Air pollution from traffic hinders the heart's ability to conduct electrical signals, a study has suggested.
Exposure to small particulates - tiny chemicals caused by burning fossil fuels - caused worrying changes on the heart traces of 48 heart patients.
Particulate pollution is already known to increase heart attack risk.
The Circulation study appears to back this up and the heart trace changes seen were characteristic of poor oxygen supply to the heart.
The electrocardiograms of the 48 patients studied, who had recently been hospitalised for heart attack, unstable angina or worsening symptoms of coronary heart disease, showed unusual changes called ST-segment depression.
Patients recovering from heart attacks had the greatest changes in pollution-related ST-segment depression over the course of the 10-month study, the Harvard University researchers found.
The American Heart Association already recommend that some heart patients, particularly those who have had a heart attack, delay driving for two to three weeks after leaving the hospital and avoid driving in heavy traffic because of the stress it creates.
Lead researcher Dr Diane Gold said: "Our study provides additional rationale to avoid or reduce heavy traffic exposure after discharge, even for those without a heart attack, since traffic exposure involves pollution exposure as well as stress."
She said more work was needed to evaluate the mechanisms behind the pollution-related ST-segment depression, but said the most likely explanation was inadequate blood supply to the heart or inflamed heart muscle.
Judy O'Sullivan of the British Heart Foundation said: "It has been established for some time that exposure to high levels of air pollution can exacerbate symptoms in people with heart and circulatory disease.
"Our advice to heart patients is to avoid prolonged exposure to high levels of air pollution."
Professor David Newby, professor of cardiology at Edinburgh University, said: "There is a whole wealth of data showing if you live in a polluted area you are more likely to get cardiovascular disease.
"The pollution levels in this study were not even that high yet they are still seeing changes, which is important.
"We should all strive to reduce pollution."
According to the World Health Organization, air pollution accounts for three million deaths worldwide every year.