Breathing in heavy amounts of traffic fumes can trigger a heart attack, according research by British scientists.
Researchers from the London School of Hygiene and Tropical Medicine found the chance of suffering an attack increased by 1.3% in the six hours after coming in contact with high levels of vehicle-related pollutants.
They singled out tiny particles known as PM10 and nitrogen dioxide which are both expelled by cars as the main culprits.
The study, published in the British Medical Journal, compared data from 80,000 heart attack patients living in 15 urban areas in England and Wales with local levels of PM10 (pollutant particles), ozone, CO (carbon monoxide), NO2 (nitrogen dioxide), and SO2 (sulphur dioxide).
The experts found found that the risk of an attack decreased again after the six hour post-exposure period, but concluded that high pollution levels can hasten rather than directly cause the medical emergency.
The authors wrote: "Higher levels of PM10 and NO2, which are typically markers of traffic related pollution, seem to be associated with transiently increased risk of myocardial infarction 1-6 hours after exposure, but later reductions in risk suggest that air pollution may be associated with bringing events forward in time rather than increasing overall risk."
Professor Jeremy Pearson, associate medical director of the British Heart Foundation which co-funded the study, said: "This large-scale study shows conclusively that your risk of having a heart attack goes up temporarily, for around six hours, after breathing in higher levels of vehicle exhaust.
"We know that pollution can have a major effect on your heart health, possibly because it can 'thicken' the blood to make it more likely to clot, putting you at higher risk of a heart attack.
"Our advice to patients remains the same - if you've been diagnosed with heart disease, try to avoid spending long periods outside in areas where there are likely to be high traffic pollution levels, such as on or near busy roads."
According to evidence presented to ministers as part of the Government's Environmental Audit Committee, air pollution could be contributing to as many as 50,000 deaths per year.
And averaged across the whole UK population it is estimated that poor air quality is shortening lives by seven to eight months.