Traffic pollution is a known trigger of asthma symptoms, but a new study suggests a much more direct link: European researchers say that traffic pollution may cause 14 percent of childhood asthma, putting it on par with second-hand smoke.

The researchers reached their conclusions by applying data from existing epidemiological research, including a key study involving traffic in southern California, to the rates of children living close to similar traffic patterns in 10 European cities.

"The fraction of people living very close to busy roads was surprisingly high," said lead author Laura Perez. "We previously evaluated this for Los Angeles where far less live along such traffic corridors. When this finding was combined with existing results from epidemiological studies to evaluate the burden of this exposure, as traditionally done, we found that this exposure could be a quite relevant contributor to chronic diseases."

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As the authors acknowledge in their study, published today in the European Respiratory Journal, applying data from the United States to European settings has some limitations. Pollution from vehicle traffic in the United States, for example, is different from traffic pollution in Europe. A higher percentage of European cars are diesel, for example, and there are different standards.

A 2010 report by the Health Effects Institute in Boston found sufficient evidence to conclude that traffic pollution triggers asthmatic symptoms, but was hesitant to make the connection to a causal effect.

"One of the reasons our panel didn’t feel the data was as strong on causing asthma is that most of these studies use two kinds of measures for exposure to traffic: How close do they live to the road, and what pollutants were they exposed to?" said Dan Greenbaum, president of HEI. "No one pollutant is a perfect marker for vehicles."

Still, that report has been used to help develop guidelines for where to situate schools, Greenbaum said, and for reducing exposure inside schools close to busy roads. It noted the biggest impact occurred within 300-500 meters from highways and major roads. In North America, that would include 30-45 percent of the population.

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"It’s well established that exposure to road pollution is related to an exacerbation of asthma," Greenbaum said. "The area that’s more developmental is the question of the degree to which living near a busily trafficked road may lead to developing asthma in the first place."

Of course, asthma likely isn’t the only condition affected by traffic pollution.

"Near-road traffic-related pollution has been associated with chronic bronchitis and with cardiovascular diseases," Perez said. "We should not forget that traffic is not only contributing pollutants near the roads, but is also a major cause of the mixture of general urban air pollution, usually characterized with the concentration of fine particles. A broad range of acute and chronic diseases have been associated with air pollution."