Breathing in fumes from traffic exhaust has long been known to raise the risk of cardiovascular disease, but the reason has remained unclear.
But the new study involving more than 6,500 adults, found it was because pollution reduces our beneficial cholesterol HDL (high density lipoprotein) which is then unable to do its job of removing bad LDL (low density lipoprotein) from the body.
As a result, arteries harden leading to heart attacks.
Although the effect was felt by both genders, it is more noticeable in women than men, said scientists.
Dr Griffith Bell, of the University of Washington, Seattle, where the research was carried out, warned that the effects from such pollution can happen at “comparatively low levels in developed countries”.
He said: “Our study helps strengthen the biological plausibility of the link between traffic related air pollution and cardiovascular disease.
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“We are slowly beginning to understand some of the biology of how that link works.”
We are slowly beginning to understand some of the biology of how that link work
According to Government estimates, about 40,000 Britons die prematurely each year because of air pollution, which has remained illegally high in 16 cities since 2010.
And heart disease remains Britian’s biggest killer, resulting in over 160,000 deaths each year.
Last night, heart research bodies in the UK welcomed the new research.
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 ten 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.”
The new study, published in Arteriosclerosis, Thrombosis, and Vascular Biology, offers an explanation for the link between dirty air and cardiovascular diseases.
These include hardening of the arteries, or atherosclerosis, which cause heart attacks and strokes, and heart failure.
Dr Bell said the connection may be explained by a reduction in the amount of HDL, meaning it cannot deplete bad LDL cholesterol as effectively.
This leads to larger quantities of the latter, harmful fat, forming in vessels and raising the risk of a clot, blocking off blood supply to the heart or brain.
It leaves the average amount of cholesterol in HDL particles higher on “a per-particle basis”, explained Dr Bell.
He said recent evidence suggests the number and functionality of HDL particles may be a better gauge of its healthy effects for the heart than their actual cholesterol content.
In the study of 6,654 middle aged and elderly Americans from diverse ethnic backgrounds, the research team found those living in areas with high levels of traffic related air pollution tended to have lower HDL levels.
Higher exposure to black carbon, the sooty emissions from diesel engines and a marker of vehicle pollution, over a period of a year was significantly associated with less ‘good’ cholesterol.
But men and women responded to air pollutants differently. While HDL was lower at higher pollution exposure for both sexes, the magnitude was greater in women.
Dr Bell said the lower HDL observed with greater air pollution “may put individuals at a higher risk for cardiovascular disease down the line.”
And changes in HDL levels may already appear after brief and medium length exposures to air pollution, he added.
Dr Bell said: “High density lipoprotein (HDL) particles possess numerous potentially cardio protective qualities.
“HDL particles transport cholesterol from blood cells and are vital in the maintenance of net cholesterol balance in the arterial wall.”
He added: “This is the first large observational study to suggest an association between air pollution exposure and HDL particle number.
“This study contributes to the hypothesis that air pollution may act through HDL to contribute to cardiovascular disease at comparably low levels found in developed countries.”
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