The study's researchers looked at asthma, bronchitis, and traffic-related air pollution in California's Alameda County, which has relatively clean air despite being near San Francisco.

Led by Bart Ostro, PhD, of California's Office of Environmental Health Hazard Assessment, the team surveyed parents of more than 1,000 kids in grades 3-5. The families lived in 10 neighborhoods near busy traffic areas.

The parents were asked if their child had been diagnosed with asthma or bronchitis. Fourteen percent said their child had been diagnosed with asthma in the past 12 months. Twelve percent of the kids had bronchitis in the previous year, and 43% of the bronchitis patients also had asthma.

The researchers also measured outdoor pollution levels, focusing on traffic pollutants near the children's schools.

Since most of the kids in the study live near their schools and don't ride a bus, the pollution levels outside their homes should be about the same.

Pollution readings were taken in the spring (March-June) and fall (September-November) of 2001.

During that time, some kids moved to different addresses. But for those living in one place for at least a year, there were modest but significant increases in bronchitis symptoms and asthma in neighborhoods with higher concentrations of traffic pollutants, write the researchers.

Wind patterns were important. Living downwind of a major road increased exposure to traffic pollutants.

The findings are in line with other American and European studies, prompting the researchers to call for wider monitoring of traffic-related air pollution.