While the exact relationship between asthma and automobile emissions is not well understood, the fact is that both are on the rise.

A recent study found that asthma hospitalization rates for children from poor, minority neighborhoods in New York City were up to 21 times higher than those for children from affluent neighborhoods and concluded that diesel exhaust was a major contributing factor.

Some children may...have a genetic predisposition to developing asthma from exposure to environmental toxins.

Now, another study suggests that some children may metabolize substances found in engine exhaust differently from others; in effect they have a genetic predisposition to developing asthma from exposure to environmental toxins.

Researchers at the University of Southern California (USC) found that children who carried variations in two genes and lived within 75 meters of a major road were up to nine times more likely to develop asthma than children who lived further away.

The study appears now in the August 2007 online version of the journal Thorax.

"This is one of the first studies to report that children with certain genetic backgrounds are even more susceptible to asthma than if they lived near major roads and did not carry the variations," says Muhammad T. Salam, Ph.D. candidate at the Keck School of Medicine of USC, and the study's lead author. "We are working to understand how traffic−related exposures may interact with these genes, leading to asthma development." Asthma is the most common chronic disease in children.

The USC study used data from the Children's Health Study (CHS), which collected health data on children in Southern California. They found that high levels of microsomal epoxide hydrolase (EPHX1) — an enzyme that metabolizes polyaromatic hydrocarbons (PAH) in vehicle emissions — was associated with an increased risk for lifetime asthma. Children with high EPHX1 levels who also carried variations in glutathione S−transferase P1 (GSTP1) genes were four times more likely to have asthma.

"This finding demonstrates the critical role of gene environment interaction in determining disease susceptibility," says David A. Schwartz, M.D., director of the National Institute of Environmental Health Sciences. "The investigators at USC have identified key genetic variations in biological pathways related to PAH metabolism that are associated with the occurrence of asthma in children who live in close proximity to traffic."

There are, however, a number of genes that are linked to asthma, and researchers are just beginning to study the associations between these and environmental factors.

"It is difficult to say that if parents with an asthmatic child move further from busy streets, the child will definitely have fewer symptoms," Salam explains. "All that can be said at this moment is that data from this and other studies show strong evidence that living near heavy traffic increases asthma risks and exacerbates symptoms in children who already have asthma."