Traffic pollution may increase the risk of heart disease by lowering levels of "good" cholesterol, a study suggests.
The discovery may help explain the well-known association between air pollution and damage to hearts and arteries, say scientists.
High density lipoprotein (HDL), also known as "good" cholesterol, helps to keep arteries clear by transporting away potentially dangerous fat molecules.
It acts to reduce the impact of "bad" cholesterol, low density lipoprotein (LDL), that plays a key role in building up harmful deposits on artery walls.
The study of 6,654 middle-aged and older Americans found that those living in urban areas with high levels of traffic pollution tended to have less HDL in their blood.
Greater exposure to black carbon, a marker of traffic pollution, and tiny sooty particles produced by diesel engines were both linked to lower HDL.
Lead scientist Dr Griffith Bell, from the University of Washington School of Public Health in Seattle, US, said: "Our study helps strengthen the biological plausibility of the link between traffic-related air pollution and cardiovascular disease.
"We're slowly beginning to understand some of the biology of how that link works."
The lower levels of HDL seen in people living in polluted cities "may put individuals at a higher risk for cardiovascular disease down the line," Dr Bell added.
While reduced HDL was linked to traffic pollution in both sexes, women were more affected than men, according to the findings published in the journal Arteriosclerosis, Thrombosis and Vascular Biology.
Even a relatively short time spent in a polluted environment appeared to alter HDL levels, said the researchers.
Professor Sir Nilesh Samani, medical director of the British Heart Foundation, said: "There is an urgent need to fund more research that looks in to the dangerous effects of air pollution on the cardiovascular system.
"This silent killer is related to 40,000 deaths in the UK each year, with eight in 10 caused by a heart attack or stroke.
"This is an interesting study showing an association between higher air pollution and lower levels of HDL-cholesterol, often called 'good cholesterol'.
"The effects are small and recent studies have questioned whether lower levels of HDL-cholesterol cause heart disease.
"Furthermore, air pollution causes a myriad of changes in the body, for example it also increases blood pressure, and therefore it is difficult to know how much contribution, if any, the observed difference in HDL-cholesterol makes to the risk associated with air pollution.
"This means it is still too early to say how these findings might fit in to the wider picture, but the underlying message is the same: air pollution poses a serious risk to heart health."