Traffic may worsen hay fever and asthma

Science News, Nov 20, 1999 by J. Raloff

As roads grow ever more congested, commutes lengthen. Even though individual vehicles spew less pollution, the air remains choked with combustion byproducts. Now, urban denizens have yet another reason to hate traffic: It pollinates their air.

A new study finds that cars and trucks stir up a lot of dust, much of it laden with allergy-causing plant pollens and molds.

Over an 8-month period, Ann G. Miguel of the California Institute of Technology in Pasadena and her colleagues vacuumed dust from paved roads at three Southern California sites--a relatively rural road in Riverside, a residential street in Long Beach, and a highway running through an industrial portion of downtown Los Angeles. Each thoroughfare was chosen because it ran directly in front of a pollution monitor measuring the quantity of particulate matter in air.

From earlier studies led by atmospheric chemist Glen R. Cass, also of Caltech, he and his colleagues knew how to calculate what share of the particles measured at a monitoring station come from road dust thrown into the air. In laboratory tests, they determined how much of the vacuumed dust could trigger an allergic response. Their new data, reported in the December ENVIRONMENTAL SCIENCE & TECHNOLOGY, indicate that up to 12 percent of the allergenic particles in air at the tested sites came from road dust kicked up there.

Some 40 to 70 percent of the particles in this road dust measured 10 micrometers in diameter or smaller, and so are known as PM-10. These are small enough to be inhaled into the lungs--or remain airborne long enough to travel miles.

Assays showed that the PM-10 fraction was every bit as allergenic as larger particles. This may surprise many pollution chemists and allergists, Cass says, "because people think of pollen grains as being larger." In fact, most pollen in the road dust was broken, but the fragments retained their potential to cause an allergic reaction.

However, Cass points out, even particles bigger than PM-10, which remain airborne for only a few hundred feet, can still travel far enough from the road to reach local homes. Moreover, the share of pollens and molds in road dust could be higher in sites where rainfall and vegetation are more abundant than in the arid sites the team studied, Cass observes.

The researchers conclude that the dust raised by traffic may trigger hay fever symptoms, such as runny nose, watery eyes, and sneezing. Inhaled more deeply, they say, these particles can cause the tissue swelling in lungs and shortness of breath that mark allergic asthma.

Traditional pollutants in vehicular exhaust, such as nitrogen dioxide, can also trigger asthma. David V. Bates of the University of British Columbia in Vancouver finds this article to be an "interesting contribution"--especially because breathing a very low concentration of nitrogen dioxide can exaggerate the asthmatic effect of subsequent exposure to an allergen, he says.

M. Eric Gershwin, chief of allergy at the University of California, Davis School of Medicine considers especially notable the new study's failure to detect latex in road dust. He says, "There had been theories that latex from tires [SN: 4/22/95, p. 244] was responsible for the increasing incidence of asthma."

The new data may also help explain reports that living near a busy road can exacerbate asthma symptoms, says Douglas W. Dockery of the Harvard School of Public Health in Boston. "We've been postulating that this might be due to diesel exhaust," he says, "but the [new study] suggests it may instead be due to the resuspension of road dust."