Traffic, susceptibility and childhood asthma

May 3 2006

Young children who live near a major road are significantly more likely to have asthma than children who live only blocks away, according to a study that appears in Environmental Health Perspectives.

The study found that children living within 75 meters (about 82 yards) of a major road had a 50 percent greater risk of having had asthma symptoms in the past year than were children who lived more than 300 meters (about 328 yards) away. Higher traffic volumes on the different roads were also related to increased rates of asthma.

"These findings are consistent with an emerging body of evidence that local traffic around homes and schools may be causing an increase in asthma," says lead author Rob McConnell, M.D., professor of preventive medicine at the Keck School of Medicine of the University of Southern California. "This is a potentially important public health problem because many children live near major roads."

More than 5,000 children ages 5 to 7 were involved in the study which was an expansion of the Children's Health Study, currently underway in 13 southern California communities. The researchers determined how far each participating child lived from a major road - a freeway, large highway or a feeder road to a highway.

"These results suggest that living in residential areas with high traffic-related pollution significantly increases the risk of childhood asthma," says David A. Schwartz, M.D., director of the National Institute of Environmental Health Sciences (NIEHS), the primary agency that funded the study. "Children with no parental history of asthma who had long-term exposure or early-life exposure to these pollutants were among the most susceptible."

Children who lived at the same residence since age 2 had slightly higher rates of asthma than those who had moved to the residence later. "That is what you would expect if the asthma was being caused by traffic," McConnell says. Risk for wheeze also decreased the further away a home was from a major road, dropping to background rates at roughly 150 meters (not quite two blocks).

Study sites included the cities of Alpine, Anaheim, Glendora, Lake Arrowhead, Lake Elsinore, Long Beach, Mira Loma, Riverside, San Bernardino, San Dimas, Santa Barbara, Santa Maria and Upland. McConnell noted that air pollution regulations typically focus on regional air pollutants rather than localized exposures within communities, such as living near a busy road, that may also be a problem. "We've taken some tentative steps to address that, for example with a law that a new school can't be built within 500 feet of a freeway. But we have to also consider whether building parks, play areas, or homes right next to a major road is a wise land use decision in terms of health."

McConnell and his colleagues plan to follow up with a subgroup of the children to measure pollutants in their homes and also to look at characteristics that may make children more susceptible (or that may be protective), such as genetic characteristics.