Heart disease is the number one killer of both men and women in the United States, and more than 610,000 Americans die of it annually. That’s one in every four deaths. If you know even just a little bit about heart health, then the key risk factors aren’t all that surprising. They include high blood pressure, high cholesterol, smoking, being overweight, and a lack of exercise. But it turns out there may be more risk factors than you realize—many that aren’t so obvious. Here are 10 strange things that may affect your heart disease risk, for better or for worse.

The altitude where you live

A recent study published in the journal Frontiers in Psychology found that people living at high altitudes (between 457 and 2,297 meters) had a lower risk of developing metabolic syndrome—a cluster of heart-disease risk factors like high blood pressure, high cholesterol, and obesity—than those who lived at sea level.

There’s less oxygen in the air at higher altitudes, which may help the heart and lungs function more efficiently, the authors speculate. The possible connection is “interesting,” says Mary Ann Bauman, MD, a spokesperson for the American Heart Association and the Go Red for Women campaign (who was not involved in any of the studies noted here), “but requires more study to determine if there is a true association.”

Women who get pregnant more than once have an increased risk of later developing atrial fibrillation, also known as a-fib, according to a new study in the journal Circulation. A-fib is a quivering or irregular heartbeat that can lead to blood clots, stroke, and other complications. In the study, women with four or more pregnancies were 30% to 50% more likely to develop a-fib compared to women who had never been pregnant.

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