The decade-long study of thousands of Americans found that people living in areas with more outdoor pollution accumulate deposits in the arteries that supply the heart faster than do people living in less polluted areas.

Previous epidemiological studies have shown associations between particle pollution, referred to as particulate matter, and heart disease.

It has been unclear, however, how exposure to particulate matter leads to diseases of the cardiovascular system.

Now, direct evidence from a 10-year epidemiological study of more than 6,000 people from six US states, shows that air pollution - even at levels below regulatory standards - accelerates the progression of atherosclerosis.

The condition, also called hardening of the arteries, can cause heart attacks. 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.

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

"The evidence supports worldwide efforts to reduce exposures to ambient air pollutants," Kaufman said.

The researchers calculated each participant's exposure to ambient fine particulate matter that is less than 2.5 microns in diameter and too small to be seen by the naked eye. In addition to PM2.5, they also measured exposure to nitrogen oxide and nitrogen dioxide, and black carbon or soot.

They collected thousands of air pollution measurements in the study participants' communities and at their homes.

Researchers also developed and applied computational models that included local information on land use, roadway and traffic volumes, weather conditions, and local sources of air pollution.

These models could generate accurate pollution concentrations at each person's home. Between the years 2000 and 2012, participants visited study clinics several times to undergo CT scanning to determine the amount of calcium deposits in their heart arteries.

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

The study found that for every 5 micrograms per cubic metre higher concentration of PM2.5, or 35 parts per billion higher concentration of oxides of nitrogen, individuals had a 4 Agatston units/year faster rate of progression of coronary artery calcium scores.

This is about a 20 per cent acceleration in the rate of these calcium deposits, researchers said.