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PHILADELPHIA, Aug. 17, 2008 —The daily exposure to free radicals from car exhaust, smokestacks and even your neighbors’ barbecue could be as harmful as smoking, according to a new study. Many combustion processes, such as those in a car engine, create tiny particles that may act as brewing pots and carriers for free radicals — chemicals believed to cause lung cancer and cardiovascular diseases.
The findings are from Barry Dellinger of LouisianaStateUniversity in Baton Rouge, who reported them August 17 in Philadelphia during a meeting of the American Chemical Society. Whether the exposure equates to smoking one cigarette or as many as two packs a day remains difficult to determine, he added.
His team’s lab experiments — first described in the July 1 Environmental Science & Technology — suggest that noxious chemicals form on soot nanoparticles in the still-hot residue of combustion, for example inside a car’s exhaust pipe and catalytic converter.
The chemicals are hydrocarbon-based free radicals. Similar chemicals usually degrade quickly if they float solo. But in this case, the chemicals stay attached to the nanoparticles, and they linger in the air for much longer than previously thought. “To our enormous surprise, the free radicals survive hours, days, even indefinitely,” Dellinger says.
To mimic the conditions in car exhaust as it cools, Dellinger’s team used silica particles 100 nanometers wide and coated them with copper oxide. The team then exposed the particles to a hot gas — experimenting with a range of different temperatures — containing hydrocarbons typically produced in flames. All those ingredients are common in the exhaust of motor vehicles and factories.
The researchers then examined the nanoparticles with magnetic fields tuned to identify unpaired electrons, the feature that makes free radicals highly reactive and potentially dangerous for living cells. The data showed a signature typical of free radicals and similar to that of semiquinone, a free radical found in cigarette smoke.
The free radicals, however, only showed up when the initial ingredients had been mixed together at temperatures between 200 and 600 degrees Celsius. That means free radicals are unlikely to form during the actual combustion, which takes place at higher temperatures. Instead, they would likely form once the exhaust begins to cool down.
David Pershing, a chemical engineer at the University of Utah in Salt Lake City, says the findings are potentially significant for human health.
Dellinger added that more research is needed to determine not only where someone would be exposed, but also how much the body would absorb.
The exact amount of risk the pollutants pose is hard to estimate, Dellinger said during his presentation. Air samples provided by the Electric Power Research Institute of Palo Alto, Calif., suggest that the risk could be equivalent to smoking as little as one cigarette a day or as much as more than two packs a day, he said. “It’s early in the game, and there’s a lot of ways of doing these calculations.”
The free radicals discovered by Dellinger’s team would not show up in ordinary smog checks, which detect molecules in the gas state and not those attached to solid nanoparticles, he said.
Even the most modern catalytic converters may be ineffective at eliminating the free radicals. Ironically, even as a catalytic converter breaks down smog-causing pollutants, it may be creating conditions (particularly high temperatures) for the free radicals to form. “You could be destroying some [pollutants] and creating some at the same time,” Dellinger says.
