High Levels of Pollution 'Bring Forward' Heart Attacks

September 21, 2011

September 21, 2011 (London, United Kingdom) — A new study shows that higher levels of air pollution appear to be associated with an increased risk of MI one to six hours after exposure, but later on there is a reduction in risk [1].

The results suggest that rather than increasing the overall risk of heart attacks, pollution merely brings forward in time MIs that would have occurred anyway, so-called "short-term displacement," say Dr Krishnan Bhaskaran (London School of Hygiene and Tropical Medicine, UK) and colleagues in their study, published online September 20, 2011 in the BMJ.

However, Bhaskaran emphasized to heartwire , "It is well-established from many studies that there are more deaths when air-pollution levels are higher"; hence, it is still important that efforts are made to limit such levels, he stresses.

In an accompanying editorial [2], Drs Simon Hales and Richard Edwards (University of Otago, Wellington, New Zealand) agree. While this new research addresses many limitations from previous studies on this topic and studies of short-term exposure may improve the understanding of the early physiological mechanisms of the effects of air pollution, "they are not the best basis for developing policy," they note.

"Given other evidence that exposure to air pollution increases overall mortality and morbidity, the case for stringent controls on pollution levels remains strong," they assert.

MINAP: A Unique Database on Timing of MIs

Bhaskaran explained that while the association between a rise in air pollution and increased mortality is established, "what hasn't been clear is whether heart attacks make a major contribution to this." Existing research on this topic has produced mixed results, he notes.

So he and his colleagues performed a case-crossover study linking clinical data from the Myocardial Ischaemia National Audit Project (MINAP) database with information on ozone, carbon monoxide, nitrogen dioxide, sulfur dioxide, and small particles from the UK National Air Quality Archive across 15 cities in England and Wales.

Bhaskaran explains that MINAP is a unique database, "particularly if you want to look at things on an hourly level and have that detail on the exact timing of when symptoms start. I don't know of any other databases on that scale that have information on the timing, down to the hour, of the heart attack."

Pollution effects were investigated with delays of one to six, seven to 12, 13 to 18, 19 to 24, and 25 to 72 hours in both single and multipollutant models, adjusted for ambient temperature, relative humidity, circulating levels of influenza and respiratory syncytial virus, day of week, holiday, and residual seasonality within calendar-month strata.

Bhaskaran says he agrees with one criticism leveled by the editorialists, that measurements of the exact pollution people are exposed to can be tricky. Ideally, it would be good to know individual details, he says, but this can only be achieved by attaching monitors to each person, a nigh-on impossible task when dealing with large numbers. Instead, the study employed regular pollution monitors stationed across the cities surveyed, and inevitably there is "a bit of a trade-off," he notes.

Effect of Air Pollution on Mortality Not Mediated Through Increasing MI

The main outcome measure was excess risk of MI per 10-µg/m3 increase in pollutant level. There were just under 80 000 MIs over a four-year period (2003–2006).

"We didn't see any convincing net impact on the overall number of heart attacks," says Bhaskaran. "What we saw was in the few hours (one to six) following a pollution peak, there was an increase in the risk of MI, but if you looked a little bit further ahead than six hours, there were fewer heart attacks than expected, so this seems to suggest that people who were going to have heart attacks in that later period maybe had their MI brought forward by a matter of a few hours or at the most a couple of days."

People who were going to have heart attacks in that later period maybe had their MI brought forward by a matter of a few hours or at most a couple of days.

The researchers conclude that the well-established effect of air pollution on cardiorespiratory mortality may not be mediated through increasing the risk of MI but through another mechanism.

Interestingly, the British Heart Foundation (BHF), which supported the research, does not make mention of the fact that overall numbers of heart attacks were not increased in this study [3]. Instead it chooses to stress the finding that the risk of heart attack goes up temporarily for around six hours after breathing in higher levels of vehicle exhaust fumes.

"Our advice to patients remains the same," says BHF associate medical director of Dr Jeremy Pearson, in the organization's statement. "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."

Bhaskaran had support for the study from the British Heart Foundation. Disclosures for the coauthors are listed in the paper. The editorialists report no conflicts of interest.

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