In experiments using harmless ultra-fine particles of gold, the scientists were able for the first time to track how such nanoparticles are breathed in, pass through the lungs and then gain access to the blood.
Most worryingly, the researchers said at a briefing in London, the nanoparticles tend to build up in damaged blood vessels of people who already suffer from coronary heart disease - the condition that causes heart attacks - and make it worse.
"There is no doubt that air pollution is a killer, and this study brings us a step closer to solving the mystery of how air pollution damages our cardiovascular health," said Jeremy Pearson, a professor and associate medical director at the British Heart Foundation charity, which part-funded the study.
Experts have long known that air pollution carries serious health risks and can trigger fatal heart attacks and strokes. According to the World Health Organization, outdoor air pollution in both cities and rural areas was estimated to cause 3.0 million premature deaths worldwide in 2012.
But until now, scientists had not been sure how particles inhaled into the lungs go on to affect heart health. The new findings, published on Wednesday in the journal ACS Nano, build on previous evidence and show that particles in the air we breathe get into blood and are carried to many different parts of the body, including arteries, blood vessels and the heart
"If reactive particles like those in air pollution ... reach susceptible areas of the body then even (a) small number of particles might have serious consequences," said Mark Miller, a senior research scientist at the University of Edinburgh, who led the study.
Miller's team used specialist techniques to track harmless gold nanoparticles breathed in by volunteers. They found the nanoparticles can migrate from the lungs into the bloodstream within 24 hours and are still detectable three months later.
The researchers also analysed surgically removed plaques from people at high risk of stroke and found that the nanoparticles tended to accumulate in the fatty plaques that grow inside blood vessels and cause heart attacks and strokes.
Nicholas Mills, a professor of cardiology who also worked on the study, said the findings showed the importance of cutting emissions and limiting peoples' exposure to nanoparticles.
(Editing by Catherine Evans)