Air pollution caused by traffic and factories may cause heart disease, a US study suggests.
Air pollution from traffic can affect neck arteries, the study says
The team found the pollution seemed to cause the narrowing of arteries - an early stage of heart disease - in a similar way to smoking.
The University of Southern California team studied almost 800 people aged over 40 living in the Los Angeles area.
Study author Professor Nino Kuenzli said the public health implications of the findings "could be immense".
Using ultrasound, the researchers measured the thickness of the inner lining of the carotid artery in the neck.
Atherosclerosis, the build-up of fatty materials in the arteries which reduces blood flow, is already linked to smoking, diabetes and obesity.
The researchers also looked at levels of pollution in the areas where study participants lived.
They measured PM2.5 particles, which are commonly produced by burning fossil fuels, such as from cars or in processing metals.
The particles are so tiny that they can be inhaled into the smallest airways.
The pollution causes the body to produce oxidants which in turn trigger inflammatory reactions in both the respiratory tract and blood vessels, triggering artery damage.
PM2.5 levels are measured in micrograms per metre cubed (ug/m3) with readings in the study ranging from 5.2 to 26.9ug/m3.
The researchers found that for every 10ug/m3 increase in pollution, the thickness of the lining of the neck artery increased by 5.9%.
After the team adjusted for factors such as age and lifestyle, artery thickness increased by between 3.9% and 4.3% for every 10ug/m3 increase in PM2.5, according to the study presented to the American Heart Association.
The strongest link was seen in women over the age of 60, with a 15.7% increase in artery thickness for every 10ug/m3 increase in pollution.
Prof Kuenzli said: "We knew that people in more polluted areas die earlier from cardiovascular disease.
"Our study found that air pollution may contribute to cardiovascular problems at a very early stage of the disease, similar to smoking, and enhances atherosclerosis, which is the underlying disease process of cardiovascular diseases."
He called for more research into the effect of air pollution.
Alison Shaw, cardiac nurse at the British Heart Foundation, said the research added to previous evidence that air pollution may contribute to heart
and circulatory disease - the UK's biggest killer.
"Certain pollutants have an inflammatory effect on the inner linings of arteries, which can trigger atherosclerosis and inflammation of the lungs, which
could aggravate existing lung problems.
"Experts believe that variations in levels of urban air pollution affect mortality rates.
"However, further large scale trials are needed to assess the health impacts of long term exposure to air pollution."
She added that the BHF was funding research into how air pollution may contribute to coronary heart disease. Results of the study are expected within three years.