When the temperature drops, the number of heart attacks goes up, according to a massive 16-year study of some 280,000 patients in Sweden. "There is seasonal variation in the occurrence of heart attack, with incidence declining in summer and peaking in winter," says the lead author in a press release, though it's "unclear whether this is due to colder temperatures or behavioral changes." The team out of Lund University announced its findings at the European Society of Cardiology Congress in Barcelona this week. Using the Swedish myocardial infarction registry (SWEDEHEART), they were able to track every heart attack treated across Sweden between 1998 and 2013. They found significantly more heart attacks in sub-zero temps and fewer in summer months, per Asian Age—to the tune of about four more incidents per day when it's below freezing than when it's above 50 degrees.
There could be various factors at play: Our bodies have ways of coping with extreme cold, such as shivering and elevated heart rates, which help raise body temperature but also increase blood pressure. That's no problem for most healthy people, but researchers think that signals trouble for those with plaques in their coronary arteries. Other potential factors tied to the seasons are decreased physical activity, dietary changes, and increased cases of respiratory tract infections and flu. HuffPost reports that extreme cold is one of several "forgotten" risk factors of heart attacks, with large meals and sudden exertion (think snow shoveling) topping the list. (Watch out for high doses of this over-the-counter drug, too.)