Those caught up in heavy traffic have three times the risk of having a heart attack within an hour, a German study presented at the American Heart Association's 49th Annual Conference on Cardiovascular Disease in Florida said.

"One potential factor could be the exhaust and air pollution coming from other cars," Annette Peters, the researcher who lead the story, said. "But we can't exclude the synergy between stress and air pollution that could tip the scale."

Researchers interviewed 1,454 people who survived heart attacks, and in the hour before their heart attack, many of them had been in heavy traffic.

They found that most of those who were in traffic prior to the onset of the attack were driving a car; however, others were riding a bicycle or taking public transit before the attack.

The study found exposure to traffic had the greatest effect on certain groups including women, the unemployed and elderly males.

Peters and colleagues are working with University of Rochester researchers to determine exactly what it is about traffic that raises heart attack risk.