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.
While the researchers also caution people with respiratory illness to avoid polluted air when exercising they say they should exercise regularly because of the benefits to overall health.
It is well known that long-term exposure to air pollution increases the risk of cardiovascular problems and this research adds support to knowledge on how air pollution harms people and what should be done to avoid it.
Lead author, Dr. Nicholas Mills of Edinburgh University's Centre for Cardiovascular Sciences says the study explains why patients with heart disease are more likely to be admitted to hospital on days in which air pollution levels are higher.
Dr. Mills says most people think of air pollution as having effects on the lungs but, this study shows it can also have a major impact on how our heart functions.
Professor Peter Weissberg, the Medical Director of the BHF,says there is already evidence that air pollution can make existing heart conditions worse and this research shows that in patients with coronary heart disease, diesel exhaust can reduce the amount of oxygen available to the heart during exercise, which may increase the risk of a heart attack.
The study was funded by the British Heart Foundation (BHF) and is published in the New England Journal of Medicine.