Winter may be far from our Minnesota minds during this sweltering week in August, but here’s a research finding that may cause a shiver or two: When the temperature drops on any particular day by 1 degree C (1.8 degrees F), the relative risk of having a heart attack increases by 2 percent for the next 28 days.
That’s a small, but significant, increase in risk. Here in the U.S., we have more than 1.1 million heart attacks each year. A 2 percent increase in risk translates into about an extra 1,700 heart attacks within those 28 days.
Yes, other studies have found an association between cold weather and an increase in the death rate. But this is apparently the first one that corrected for two important confounding factors: influenza outbreaks and air pollution.
In other words, when you take the flu and air pollution out of the picture, cold weather is still more likely to be associated with an increased risk of heart attack.
Data for this study, which was published this week in the British Medical Journal, came from hospital admissions records for 84,000 heart attacks that occurred in England and Wales between 2003 and 2006. Not unsurprisingly, the researchers (all from the London School of Hygiene and Tropical Medicine) found that older people (those over the age of 75) and those with a previous personal history of heart disease were most susceptible to the effects of plummeting temperatures.
The study also found that, as the authors expected (based on previous studies), the risk of heart attacks is higher on weekdays, particularly Monday. Specifically, the Brits in the study were 14 percent more likely to have a heart attack on Monday than on Sunday. Holidays, however, were not higher-than-normal heart attack days.
No link between high temperatures and heart attacks was discovered, but that may be because the weather in the U.K. is seldom steamy for very long.
Nor was relative humidity associated with an increased risk of heart attack in this study — a comforting finding, perhaps, during this sweltering August week here in Minnesota. (The study did find, however, a nonsignificant increase in risk on days with both the highest and the lowest levels of humidity.)
Like all studies, this one has its limitations. First and foremost, it’s an observational study, which means it can show only an association between two factors (cold temperatures and increased risk of heart attacks) not a cause-and-effect. And although it adjusted for the effects of pollution and the flu, other nontemperature factors may explain this study’s findings. As the authors themselves point out, the study included only data from patients who were admitted to a hospital, not those who died before reaching the hospital. That data restriction may have skewed the findings.
Just why cold weather may increase the risk of heart attacks is not known, but researchers hypothesize that it may have to do with the cold’s ability to raise blood pressure and/or release proteins that make the blood more likely to clot.
This study re-emphasizes that people at increased risk of heart attacks (and stroke) should take extra precautions during cold weather to stay warm. The rest of us can help by volunteering to shovel snow and do other cold-weather tasks for our more vulnerable neighbors.