The closer that children live to Southern California freeways, the greater their risk of being diagnosed with asthma, USC researchers have found in a study that bolsters growing evidence that air pollution can cause asthma.
Children who lived a quarter mile from a freeway, for example, had an 89% higher risk of asthma than children living about a mile from a freeway, according to the new research.
Even in areas such as Santa Maria, with generally good air quality, the researchers found that the risk of asthma increased for children who lived near freeways.
Separately, a different team of University of Southern California researchers has concluded that the chronic health effects of smog among adults are two to three times greater than earlier research showed. The team pinpointed a link between the tiny particles contained in air pollution and increased deaths from heart disease.
Articles on the two studies, conducted in Southern California, appear in the November issue of the journal Epidemiology.
USC released the findings Tuesday.
The freeway article is part of an ongoing landmark study of how air pollution affects children's respiratory health. That study, which began in 1993, produced findings last fall that showed smog can permanently stunt lung growth in children and lead to lifelong health problems.
Dr. Elisa Nicholas, project director for the Long Beach Alliance for Children with Asthma, called the freeway study significant. "There's increasing evidence demonstrating a link between air pollution and the development of asthma," Nicholas said Tuesday. "The more evidence we have, the more political will there will be to clean up emissions from the freeways."
Earlier studies have demonstrated a relationship between children's asthma and traffic exposure, but results have not been consistent as to whether air pollution causes asthma, according to the article by a team of seven researchers at the USC Keck School of Medicine.
Nor has research been conducted in Southern California, said lead author James Gauderman, a USC associate professor of preventive medicine.
So researchers tracked 208 children living in 10 cities in the region, including 31 children, or 15%, with asthma.
They installed air samplers outside the children's homes to measure nitrogen dioxide for two-week periods in the summer and fall of 2000. Nitrogen dioxide is produced by pollutants from cars and trucks.
Researchers measured the distance between each home and freeways, and counted how many vehicles traveled within 164 yards of the homes.
They found that children with higher levels of nitrogen dioxide near their homes were more likely to have asthma. For each increase of 5.7 parts per billion of the pollutant, the risk of asthma increased by 83%, the study states.
The researchers have not determined that nitrogen dioxide is causing asthma, but it is found with other pollutants -- including particulate matter that has been tied to other diseases.
Researchers also found that air pollution from freeways influenced nitrogen dioxide levels more strongly than pollution from smaller roads. Gauderman said that the current findings do not allow researchers to determine at what distance from a freeway children can avoid an increased asthma risk.
He emphasized that the study does not show that every child living near a freeway gets asthma. "We have to realize that even for a kid to live very close to a freeway, odds are that they're not going to get asthma. There's only a fraction of kids that get asthma," he said Tuesday.
Gauderman also said the study does not provide the type of information that researchers can use to advise individual parents.
"The message is probably more general, in terms of thinking about not planning tracts or schools close to a major freeway," he said. The findings might also be useful for government regulators studying the impacts of air pollution.
"From a regulatory standpoint, it might suggest that we need to look not only at background air quality but also the more local exposures that one might have by living next to a major roadway," Gauderman said.
The study involved children living in the cities of Alpine, Atascadero, Lake Elsinore, Lancaster, Long Beach, Mira Loma, Riverside, San Dimas, Santa Maria and Upland.