Traffic pollution revs up allergens
Reaction between smog and pollen could explain allergy increases.
Exhaust fumes from traffic could be turning airborne proteins into more powerful allergens, explaining why asthma and other allergies are on the rise in urban areas.
Researchers have found that the mixture of nitrogen dioxide and ozone produced by vehicles can add the chemical group nitrate to the protein molecules that account for up to 5% of the particles in our air.
Nitration could boost the power of existing allergens, or even make benign proteins allergenic, says Ulrich Pöschl, an atmospheric chemist from the Technical University of Munich, Germany, who worked on the study. Previous research has shown that nitrated proteins bind more strongly to the antibodies that cause allergic reactions.
"There is clear evidence that allergies are on the increase," says Pöschl. "The question is: 'Why?'."
Medical studies have shown a link between air pollution and rising allergy rates. But scientists have not been able to pin down how one causes the other. Nitration is now a prime suspect, Pöschl says.
Technical University of Munich, Germany
But for allergenic proteins from birch pollen left at a busy Munich road junction for a few days, that figure rises to 10%. And for allergenic proteins exposed to smog in the laboratory, nitration rose to 20%, they report in Environmental Science and Technology1.
The smog reacts with the amino acid tyrosine, a common component of proteins. Birch-pollen protein has seven tyrosine components and was readily nitrated by traffic smog. Proteins that are easiest to nitrate might trigger the strongest allergic reaction, says Pöschl.
It's not clear how nitrate groups increase the allergic response, he adds, but previous studies have suggested that the body uses tyrosine nitration as a marker to attract antibodies to inflamed tissue.
Pöschl thinks that the finding could help to develop drugs that stop nitrated proteins disrupting our immune systems. And it strengthens the case for reducing nitrogen-dioxide emissions, he adds.
Scientists are still unsure what makes a protein allergenic, and nitration is unlikely to be the only factor. Recent studies (see " Allergens reveal common contours" ) have suggested that allergens tend to share shapes that are particularly good at activating the body's immune response.
Clare Mills, an allergy expert at the Institute of Food Research, Norwich, UK, who led the research on shape, thinks that traffic pollution affects allergies in several different ways. Others have suggested that traffic pollution could make urban trees emit more allergens, she says.
"Life is often a mixture," says Mills. "I think research like this will help us unpick that."
Pöschl's team are trying to pin down how much traffic smog boosts proteins' power to cause allergies by giving normal and nitrated proteins to mice. Preliminary results suggest that the nitrated proteins are significantly more active. "These results will be very important when they come out," says Mills.
- Franze T., Weller M. G., Niessner R. & Pöschl U. Environ. Sci. Technol. published online doi:10.1021/es0488737 (2005).