People may be more likely to suffer a heart attack or stroke if they are exposed to air pollution over a long period, a new study suggests.

According to US scientists, long-term exposure to air pollution appears to speed up atherosclerosis (hardening of the arteries), a major risk factor for heart-related events such as heart attacks.

The scientists monitored over 5,300 people aged between 45 and 84 from six different areas.

They found that higher concentrations of fine particulate air pollution appeared to speed up the thickening of the inner two layers of the common carotid artery, which is responsible for providing oxygenated blood to the neck and head.

Particulate is a general term used to describe any tiny solid or liquid particples. Particulate air pollution includes the pollution emitted by motor vehicles (in exhaust fumes) and some power plants.

The scientists also found that if pollution levels fell over time, the thickening of blood vessels slowed down.

"Our findings help us to understand how it is that exposures to air pollution may cause the increases in heart attacks and strokes observed by other studies," the team from the University of Michigan said.

After taking into account other factors, such as smoking, the scientists found that the thickness of the carotid artery increased by 14 micrometers per year. However, in people exposed to higher levels of residential air pollution, this thickening occurred at a faster rate.

"Linking these findings with other results from the same population suggests that persons living in a more polluted part of town may have a 2% higher risk of stroke as compared to people in a less polluted part of the same metropolitan area," they noted.

Details of these findings are published in the journal, PLOS Medicine.

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