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Beware this chilling truth: Cold weather brings heart risks. It’s a reality Sue Gaston discovered firsthand.

In January 2017, the Anchorage, Alaska, resident had come in from shoveling her walk as she had done many times before. She was an active person, of proud Norwegian heritage.

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“We’re just kind of … worker bees,” she said. “And when the snow needs shoveling, you go out and you shovel it.”

But this time, something felt different. She felt tired, sore, clammy and nauseated, and realized she was having a heart attack.

She was able to call for help and soon was at a hospital, where doctors inserted a stent to open a blocked heart artery. Since then, she has been more careful. “I’m not quite as much in a rush and not quite as much pushing like I might have in the past,” said the 66-year-old.

Besides snow shoveling, stress, artery constriction, flu and lack of exercise can also cause heart problems in the winter months.

Dr. Robert A. Kloner has worked on studies showing cardiac deaths begin to climb around Thanksgiving, peak early in the year, then decrease as warmer weather returns. He is chief science officer of the Huntington Medical Research Institute and a professor of medicine at the University of Southern California in Los Angeles.

A 2015 study he worked on showed death rates from circulatory problems, coronary heart disease and heart attacks all rose as temperatures fell. Every 1-degree Celsius drop in temperature (that’s 1.8 degrees Fahrenheit) came with a 0.49% increase in deaths from all causes. Other studies have shown that when temperatures go down, strokes increase.

Many factors are at work, Kloner said. Cold makes arteries constrict. That can decrease blood flow and delivery of oxygen. Cold also tends to increase the formation of blood clots. Winter also is flu season, and studies have shown an increase in heart attacks shortly after people get the flu.

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Cold-weather months also bring holidays that can stress people out, and the festivities can cause them to overindulge, said Kloner, whose work helped uncover a spike in cardiac deaths on Christmas and New Year’s Day.

Another seasonal factor is the lack of sunshine, which might deter people from exercising. And, wood-burning fireplaces produce air pollution that can hurt the lungs and heart.

Kloner also said, “When it’s dark, people tend to be more depressed. We know that there’s a correlation between emotional upset, depression and cardiac disease.”

Dr. Nathalie Auger of the University of Montreal Hospital Research Center led a study showing that, in Quebec, the more it snowed, the more men had heart attacks. Researchers suspect shoveling snow was the link.

“It’s quite difficult physical activity,” she said. “It can happen quite suddenly at the beginning of the season.”

Kloner said shoveling is a risk particularly for people who have heart disease. Trudging out in the snow increases oxygen demand at the same time your arteries are tightening up in the cold. “It’s sort of a double-whammy.”

Auger said climate change, and the increase in extreme weather it brings, will mean more people need to be on alert.

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To protect the heart in the cold, both doctors recommend common-sense measures such as eating a healthy diet and avoiding smoking. “But also, in the cold weather, especially if you have known coronary artery disease – stay warm,” Kloner said. “Try to bundle up.”