Doctors have known for a while that the development of blood clots, often in the legs—a condition called deep vein thrombosis or DVT—is one of the major health risks of flying. While these clots can cause pain and swelling in the area where they form, they tend to be harmless if they stay put. But if they travel up through the bloodstream to the lungs, they can cause a blockage—known as a pulmonary embolism—which prevents blood from flowing through and getting oxygenated, and can be life-threatening if not treated promptly.

If our legroom continues to shrink, will our risk of DVT and pulmonary embolism go up? Allen Stewart, a cardiovascular surgeon and director of aortic surgery at Mount Sinai Health System in New York, says the risk of blood clots and DVTs will increase as long as the seats remain small (or get smaller) and we continue to disregard the problem. However, he says, for nearly everybody, “blood clots on airplanes are completely preventable.” Knowing how they form and how to prevent them could significantly reduce—or even eliminate—your risk. It’s simple, though perhaps easier said than done: you just have to get up and walk around.

Blood clots are more likely to form while people are on airplanes for a variety of reasons. The decreased oxygen of higher altitudes makes the blood clot more quickly, for starters. Combine that with the fact that people often spend most of their time on airplanes stuck in the same position, causing their blood to circulate less. Any flight longer than four hours puts you at an even greater risk. And then there are risk factors you carry onto the plane with you, says Stewart: if you’re overweight, over 40, post-menopausal, using certain contraceptives, dehydrated, or have cancer, you’re already more susceptible to blood clots.

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