Air pollution could be putting people with heart disease at greater risk, according to new research.
Experts measured the effects of diesel exhaust fumes
A joint study by Edinburgh University and Swedish experts measured the effects of diesel exhaust fumes on men who had suffered a heart attack.
Researchers said the results were an important step in finding out how pollution affects the human body.
The study revealed that pollution reduces the amount of oxygen available to the heart during exercise.
The research was funded by the British Heart Foundation and will be published in the New England Journal of Medicine.
During the study, a group of men who had experienced a heart attack exercised in a room with a diluted amount of diesel fumes which were equal to about 10% of the pollution levels found in a typical city centre.
Dr Nicholas Mills, of the university's Centre for Cardiovascular Sciences, said: "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 men who took part in the study were exposed for one hour to either filtered air or dilute diesel exhaust while intermittently riding a stationary bicycle in a carefully monitored exposure chamber.
Heart function was monitored continuously and blood tests taken six hours after leaving the chamber.
Electrical monitoring of the heart by experts from Edinburgh and Umea Universities showed that inhalation of diesel exhaust caused a three-fold increase in the stress of the heart during exercise.
Professor Peter Weissberg, medical director of the British Heart Foundation, said there was already evidence that air pollution could make existing heart conditions worse.
He added: "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."