Air pollution thickens the blood and increases the likelihood of inflammation, according to research.
Pollution can penetrate deep into the body
The study may help explain why poor air is linked to an increased risk of heart attacks and stroke, as well as worsening respiratory problems.
University of Edinburgh researchers focused on ultra-fine pollutants known as particulate matter, which they say may be able to alter cell function.
Details are carried in the Occupational and Environmental Medicine journal.
The researchers tested the inflammatory and blood clotting responses of human immune cells called macrophages, lung cells and cells taken from the umbilical cord.
Each was tested six and 24 hours after exposure to particulate matter.
The results showed that levels of clotting factors, which thicken the blood, were raised in almost all the cell types.
The rate of death in immune cells also significantly increased, and exposure to the pollutants boosted inflammatory activity.
The researchers say their findings strongly suggest that particulate matter has the ability to alter cell function.
They believe that factors which trigger clotting may also trigger inflammation, and vice versa, so that if one begins to take hold, it is highly likely that the other will follow.
Recent research has shown that particulate matter is so tiny that, when inhaled, it can pass through the lungs directly into the bloodstream.
This may mean that its effect on macrophages could be deadly in people who are at risk of heart disease.
Macrophages are a major component of the plaque deposits which can build up on the walls of the arteries.
These plaques can obstruct blood flow, and it they rupture may lead to the formation of a clot which can trigger a heart attack, or stroke.
Lead researcher Professor William MacNee told the BBC News website that particulate matter produced high levels of charged particles called free radicals, which can damage the body's tissue.
"Our research points to the fact that they stimulate a change in the blood which make clots more likely to form," he said.
Judy O'Sullivan, a cardiac nurse at the British Heart Foundation, said: "This research adds to the debate that air pollution may contribute to heart and circulatory disease, the UK's biggest killer."
She said there was no direct evidence to suggest that there is a direct causal link between air pollutants and coronary heart disease.
However, research funded by the BHF is examining the issue further.
In a statement, the Stroke Association called for "more attention to be given to the issue of reducing air pollution in an effort to reduce the risk of stroke and to create a safer environment for people to live in".
Dr Richard Russell, of the British Lung Foundation, said it was well established that particulate matter increased inflammation in the lung tissues.
However, while it was clear that this made existing respiratory disease worse, there was, as with heart disease, still no direct evidence that it triggered the onset of disease.
Dr Russell said it was probable that free radicals caused damage to the lung tissue, thus triggering inflammation.
It was also possible that inflammation was triggered by the lungs reacting to the carbon deposits which were also a major component of particulates.