Exposure to high levels of air pollution could increase the risk of having a heart attack for up to six hours after exposure, finds research published Wednesday in the British Medical Journal.

Traffic jam in London, UK exposes pedestrians to air pollution. [Environment News Service]

Yet, the researchers found no increased risk of heart attack after the six hours had passed.

For the study, Krishnan Bhaskaran, an epidemiologist from the London School of Hygiene and Tropical Medicine, and his colleagues reviewed 79,288 heart attack cases that occurred from 2003 to 2006.

They studied exposure to pollution, by the hour, of people who had heart attacks while residing in 15 urban areas of England and Wales.

The authors 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 ground-level ozone.

Higher levels of PM10 and NO2 are well-known markers of traffic-related pollution, said Bhaskaran.

In single pollutant models, PM10 and NO2 levels were associated with a very short term increase in risk of heart attack, between one and six hours after exposure.

Given the transient nature of the increased risk, the researchers speculate that the heart attack could have happened anyway and was merely brought forward in time by a few hours. This is known as a "short-term displacement effect" of pollution.

The study's authors say while established research has concluded that high pollution levels are associated with premature death from heart disease, the link is less clear to an increased risk of heart attack, also called myocardial infarction.

"Most studies to date have investigated associations on a daily time scale (often referred to as 'short term'), but a small number of studies using data at a finer temporal resolution have observed effects on risk of myocardial infarction within a few hours of exposure to particulate matter and more generally to traffic," the authors state.

"Only a handful of studies have looked at the associations between pollution and myocardial infarction risk at an hourly temporal resolution," they state.

The researchers acknowledge that there may be "limited potential for reducing the overall burden of myocardial infarction through reductions in pollution alone."

Still, they argue that their findings "should not undermine calls for action on air pollution, which has well established associations with broader health outcomes including overall, respiratory and cardiovascular mortality."

In an accompanying editorial in the "British Medical Journal," Professor Richard Edwards and Dr. Simon Hales from the University of Otago in New Zealand say that "despite the strengths of the study, it is possible that a true effect was missed because of imprecise measurements and inadequate statistical power."

They conclude that "given other evidence that exposure to air pollution increases overall mortality and morbidity, the case for stringent controls on pollutant levels remains strong."

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