Temperature extremes have been known to have an ill effect on populations' health and mortality rates. Heat stroke can cause organ failure, and cold weather has been linked to increased hospitalization and death rates. Little firm data exists, however, to draw exacting predictions about how a change in the weather can influence the risk of cardiovascular problems. A new study reveals that even seemingly tiny daily temperature drops have a sizable impact on the number of heart attacks in a large geographic area.

Researchers behind the new work assessed four years of hospital admissions from 15 sites across the U.K. and found that a drop in the mean daily temperature of as little as 1 degree Celsius increased the number of heart attacks for the proceeding four weeks. The results were published online August 10 in the British Medical Journal (BMJ).

Previous studies have come to conflicting conclusions on the subject of cardiac deaths and temperature changes—many finding increases in both cold and hot weather. But the new analysis did not find a similar increase in cardiac events during temperature increases.

The cooling increased the heart attack rate by just 2 percent. "Two percent sounds small," says Krishnan Bhaskaran, a researcher at the London School of Hygiene and Tropical Medicine and lead author on the new study, "but the reason it's important is the whole population is exposed to the weather, and heart attacks are common in the first place."

And on a broad scale the small numbers can make a big difference in what is the most common form of death for both men and women worldwide. "From a public health perspective, even a weak effect of temperature on health constitutes an important problem due to ubiquity of exposure," Manuela De Sario and Paola Michelozzi, both of the Lazio Region Department of Epidemiology in Rome and co-authors of an editorial that appears in the same issue of the BMJ, explained in an e-mail to Scientific American.

The new multisite study analyzed 84,010 hospitalizations reported for heart attacks from 2003 through 2006 (88 percent of which were confirmed with an electrocardiogram or other lab test). Extrapolating from the data, Bhaskaran and his colleagues estimated that for every 1 degree C drop in the mean temperatures in the U.K. over a 24-hour period—where about 11,600 heart attacks occur each month—an additional 200 heart attacks are likely to occur.

Both men and women seemed to be equally vulnerable to this change in the weather, and those at greatest risk were people aged 75 to 84 years, along with those who had previous heart troubles. People taking aspirin had a slightly reduced risk, although the authors do not recommend starting on an aspirin regimen based on their findings (and even this drug did not fully mitigate the risk, they noted).

The cold facts
Despite perennial tales about snow shoveling-induced heart attacks, there seems to be more to the cold story than physical exertion. Researchers are still parsing out the precise physiologic effects of cold on the cardiovascular system, but some controlled lab studies run on volunteers have provided clues. These lab tests have shown that when the ambient temperature drops, blood viscosity increases, blood pressure changes, and the heart has to work harder, likely increasing the risk for clots and ultimately a heart attack.

Although overexertion and deep freezes do seem to contribute to heart attack risk, the new data suggests that overall temperature matters less than its variation. In other words, Bhaskaran explains, there did not seem to be a chilly threshold temperature after which the one-degree rule took hold. That means that even if the mean daily temperature is 10 degrees C one day, 9 the next, and 10 again the following day, there are still likely to be an extra 200 heart attacks during the next month.

The striking lag time, which tended to be about two weeks after a temperature drop, although some extended to 28 days, seem to be more pronounced with cold weather heart troubles than in those spurred by heat waves. But "why that is, we're not entirely sure," Bhaskaran says. The cold might simply start broader changes in the body that can eventually lead to a heart attack, he suggests.

Because the study was based on a database of hospital admissions, the calculations do not take into account heart attacks in which a person died before reaching the hospital. But if a greater number of deaths occurred due to emergency vehicle delays caused by bad winter weather, the present estimates could be a conservative tally.

A matter of degree
The disparity in the impact of temperature ups and downs might be because many earlier studies have looked at shorter time periods—days rather than weeks—or smaller study populations. Many have also examined general mortality or cardiac events, rather than heart attacks specifically. "A previous study in Paris during the 2003 heat wave found an increased risk of sudden cardiac death but no increase in myocardial infarction [heart attack]," De Sario and Michelozzi noted as an example.

Bhaskaran and his team suggest that their results might not have yielded results for hot weather because the temperatures in the U.K. infrequently climb above the high 20s C. De Sario and Michelozzi also noted that the effects of heat on the body are also likely more immediate and thus might lead to more fatal heart attacks before reaching the hospital (in which case they would not have been counted by the data set used in the new study). This scenario would mean that heat-related heart attacks are "probably underestimated by using hospital admissions data," De Sario and Michelozzi noted.

Confounding factors, such as relative humidity, viral infection, pollution and holidays can also impact heart attack rates, and other studies have controlled for these by varying degrees. (Bhaskaran and colleagues included these rates in their statistical analysis.) And as De Sario and Michelozzi noted, the increase in risk from the modest temperature drop reported in the study is about "10 times greater than that estimated for air pollution effect."

Warming trend
The grim numbers might be of little immediate consolation, despite predictions of climbing global temperatures due to climate change. But as the researchers point out, every degree seems to make a difference. Temperature fluctuations, however, are poised to become more extreme globally, wrote De Sario and Michelozzi in their editorial, which means more research will be crucial in order to tease out the physiological effects of temperature swings.

Simple stop-gaps such as air-conditioning have been shown to beat back the dangers of heat exhaustion. As for cold weather, Bhaskaran looks forward to examining "the role of clothing and heating—can we get rid of the excess risk just by taking care of these simple measures?"

Also worth examining, he notes, is the finding that the risk for those 85 and older was not as high for cooling-induced heart attacks than was that for those 75 to 84 years old. (This trend might be explained by the assumption that they are less likely to spend as much time outdoors and are more likely to live in care facilities that supply ample heat during the cooler months.)

In the meantime, Bhaskaran and colleagues propose that the new findings could be used to help bolster an early-warning system for at-risk populations when the forecast predicts a drop in the mercury. But more immediately, Bhaskaran says, he recommends that when it starts to cool off, people—especially older adults and those with previous heart problems—should start "wrapping up better and limiting the times outdoors."

The temperature, however, seems to be only a small player in the larger field of heart attack risk. More powerful determinants, such as smoking, drinking and diet, Bhaskaran points out, are factors that people can modify in their daily lives to decrease their odds. After all, he says, "we can't control the weather."