Don't blame road rage. Traffic triggers heart attacks (search), a German study shows.

Yes, it appears a heart attack is imminent for the beefy guy with the red face shouting from his SUV, but what hurts our hearts isn't seething in traffic — it's breathing in traffic, says Harvard researcher Peter H. Stone, MD. He is the co-director of the cardiac unit at Brigham and Women's Hospital in Boston.

"This is totally more due to air pollution (search) than to stress," Stone tells WebMD. "When driving, we are so used to thinking that the principal health jeopardy is from rage, but that is not true at all."

Stone's editorial in the Oct. 21 issue of The New England Journal of Medicine accompanies a study by Annette Peters, PhD, of the GSF-National Research Center for Environment and Health, Neuherberg, Germany, and colleagues.

Peters' team interviewed nearly 700 people who survived for at least 24 hours after having a heart attack. During the hour before their heart attacks, they were more likely to have been in traffic than at any other hour of the day. Twelve percent of the heart attacks occurred within an hour of being in traffic. Analysis of the data showed that 8 percent of the heart attacks were directly linked to traffic exposure.

Yes, most of the heart attack victims were exposed to the stress of driving. But some were exposed to traffic while riding on streetcars or buses — and their traffic risk was just as high as that of the drivers. Moreover, people rushing to work had slightly fewer traffic-related heart attacks than women, unemployed people, and retirees.

"If these findings can be replicated — if this is really true — we might have been underestimating the short-term risk of air pollution," Peters tells WebMD. "Most air pollution studies rely on 24-hour averages from ambient air monitors not influenced by traffic. But for people who spend a considerable time in traffic, their personal exposure might be much higher."

It's not the first time researchers have linked traffic air pollution to heart attacks.

"Many studies in the past have looked at passive bystanders to traffic, like highway patrolmen and gas station attendants," Stone says. "They have an increased cardiovascular event rate and increased jeopardy simply due to their proximity to traffic. It is not driving that increases their risk, but the additional element of exposure to particulate matter and gasses related to traffic."

How Air Pollution From Traffic Causes Heart Attacks

One of the ways air pollution does its damage is by stimulating the body's inflammatory defenses. The tiny particles and gasses in air pollution set off alarms throughout your body. The alarms trigger cascades of chemical messengers that arm immune defenses. Over time, this process speeds the buildup of sticky plaque in the walls of your arteries.

But increasing the long-term risk of heart disease isn't the only heart threat from air pollution. For those who already have plaque building up in their arteries, there's a more immediate danger: Air pollution can trigger the rupture of these plaque pockets. The gooey stuff exploding from these pockets can block smaller blood vessels and cause a heart attack.

"If you think of the plaque as a mud flat in the blood vessel, there's a thin crust overlying a more liquid, gooey mud within," Stone explains. "How hard would it be to rupture or break that top crust? When you breathe in air pollution, it causes the whole artery to contract. This mechanical force can rupture the plaque. But the other thing air pollution does is cause trouble from within. It promotes inflammation that erodes the inner surface of the plaque. So plaques can rupture from without or from within — and air pollution can potentially do both."

Take Better Care 'Til There's Better Air

The vast majority of us can no more avoid traffic than we can avoid breathing. So what's to be done? Better air quality is an obvious answer — but not one we can accomplish overnight.

"The irony is there is nothing to avoid, because it is not feasible or practical to avoid traffic," Stone says. "The question becomes, 'How do we reduce the air pollution we are exposed to?' And that gets into the political question of limiting the pollutants that get into our environment."

Peters calls for cleaner vehicles and better city planning. Meanwhile, she says people with heart disease have to do whatever they can to reduce their risk.

"The individual cannot do much about air pollution," she admits, "but if you know you have heart disease, it is advised that you get the best treatment you can. The pathological mechanisms we think traffic triggers are counteracted by current therapy for heart disease. So people can get protection through their medication."

By Daniel J. DeNoon, reviewed by Brunilda Nazario, MD

SOURCES: Peters, A. The New England Journal of Medicine, Oct. 21, 2004; vol 351: pp 1721-1730. Stone, P.H. The New England Journal of Medicine, Oct. 21, 2004; vol 351: pp 1716-1718. Annette Peters, PhD, GSF-National Research Center for Environment and Health, Neuherberg, Germany. Peter H. Stone, MD, associate professor, division of cardiology, Brigham & Women's Hospital, Boston.