Higher levels of air pollution can increase the risk of having a heart attack for up to six hours after exposure, warn UK researchers.
Even moderate levels of pollution from traffic carry an extra risk, according to a new study in the British Medical Journal.
The findings come from a detailed analysis of almost 80,000 heart attack cases and the level of pollution to which they were exposed.
Air pollution is currently estimated to reduce the life expectancy of everyone in the UK by an average of seven to eight months, probably by affecting the heart and lungs.
In the latest study Krishnan Bhaskaran, an epidemiologist from the London School of Hygiene and Tropical Medicine, and colleagues found rising air pollution was linked to a rise in heart attacks up to six hours after exposure.
However, there was no increased risk after the six-hour period, with the number of heart attacks then falling to a lower level than expected.
The researchers reviewed 79,288 heart attack cases from 2003 to 2006 and exposure, by the hour, to pollution levels.
They used the UK National Air Quality Archive to investigate the levels of specific pollutants in the atmosphere. These included pollutant particles (PM10), carbon monoxide (CO), nitrogen dioxide (NO2), sulphur dioxide (SO2) and ozone.
Higher levels of PM10 – tiny toxic particles – and NO2 are well-known markers of traffic related pollution from vehicle exhaust fumes, said Dr Bhaskaran.
National air pollution warnings on weather reports alert people to changes using bands ranging from low, to moderate, to high, to very high.
People who are sensitive to air pollution are advised to spend less time outdoors during high and very high episodes of air pollution, and not to exercise, along with those suffering asthma and heart disease.
Dr Bhaskaran estimated there would be an extra five per cent risk of a heart attack caused by a change in air pollution from ‘low’ to ‘moderate’ in the following six hours.
The risk would increase further during ‘high’ and ‘very high’ episodes but they are uncommon in Britain, he added.
Dr Bhaskaran said the data suggested that after the first six hour period following pollution peaks, the number of heart attacks was lower than expected.
Some people who were going to have a heart attack in that later period may have simply had their heart attack brought forward by a few hours as result of the pollution exposure, he said.
‘We know from many studies that there are more deaths when pollution levels are higher, but whether heart attacks make a major contribution to this is not clear.
‘Although we found a short period of increased risk of heart attacks in the few hours after air pollution peaks, the risk was small and had little net impact on the overall number of heart attacks’ he added.
Professor Jeremy Pearson, Associate Medical Director at 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.’
The study looked at heart attacks in England and Wales.