Air pollution pumped out from the exhausts of diesel vehicles may be responsible for thousands of extra deaths from stroke each year.
A new study has shown tiny soot and dust particles released into the atmosphere may increase the risk of suffering a stroke.
Analysis of data from the US and China has shown the number of people suffering strokes increases by 1.19 per cent for every 10 micrograms of the particles in the air.
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Tiny particles of soot known as PM2.5 released by car exhausts and the burning of fossil fuels may be increasing the risk of stroke. Researchers found the number of stroke cases increases by 1.19 per cent for every 10 micrograms of PM2.5s in a cubic metre of air. Stock image of air pollution in Beijing is pictured
If this was applied worldwide, an increase in pollution of this magnitude would account for around 71,000 extra deaths each year.
Particulate pollution, however, tends to vary at a local level depending on the weather, traffic and industrial activity in an area.
The study showed that warm weather could also impact on air quality and the risk of stroke.
Dr Longjian Liu, an epidemiologist at Drexel University in Philadelphia, Pennsylvania and the lead author of the study, said: 'Seasonal variations in air quality can be partly attributable to the climate changes.
'In the summer, there are lots of rainy and windy days, which can help disperse air pollution.
'High temperatures create a critical thermal stress that may lead to an increased risk for stroke and other heat- and air quality-related illnesses and deaths.'
Previous research has shown that particulate matter less than 2.5 micrometres in size, known as PM2.5s, can have a wide range of impacts on human health.
Scientists have found the particles are small enough to pass into people's blood streams where they can impact on the cardiovascular system.
They have been linked to the hardening of arteries and other damage that increase the risk of cardiovascular disease.
PM2.5s are tiny particles of soot and dust released into the atmosphere from diesel vehicle exhausts (stock image pictured), forest fires and fossil fuel power stations. The US and China have some of the highest levels of PM2.5 pollution in the world
PM2.5s, which are created by forest fires, car exhausts and fossil fuel burning power plants, have also been implicated in lung conditions such as asthma and lung cancer.
In the latest study, researchers examined the air quality data from 1,118 counties in 49 states in the United States and 120 cities in China between 2010 and 2013.
The study found the southern regions of the US had the highest average annual PM2.5 levels while the west had the lowest.
It showed that stroke prevalence in the south was 4.2 per cent while in the west it was 3 per cent.
Stroke is the fifth leading cause of death in the US, killing around 129,000 people.
China regularly issues smog warnings in its capital Beijing as the city becomes choked with pollution.
An estimated 15 million people suffer a stroke worldwide, with around six million dying as a result of the condition.
Strokes are caused when the brain is starved of oxygen - either because of a blockage in the blood vessels feeding it or due to bleeding within the brain itself.
While it has still to be confirmed, it is feared PM2.5s may increase the risk of blockages by causing blood vessels to harden and clots to occur.
Dr Liu said the findings of the study provided evidence that public health authorities needed to do more to reduce air pollution.
Warnings for patients who are at high risk could also be issued on days when air pollution is at its worse, so they may take steps to avoid breathing in the harmful particles.
Dr Liu said: 'Patients with stroke are in danger of dehydration due to high temperatures in the summer, and are in danger of suffering from pneumonia, influenza and other respiratory diseases in winter.
'Women and the elderly also appear more vulnerable to stroke risk due to air quality and heat-related diseases.'
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