant, typically used east of the Continental Divide, contains 1,000 times more polycyclic aromatic hydrocarbons (PAHs), which are byproducts of fuel burning, than its asphalt-based counterpart used throughout the West. What's more, these known cancer-causing compounds might be tracked indoors.

After sweeping up samples of pavement dust from across the U.S., a USGS team discovered much higher levels of PAHs in the dust from pavement coated with coal-tar sealant than from pavement coated with asphalt-based sealant or no sealant at all.

"We thought that concentrations of PAHs were so high in parking lots that they may be affecting house dust," says Barbara Mahler, a hydrologist involved in the research, which was published in Environmental Science & Technology in November 2008.

Her hunch was right. For a follow-up study published this month in the same journal, the team found that PAH concentrations are 25 times higher in homes next to parking lots with coal-tar sealant because people naturally ferry road dust inside on their shoes and clothes. Based on human toxicology data, Mahler says the level of PAHs detected in these homes translates, minimally, to an increased cancer risk for preschoolers, who often crawl around and ingest things on floors and whose young bodies are still developing.

Another team of researchers, working for Austin, Texas, began studying PAHs several years ago; they quickly determined that the compounds are also an environmental health problem because they drain into aqueous environments as runoff. Members of that team began talking to the Environmental Protection Agency about their findings in 2005, but say it took the agency five years to take their warning seriously. "Coal-tar seal coat needs to be phased out," team member Nancy McClintock says. "It's a nationwide problem."

Austin, Washington, D.C., and Dane County, Wis., meanwhile, have banned its use, and Minnesota has banned the purchase of coal-tar seal-coat products by state agencies by July 1.

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Such phase-outs don't come easily, but there is precedent for them. Back in the 1940s, manufacturers began chemically treating lumber used in playgrounds and decks with a preservative called chromated copper arsenate to protect against rot and insect damage. CCA contains chromium and arsenic, toxic compounds that leach out of the wood and into soil — as with PAHs, putting children who crawl in the contaminated area at risk. After an EPA assessment, manufacturers phased out the use of CCA for virtually all residential wood products by the end of 2003. (Existing nonresidential uses include utility poles and crossties.) It has been replaced successfully by new water-based preservatives and composites such as recycled plastic lumber.

According to McClintock's team in Texas, asphalt-based seal coat is currently the only widely available, affordable alternative to coal tar. A product from Carbon Plex, made from a blend of asphalt-based seal coat and recycled tires, appears promising, but a gallon is about 15 times more expensive than a similar amount of coal tar — and, the researchers say, it's not commercially available, especially to smaller cities like Austin. Ushering in a viable alternative is a manufacturing and investment hurdle, compounded by a bad economy, they say.

BingIs asphalt-based seal coating safer?

The Austin team isn't certain that an alternative to seal coat is even needed. Manufacturers claim that coal tar-based sealants preserve the appearance of pavement, as well as protect it from water, salt, gas and oil penetration, extending its life span. But scientific studies about seal coat's efficacy have thus far been minimal. "We're not convinced seal coat is a terribly important product," Austin research team member Mateo Scoggins says. "But the PAHs in it are a serious problem."