The findings showed that air pollution accelerates the progression of atherosclerosis -- the condition called hardening of the arteries, which can cause heart attacks.

People living in areas with more outdoor pollution, even at lower levels, accumulate deposits in the arteries that supply the heart faster than do people living in less polluted areas.

"The study provides important new information on how pollution affects the main biological process that leads to heart disease," said lead author Joel Kaufman, professor at University of Washington.

Results were strongest for fine particulate matter (PM2.5) and the traffic-related pollutant gases called oxides of nitrogen.

For every 5 g/m3 higher concentration of PM2.5, or 35 parts per billion higher concentration of oxides of nitrogen, individuals had a faster rate of progression of coronary artery calcium scores.

This is about 20 percent acceleration in the rate of these calcium deposits, the researchers warned in the study published online in The Lancet.

The team conducted a 10-year epidemiological study of atherosclerosis and air pollution on more than 6,000 people from six US states.

Researchers repeatedly measured calcium deposits in the heart's arteries by using CT scans. They also assessed each person's exposure to pollution based on home address.