"Those infants who had more exposure to wood-burning appliances were more likely to show up in doctors' offices or be hospitalized for bronchiolitis," said Dr. Catherine Karr, an assistant professor of pediatrics at the University of Washington, and lead author of a report in the Nov. 15 issue of the American Journal of Respiratory and Critical Care Medicine.

There hasn't been much research done on the effects of air pollution on very young children, Karr said. Such studies have typically focused on older children, in whom asthma is a more prevalent problem related to air pollution.

Karr and Canadian researchers analyzed nearly 12,000 cases of infant bronchiolitis between 1999 and 2002 in the province of British Columbia, checking on exposure to air pollutants such as nitric oxide, nitrogen dioxide, carbon monoxide and particulate matter. They also looked at the source of those pollutants.

Infants who lived within 50 meters -- about 55 yards -- of a highway had a 6 percent increased risk of bronchiolitis, while those with higher exposure to wood smoke had an 8 percent increased risk, compared to those with the lowest exposure.

"Bronchiolitis is the number one reason why a child ends up in a hospital in the first year of life," Karr explained. "It is responsible for 13 percent of those hospitalizations."

Bronchiolitis is a respiratory condition that starts out looking like a common cold but can become "quite severe," she said. It can be caused by viruses, and is often the first infection a child experiences early in life.

The study "lets families know about concerns about infant exposure to traffic and wood-burning appliances," Karr said. "If they can avoid those things, they should. If they do use wood-burning appliances, they should use safety practices, making sure the appliances are properly vented and burn efficiently."

The study included such pollution because "here in the Pacific Northwest, we have more exposure to wood-burning stoves than in other places," Karr said.

"This study extends some past findings that wood smoke can be very irritating to the respiratory system, and has been shown to have effects on the lungs of children," said George Thurston, director of the Particulate Matter Health Effects Research Center in New York.

"Wood smoke seems to have the biggest effects on respiratory health, whereas fossil fuels seem to have the biggest effects on cardiac health, because they are more laden with metals," Thurston said.

The Pacific Northwest is unusual because of a higher concentration of wood-burning appliances, he said. "In other areas, traffic may dominate more," Thurston noted.

More information

The basics of bronchiolitis are explained by the American Academy of Family Physicians.