People who live in areas with high levels of traffic-related air pollution might have a slightly increased risk of dying from stroke, Danish researchers suggest in a new study.

They found people living in urban zones with high estimated concentrations of nitrogen dioxide were 22 percent more likely to suffer a fatal stroke than people in less-polluted neighborhoods.

Nitrogen dioxide is a component of car exhaust and is known to cause lung damage. Previous research also suggests spikes in air pollution over the course of days or weeks can trigger death and hospitalization from stroke.

Last year, the U.S. Environmental Protection Agency strengthened its standards to prevent brief exposures to high levels of the pollutant, but the average threshold has remained the same for decades.

The latest study, published in the journal Stroke, is one of the largest and most comprehensive to link chronic exposure to everyday levels of air pollution to strokes, said Dr. Jiu-Chiuan Chen, who studies the effects of air pollution at the University of Southern California Keck School of Medicine in Los Angeles.

"Policy-makers need to realize the danger of living and walking near a high traffic area," said Chen, who was not involved in the new work.

The Danish researchers looked at data for more than 52,000 residents of two major cities in Denmark.

Over the course of a decade, nearly 2,000 participants, aged 50 to 65 at the start of the study, experienced a first-time stroke landing them in the hospital, and 142 died within 30 days.

People exposed to the highest levels of nitrogen dioxide at their homes were five percent more likely to experience a stroke than those who breathed the cleanest air. And they were 22 percent more likely to die from a stroke, report Zorana J. Andersen, of the Danish Cancer Society in Copenhagen, and colleagues.

Those numbers take into account several factors related to stroke risk, such as how heavy people were and whether or not they exercised or smoked.

But from a statistical point of view, it is still possible that chance may have played a role in the findings.

And there is no proof that nitrogen dioxide itself -- and not some unknown factor -- caused the rise in strokes.

The researchers based estimates of nitrogen dioxide exposure dating back nearly four decades on factors such as the proximity of a major roadway to a participant's home.

Previous studies have found that air pollution particles, which are small enough to make their way into the blood stream, contribute to inflammation of the blood vessels, which ultimately may lead to an increased risk of heart attack, stroke and heart failure.

In addition to higher long-term exposure to air pollution, stroke patients in this study were also more likely to be male, obese and have other risk factors for heart disease, such as high blood pressure, high cholesterol, and a history of smoking.

Though the authors estimated outdoor nitrogen dioxide concentrations at people's home address, they did not account for commuting habits or air pollution at the job.

"Commuting is also an important source of air pollution exposure," Chen noted.

While limiting commute time in heavy traffic and exercising away from busy roadways may help people avoid the worst smog, Chen said air pollution is ultimately a public health problem that policy-makers need to deal with.