Talk about a killer commute: The risk of a heart attack appears to triple within an hour of being in traffic.

And it doesn't matter whether you're driving a car, riding on public transit or pedaling a bicycle, according to a provocative new study published in today's issue of the New England Journal of Medicine.

Something about traffic seems to trigger heart attacks.

A number of factors, such as stress, may be to blame, but the analysis by Annette Peters and her colleagues at the Institute of Epidemiology in Neuherberg, Germany, suggests that particulate air pollution from vehicle exhausts may be the most important.

"This is a hot issue in many, many countries right now," said Dr. Lewis Kuller, a noted epidemiologist at the University of Pittsburgh Graduate School of Public Health.

A number of studies have suggested that fine particles can contribute to heart disease and might also serve as the trigger for an attack. In some large cities around the globe, central city traffic has been restricted in attempts to reduce particulate pollution, he noted.

Most studies have focused on stationary sources of particulates, such as power plants.

"But mobile sources ---- pollution from cars ---- are probably the No. 1 source of pollution now," Kuller said.

People often assume that air pollution has the greatest health impact in terms of respiratory diseases.

"But actually the biggest impact is on heart disease," said Dr. Murray Mittleman, a cardiovascular disease epidemiologist at Harvard University, if only because heart disease is more widespread.

Previous studies of particulate pollution have usually focused on communitywide exposures, using central air monitors, and then compared pollution levels with hospital admissions, he noted. Those studies have suggested that particulates might increase the risk of heart attacks by 10 percent or 15 percent.

But by focusing on traffic and the presumed exposure to particulates by people in traffic, the Peters team found a much stronger effect, he said.

"It's a unique study at this point," said Mittleman, who has collaborated with Peters in the past. "I think that this work is really important."

The researchers interviewed 691 people who survived heart attacks in the Augsburg region of southern Germany from February 1999 to July 2001 and reviewed diaries in which they described their activities for four days before the attacks. Increased time spent in traffic was consistently linked with increased risk of heart attack.

Most of the patients had been in cars, but bike riders and bus passengers also faced an increased risk within an hour of being in traffic. So, if stress was a factor, it apparently was not the stress of driving.

And the effect didn't seem to involve necessarily commuting to work. The researchers found that unemployed people had an even stronger response to traffic.

When the researchers adjusted their findings to account for other factors known to increase the risk of heart attacks, such as morning hours or strenuous exercise, that only slightly reduced the estimated effect of traffic.

The researchers didn't make any measurements of particulate pollution levels, but assumed in their analysis that people in traffic are exposed to higher levels of particulates than other people in a community.

Nevertheless, the study provides compelling evidence that particulate air pollution may trigger heart attacks, according to an accompanying editorial by Dr. Peter Stone, co-director of the cardiac care unit at Brigham and Women's Hospital in Boston.

"The physiological mechanisms are something that are still under investigation," added Mittleman, noting the inhaled particles might inflame narrowed coronary arteries or might increase blood clotting.

Exposure to traffic was identified only as a possible trigger to heart attack and not necessarily the underlying cause of heart disease.

Dr. Tony Farah, a cardiologist at Allegheny General Hospital, said the trigger for a heart attack is thought to be a rupture of existing plaque that has narrowed an artery. And some plaque may be more vulnerable to such rupture than others, probably because of inflammation.

"We don't really have a good way of identifying these so-called vulnerable plaques," Farah said.

Cardiologists, of course, would like to reduce heart attacks by identifying these people early, but lowering air pollution levels might well be an alternative and complementary strategy, he added.

Farah noted that the authors couldn't rule out stress as a confounding factor in their analysis and also noted that recent studies have shown that stress may be a much larger trigger for heart attacks than previously thought. The association between heart attacks and people who are in or have been in traffic is not something that necessarily jumps out at physicians who treat these patients, he added.

In fact, Kuller noted that very few people actually suffer heart attacks and sudden cardiac deaths while driving. That would be one argument against the idea that particulates trigger heart attacks, he said.

Mittleman, however, contends that the increased risk associated with traffic might be hard for most people to discern. Even though traffic might double or triple a person's risk of heart attack as compared with the "absolute" risk that person would face under normal conditions, "the absolute risk is probably quite small."

A 50-year-old man, for instance, might face a one-in-a-million chance of having a heart attack in any random hour, Mittleman said. So if his risk was tripled because of traffic exposure, he still would have only a three-in-a-million risk.

That might not mean much from an individual's point of view, Mittleman acknowledged. But when viewed from a societal perspective, such an increase in the relative risk of heart attack could involve large numbers of patients and would be a major public health concern, he said.