Researchers Link Childhood Asthma To Exposure To Traffic-related Pollution

Scientists studying air pollution levels in 10 Southern Californiacities found that the closer children live to a freeway, the greatertheir chance of having been diagnosed with asthma. They report theirfindings in the November issue of the journal Epidemiology.

Researchers also found that children who had higher levels ofnitrogen dioxide, or NO2, in the air around their homes were morelikely to have developed asthma. NO2 is a product of pollutants emittedfrom combustion engines, such as those in cars and trucks.

"These results suggest that tailpipe pollutants from freewaytraffic are a significant risk factor for asthma," says lead authorJames Gauderman, Ph.D., associate professor of preventive medicine atthe Keck School. "Considering the enormous costs associated withchildhood asthma, today's public policy toward regulating pollutantsmay merit some re-evaluation."

"These results have both scientific and public healthimplications," says David A. Schwartz, M.D., director of the NationalInstitute of EnvironmentalHealth Sciences, the federal agency that funded the study. "Theystrengthen an emerging body of evidence that air pollution can causeasthma, and that exposure to outdoor levels of nitrogen dioxide andother traffic-related air pollutants may be a significant risk factorfor this illness."

Researchers looked at the pollution-asthma link in 208 childrenwho were part of the USC-led Children's Health Study, the longestinvestigation ever into air pollution and kids' health. The study hastracked the respiratory health of children in a group of SouthernCalifornia cities since 1993.

The investigators placed air samplers outside the home of eachstudent to measure NO2 levels. In addition, they determined thedistance of each child's home from local freeways, as well as how manyvehicles traveled within 150 meters (about 164 yards) of the child'shome. Finally, they estimated traffic-related air pollution levels ateach child's home using models that take weather conditions, vehiclecounts and other important factors into account.

In all, 31 children (15 percent) had asthma. Scientists founda link between asthma prevalence in the children and NO2 levels attheir homes. For each increase of 5.7 parts per billion in averageNO2-which represents a typical range from low to high pollution levelsamong Southern California cities-the risk of asthma increased by 83percent. Risk of wheezing and current asthma medication use also roseas NO2 levels increased.

They also found that the closer the students lived to afreeway, the higher the NO2 levels outside their homes. NO2 levels alsocorresponded with traffic-related pollution estimates from the group'sstatistical model.

It was not surprising, then, when they found that the closerthe students lived to a freeway, the higher the students' asthmaprevalence. For every 1.2 kilometers (about three-quarters of a mile)the students lived closer to the freeway, asthma risk increased by 89percent. For example, students who lived 400 meters from the freewayhad an 89 percent higher risk of asthma than students living 1,600meters away from the freeway.

Interestingly, the researchers saw that air pollution fromfreeway traffic influenced NO2 concentrations at homes more stronglythan pollution from other types of roads. Traffic counts within 150meters of homes (which primarily comprised traffic from smallerstreets) were only weakly correlated with measured NO2.

In any community, a freeway is a major source of air pollution."Cars and trucks traveling on freeways and other large roads may be abigger source of pollutants that matter for asthma than traffic onsmaller roads," Gauderman says. Scientists also find it difficult toget good data on traffic on smaller streets, which may make it harderto find associations between asthma and local traffic.

Gauderman cautions that researchers do not yet know that NO2 isto blame for the asthma. NO2 travels together with other airbornepollutants, such as particulate matter, so it may be a marker for otherasthma-causing pollutants.

Study sites included the cities of Alpine, Atascadero, LakeElsinore, Lancaster, Long Beach, Mira Loma, Riverside, San Dimas, SantaMaria and Upland.

The Children's Health Study is supported by the NIEHS, CaliforniaAir Resources Board, the Southern California Particle Center andSupersite, the Environmental Protection Agency and the HastingsFoundation.

W.J. Gauderman, E. Avol, F. Lurmann, N. Kuenzli, F. Gilliland,J. Peters and R. McConnell, "Childhood Asthma and Exposure to Trafficand Nitrogen Dioxide," Epidemiology. Vol. 16, No. 6, November 2005