Extreme temperatures and heavy air pollution boost heart attack risk, according to a major new study.

And on days when the air is extra dirty and the temperature is unusually hot or cold, the effects are likely to be particularly bad, given that temperature and pollution seem to harm the body in different ways, Dr. Krishnan Bhaskaran of the London School of Hygiene and Tropical Medicine in the UK, the lead author of the research, told Reuters Health.

Several studies have linked changes in temperature to increases in deaths due to any cause, as well as heart disease mortality, Bhaskaran and his team note in their reports. But looking at heart attacks-not just deaths from heart disease-could offer a more accurate picture of the overall health risks of temperature changes and air pollution, they say, and might also offer clues to why they may trigger heart attack in high-risk people.

In two separate reports, the researchers reviewed 19 studies on temperature and heart attack and 26 examining air pollution and heart attack.

In the 12 temperature studies that collected winter data, eight showed short-term increases in heart attack risk with colder temperatures. Seven of the 13 studies that looked at the effects of warmer temperatures found increased heart attack risk in hotter weather.

Cold temperatures seemed to have a greater effect on heart attack risk in areas that were warmer, on average, Bhaskaran and colleagues note, suggesting that people living in colder areas may be better adapted to dips in temperature. But hot days boosted heart attack risk whether they happened in Sweden or Brazil.

In a city that normally sees 10 heart attacks a day, Bhaskaran explained, the findings show there might be an extra one to four heart attacks on the hottest or coldest days.

There was a lot of variation in the methods and quality of the studies we reviewed, so more work is needed in this area, but we thought the results were consistent enough to suggest that these effects are real, he added.

The evidence from the pollution studies was less clear-cut, the researcher said, but overall suggested that the risk of heart attack increases with levels of several different pollutants. Also, he noted, there appeared to be no safe level of air pollution at which no effect on heart attack risk was seen.

Our findings would suggest that further lowering limits would likely further reduce the health burden associated with pollution, which is of course a desirable outcome, he said.

Governments can also make an effort to alert at-risk people when extreme temperatures or high levels of pollution are expected, the researcher said. He noted that the UK now makes automated phone calls to people with emphysema when they are at risk due to changes in temperature; this, he said, has led to a reduction in hospital admissions for lung disease patients.

In an editorial accompanying the study, Professor David E. Newby of the University of Edinburgh and colleagues note that efforts to control air pollution are likely to help reduce carbon dioxide emissions as well, possibly helping to alleviate the effects of climate change down the road.

SOURCE: Heart, November 2009.

United kingdom Medicine