A new study by British and Swedish researchers has found that air pollution could potentially harm the blood vessels and clotting abilities of people with heart disease.
The study by scientists at Edinburgh and Umea Universities found that inhaling diesel exhaust caused changes in the heart's electrical activity.
The researchers say the air pollution also reduces the amount of oxygen available to the heart during exercise and warn heart patients to avoid exercising in areas of heavy traffic.
For the study the researchers used 20 men age around 60 years who had experienced a heart attack.
The men were screened to ensure they were stable and being treated to prevent a further heart attack.
The men had no signs of angina or heart rhythm problems and were able to tolerate exercise.
The men were asked to exercise on a stationary bicycle inside a chamber where they were exposed, on two separate visits, to filtered air and diluted diesel fumes which equated to being in heavy traffic.
They exercised for an hour with rest breaks every 15 minutes while still inside the chamber.
Electrodes attached to their chests monitored their hearts while they were in the chamber and blood samples were taken six hours after leaving it.
The air pollution in the chamber was also carefully measured.
The results showed that while they were inhaling diesel fumes the men experienced a three fold increase in stress on the heart.
The scientists say a lack of oxygen flow to the heart results in exercise-induced ischemia even when the patient is exposed to a small amount of diluted diesel exhaust during exercise.
This in turn causes the reduction of a key anti clotting substance in the blood and the changes can trigger a heart attack and even death.
They advise heart patients to workout indoors to avoid breathing polluted air.
The American Heart Association estimates that as many as 16 million Americans have heart disease and a large number of Americans also suffer from asthma, bronchitis and chronic obstructive pulmonary disease.