Exposure to traffic fumes increases the risk of heart attack, or strokes and potentially fatal blood clots in the leg. Pollution from cars and trucks and industrial plants has been shown to harm people with coronary artery disease, causing worrying changes on the heart traces of patients recovering from heart attacks. Previous studies have shown it alters the blood’s coagulation properties and heightens the risk of deep vein thrombosis.

Particulate pollution is already known to increase the risk of heart attacks and other serious diseases. The risk of heart attack increases exponentially after exposure to even slightly higher amounts of metal and dust.

Harvard researchers monitored 48 patients from the Boston area with coronary artery disease using portable electrocardiograph machines. The studied electrocardiograms of the patients showed unusual changes in the electrical conductivity of the heart called ST-segment depression. Increases in ST-segment depression were higher in patients recovering from a heart attack compared with other patients, the researchers said.

Guidelines from the American Heart Association and the American College of Cardiology advise patients who have just been discharged from hospital after a heart attack to avoid heavy traffic because of the stress of driving. Traffic exposure “involves pollution exposure as well as stress,” researchers said. Even people without a heart attack should avoid or reduce heavy traffic exposure after discharge.

“If the air pollution-associated ST-segment changes represent either myocardial inflammation or risk of ischemia, then it is possible that reduction in regional traffic and non-traffic associated air pollution may reduce heart attack or risk for either ischemia, arrhythmia or heart failure in patients with coronary artery disease in the period after hospitalization,” said lead researcher Dr. Diane R. Gold, and associate professor of medicine and environmental health.

The report was published in Circulation: Journal of the American Heart Association. According to the World Health Organization, air pollution accounts for three million deaths worldwide every year. Cars, trucks and industrial plants are the greatest source of air pollution emissions that increases the rates of heart attacks.

Other studies found that when particulates are cut even for a short period of time, death rates fall. As an example, when Hong Kong imposed reductions in sulfur dioxide, or when Dublin imposed a coal ban, they saw immediate reductions in death rates from cardiovascular diseases.

Another study conducted by the Harvard School of Public Health which focused on air pollution’s effects on clotting in the veins found that exposure to air pollution from traffic fumes raises risks of potentially fatal clots in the leg; it alters the blood’s coagulation properties and heightens the risk of deep vein thrombosis (DVT). A study published in 2007 in Stroke: Journal of the American Heart Association found that breathing fine particle pollution during warm weather months can increase stroke risk.

However, a person’s relative risk due to air pollution is small compared with the impact of established cardiovascular risk factors such as smoking, obesity, or high blood pressure, according to the American Heart Association.

More research is needed to find out whether the pollution-related ST-segment depression is related to increased heart muscle inflammation, reduced oxygen flow, oxidative stress, or increased risk of arrhythmias, researchers said.