Regular exposure to even low levels of air pollution may cause changes to the heart similar to those in the early stages of heart failure, experts say.
A study of 4,000 people in the UK found those who lived by loud, busy roads had larger hearts on average than those living in less polluted areas.
This was despite the fact people in the study were exposed to pollution levels below the UK guidelines.
Researchers called on the government to reduce air pollution more quickly.
A team of scientists, led from Queen Mary University of London, analysed health data of people who had no underlying heart problems and were part of the UK Biobank study, including the size, weight and function of their hearts.
Researchers also looked at the pollution levels in the areas they lived in.
Their study found a clear link between exposure to higher pollution levels and larger right and left ventricles - important pumping chambers in the heart.
For every extra one microgram per cubic metre of PM2.5 - small particles of air pollution - and for every 10 extra micrograms per cubic metre of nitrogen dioxide, the heart enlarged by about 1%.
The changes were comparable to being consistently inactive or having elevated blood pressure, said Dr Nay Aung, who led the study's data analysis.
"Air pollution should be seen as a modifiable risk factor," he said.
"Doctors and the general public all need to be aware of their exposure when they think about their heart health, just like they think about their blood pressure, their cholesterol and their weight."
While the exact locations where people lived were not included in the study, most were outside of the major UK cities and all of them were exposed to levels of PM2.5 air pollution well below current UK limits.
In the study, average annual exposures to PM2.5 ranged from eight to 12 micrograms per cubic metre.
This is lower than the UK limits of 25 micrograms per cubic metre but closer to the World Health Organization's recommended limit of 10 micrograms per cubic metre.
This fine particle pollution is particularly dangerous because it can penetrate deep into the lungs and cardiovascular system.
Exposure to nitrogen dioxide in the study ranged from 10-50 micrograms per cubic metre - the UK and WHO limits are 40 micrograms per cubic metre.
Dr Aung said the changes in the heart were small and potentially reversible.
But he said the fact any change at all was detectable suggested even relatively low levels of air pollution may have a harmful effect on health.
"If you think the current levels of air pollution are safe, then in theory we shouldn't be able to detect any changes," Dr Aung added.
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