Every year, 300 million people take long distance flights that are four hours or longer. Each one of these passengers may be at an increased risk for developing a deep vein thrombosis (DVT), a potentially deadly condition where a clot forms in one of the veins in the extremities. These clots are typically found in the thighs or lower legs.

And this risk isn’t limited to the friendly skies: traveling for periods longer than four hours, whether you’re traveling by plane, car, bus, or train, can put you at an elevated risk for developing a blood clot, according to the Centers for Disease Control and Prevention (CDC).

How Do These Clots Form?

Blood clots form when the blood clumps or sticks together, often because of slowed or restricted blood flow. When you’re sitting in a confined space, whether it’s a cramped coach seat on a plane or positioning yourself around backpacks, purses, and other personal items, you limit blood flow to the legs. This can cause clots to form in the deep veins. The risk increases with the amount of time you spend in the car or on the plane.

For many people, even when a blood clot forms, it will dissolve on its own, and you may never be aware that it existed in the first place. However, once a DVT has formed, you have the risk that the clot will become dislodged from wherever it has formed in the leg, and travel through the body to the lungs, where it can block blood flow and oxygenation to the major organs, causing massive organ failure or death.

The good news, however, is that there are several easy steps that you can take to minimize your risk of developing a DVT or life-threatening PE on your next trip, whether it’s for work or play.