Science News
August, 2008
August, 2008
Cancer-causing agents' interaction with nanoparticles could make the
chemicals as harmful as cigarette smoke, lab study suggests
By Davide Castelvecchi
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, 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 Louisiana State University 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 called semiquinones.
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. Data on atmospheric pollution
provided by the Electric Power Research Institute of Palo Alto,
Calif., suggests 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.
Citations & References:
Lomnicki, S.... and B. Dellinger 2008. Copper oxide-based model of
persistent free radical formation on combustion-derived particulate
matter. Environmental Science & Technology 42(July 1):4982.
Posts
