U.S. researchers have found a close link between air pollution from city traffic and asthma risk for children who require an emergency room visit.

In the study, researchers at the Rollins School of Public Health at Emory University were trying to pinpoint which components of pollution play the biggest role in making asthma worse.

The researchers examined the medical records of children 5 to 17 years old who had been treated in Atlanta-area emergency rooms from 1993 to 2004 because of asthma attacks. Data were gathered from more than 90,000 asthma-related visits.

They then analyzed connections between the visits and daily data on the levels of 11 different pollutants.

The researchers found signs that ozone worsens asthma, as they had expected. But they also found indications that components of pollution that comes from combustion engines, such as those in cars and trucks, were also linked to serious asthma problems in kids.

The effect was found to be strongest during the warmer parts of the year.

"Characterizing the associations between ambient air pollutants and pediatric asthma exacerbations, particularly with respect to the chemical composition of particulate matter, can help us better understand the impact of these different components and can help to inform public health policy decisions," said the study's lead author, Matthew J. Strickland, an assistant professor of environmental health at the school.

The researchers reported their findings in the April issue of the American Journal of Respiratory and Critical Care Medicine.