By Nicola Jones
If you're thinking about sprucing up your driveway with a fresh coat of black sealant, consider this: some homes with black parking lots have been found to have surprisingly large doses of carcinogens in their household dust.
Some of the sticky, black sealants used to coat asphalt are made of coal tar, which contains polyaromatic hydrocarbons (PAHs), some of which are known or suspected carcinogens. Barbara Mahler of the U.S. Geological Survey in Austin, Texas, and colleagues have been tracking a link between high quantities of these compounds in the environment and sealed parking lots. "Scientists who work with these compounds--their jaws drop open when they see our numbers," says Mahler.
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Their work has led to the banning of coal-tar sealants in some cities, including their home town of Austin and, in 2009, Washington D.C. Now they show that the use of coal tar sealant in household driveways makes a big difference to the amount of PAHs in household dust.
Although there's no direct evidence that PAH-loaded dust is doing anyone harm--and there are many other hazards in household dust to worry about, from pesticides to flame retardants--the group says there is cause for concern. "Kids eat dust. They're on the floor and they put everything in their mouths," says Mahler. The group wants to team up with health experts to study the possible health effects.
Dust devils
The researchers looked at 23 ground-floor apartments in Austin, about half of which still had coal tar-sealed parking lots left over from before the 2006 ban.
The total amount of PAHs was on average 25 times higher inside apartments with coal tar-sealed parking lots than those with without, they report in Environmental Science & Technology. Surprisingly, there was no significant difference between households on the basis of any other factor they could think of: "Not cooking habits, not candles, not vacuuming, not pets or bicycles inside the house. None of those had any statistical significance," says Mahler.
There are few guidelines for PAH levels in household dust. Germany, where there were worries about exposure to PAHs from a coal tar-based glue used for wood floors, set a guideline of 10 micrograms of benzopyrene (the most worrisome PAH) per gram of dust. In the Texas study, 4 of the 11 homes with sealed driveways were above this threshold, with the highest at 24 micrograms per gram.
"On the parking lots themselves, the concentrations all exceed that number, some by a factor of 60," notes Mahler. That in itself is worrying because kids and pets often play on driveways in summer. Games such as basketball might particularly throw up PAH-ridden dust.
Paint it black
Coal-tar sealant is available from home-improvement stores and is used by both cities and homeowners to give an attractive black finish to asphalt. "It's also marketed as increasing the longevity of the asphalt underneath," says Mahler. "I've not been shown any studies to prove that." She notes that asphalt tends to degrade faster if it's hot, and a blacker surface might be expected to heat up faster.
Mahler says the concentration of PAHs in dust is truly surprising. It's more, she notes, than concentrations found at old coal-tar industrial sites that have been marked by the government as toxic zones for cleanup.
Anne LeHuray, a scientist with the Pavement Coatings Technology Council in Alexandria, Virginia, says the council has previously criticized the researchers for not looking at all the possible sources of PAHs in their environmental samples. Nevertheless, the council recommends not applying coal-tar sealants within 48 hours of rain, in order to limit any possible PAH release.
Mahler says that having a doormat or taking off shoes is always good advice to keep pollutants out of the home, but that, in their study, vacuuming habits didn't seem to make a difference to PAH concentrations.
