Chemical particles in diesel exhaust fumes could increase the risk of heart attacks, new research has suggested.
Edinburgh University scientists found minuscule particles produced by burning diesel can increase the chance of blood clots forming in arteries.
The blood clots can then lead to heart attacks or stroke.
The team measured the impact of diesel exhaust fumes on a group of healthy volunteers at levels found in heavily polluted cities.
The volunteers' reaction to gases found in diesel fumes, such as carbon monoxide and nitrogen dioxide, were compared with their reactions to tiny chemical particles found in the exhausts.
It was found that the particles, and not the gases, impaired the function of blood vessels.
Dr Mark Miller, of Edinburgh University's centre for cardiovascular science, said: "While many people tend to think of the effects of air pollution in terms of damage to the lungs, there is strong evidence that it has an impact on the heart and blood vessels as well.
"Our research shows that while both gases and particles can affect our blood pressure, it is actually the minuscule chemical particles that are emitted by car exhausts that are really harmful.
"These particles produce highly reactive molecules called free radicals that can injure our blood vessels and lead to vascular disease."
He added: "We are now investigating which of the chemicals carried by these particles cause these harmful actions, so that in the future we can try and remove these chemicals, and prevent the health effects of vehicle emissions."
The particles, which are thinner than a millionth of a metre, can be filtered out of exhaust emissions by fitting special traps to vehicles.
The researchers said environmental health measures designed to reduce emissions should now be tested to determine whether they reduce the rate of heart attacks.
Professor Jeremy Pearson, associate medical director at the British Heart Foundation, said: "We've known for a long time that air pollution is a major heart health issue, and that's why we're funding this team in Edinburgh to continue their vital research.
"Their findings suggest that lives could be saved by cutting these harmful nanoparticles out of exhausts, perhaps by taking them out of the fuel, or making manufacturers add gadgets to their vehicles that can trap particles before they escape.
"The best approach isn't clear yet.
"For now our advice remains the same, people with heart disease should avoid spending long periods outside in areas where traffic pollution is likely to be high, such as on or near busy roads."
The research, funded by the British Heart Foundation, has been published in the European Heart Journal.