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Environment

Vehicle Tailpipes Spew An Overlooked Pollutant

Air Monitoring: Urban traffic emits a significant amount of toxic isocyanic acid

by Janet Pelley
July 11, 2013

BAD AIR
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Credit: Shutterstock
Vehicle exhaust contains significant amounts of isocyanic acid, a compound linked to cataracts, rheumatoid arthritis, and hardened arteries.
Photo of exhaust from vehicles in traffic
Credit: Shutterstock
Vehicle exhaust contains significant amounts of isocyanic acid, a compound linked to cataracts, rheumatoid arthritis, and hardened arteries.

Vehicle exhaust sends a gaseous soup of small molecules into the air around cities, including acidic chemicals that can cause acid rain or smog. Canadian researchers now report that automobile emissions are a significant source of a once overlooked gaseous acid called isocyanic acid. The scientists estimate that urban traffic releases the chemical at levels high enough to possibly trigger health problems in people (Environ. Sci. Technol. 2013, DOI: 10.1021/es401127j).

Toxicologists have studied isocyanic acid (HNCO) in the blood and linked it to health problems observed in smokers, such as cataracts, rheumatoid arthritis, and hardened arteries. The acid causes harm by reacting with amine groups on proteins in the body through carbamylation. These covalent modifications cause the proteins to unfold, which then triggers a damaging inflammatory response.

Atmospheric chemists have only recently started to study isocyanic acid. “Even though isocyanic acid is a simple molecule—[it’s] just HNCO—researchers couldn’t measure it with existing equipment, and so it was ignored,” says Joost de Gouw, an atmospheric scientist at the University of Colorado, Boulder. De Gouw and his team recently developed an instrument called an acetate ion chemical ionization mass spectrometer (Acid-CIMS), which can measure levels of a wide range of gaseous acids, including isocyanic acid. In 2011, they used it to detect isocyanic acid for the first time in wood and tobacco smoke (Proc. Natl. Acad. Sci. USA 2011, DOI: 10.1073/pnas.1103352108). They measured the acid in the air around Boulder, Colo., at levels up to 200 ppt by volume. They attributed the presence of isocyanic acid to nearby wildfires.

“We thought that if forest fires can produce isocyanic acid, it was likely that vehicles could produce it, too,” says John Liggio, a research scientist with Environment Canada and an author of the new study. To estimate how much of the acid is emitted by vehicles every year, Liggio and his colleagues first ran a light-duty diesel engine from a passenger car in their lab and measured levels of pollutants in the exhaust using de Gouw’s mass spectrometer. They next calculated the ratio between isocyanic acid and carbon monoxide in the exhaust. Using data on annual carbon monoxide releases from vehicles, the researchers estimated that light-duty diesel vehicles emit roughly 6 tons of isocyanic acid per year across Canada.

The scientists also measured levels of the acid in the air near a major roadway in Toronto. Based on those measurements, they estimated that all vehicular traffic generates 770 tons of isocyanic acid per year throughout Canada. The large difference between the lab-based and field-based estimates suggests that “a lot more vehicles than just light-duty diesels are releasing isocyanic acid,” Liggio says.

De Gouw points out that the isocyanic acid concentrations measured in Toronto’s air spiked as high as 990 ppt by volume. Previous studies have estimated that inhaling isocyanic acid at 1 ppb by volume could lead to levels in the blood that favor protein carbamylation. So urban air may contain enough of the acid to cause health problems, he says.

“The bottom line is that we know the compound is in the atmosphere, it has potential health impacts, and we need to know more about the sources,” de Gouw says. Liggio and de Gouw agree that toxicologists and epidemiologists need to get involved to determine the chronic health effects of exposure to isocyanic acid.

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