Researchers at the University of Edinburgh and Umea warned that air pollution could be hazardous for patients with heart disease. Polluted air could put heart disease sufferers at risk by affecting the blood vessels and clotting.
At the University, a study was conducted by measuring the effects of diesel exhaust on heart and blood vessel function in men who have previously experienced a heart attack, and it was found that inhalation of diesel exhaust resulted in changes in the heart's electrical activity.
This study suggested that the air pollution reduces the amount of oxygen available to the heart during exercise.
"This study provides an explanation for why patients with heart disease are more likely to be admitted to hospital on days in which air pollution levels are increased. Most people tend to think of air pollution as having effects on the lungs but, as this study shows, it can also have a major impact on how our heart functions," the New England Journal of Medicine quoted Dr Nicholas Mills, of the University's Centre for Cardiovascular Sciences, as saying.
The study was conducted on a group of twenty men who had suffered a previous heart attack. The volunteers were screened for angina or heart rhythm problems.
They were then exposed to either filtered air or dilute diesel exhaust for one hour while sporadically riding a stationary bicycle, which was carefully monitored in an exposure chamber in Umea University.
Simultaneously, heart function was also being monitored and blood tests were taken six hours after leaving the chamber. When the heart was electrically monitored it showed that inhalation of diesel exhaust caused a three-fold increase in the stress of the heart during exercise.
In supplement to this, the body's ability to release t-PA (tissue plasminogen activator), a "guardian" protein, which can prevent blood clots from forming, was also reduced by more than one third. For the fine exhaust particles produced by road traffic, the link between air pollution and heart disease is the strongest.
Researchers are mainly interested in diesel engines as they generate 10-100 times more pollutant particles than petrol engines. "Diesel exhaust consists of a complex mixture of particles and gases. Before we can recommend the widespread use of particle traps in diesel engines, we need to show that particles are the responsible component," Dr Mills said.
"If we do that, then it is likely that devices to filter particles from exhaust, will reduce exposure and benefit public health," he added.
Professor Peter Weissberg, Medical Director of the British Heart Foundation (BHF), said: "There is already evidence that air pollution can make existing heart conditions worse. This research is helping us work out why. It 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.
"Because of the overwhelming benefits of exercise on heart health, we would still encourage heart patients to exercise regularly, but preferably not when there is a lot of local traffic around. Heart patients can look out for pollution levels on their local weather forecasts."
The research was funded by the British Heart Foundation and published in the New England Journal of Medicine.