The problem starts when a blood clot forms within one of the body’s large deep veins, usually in a leg. The area around the blocked vein may swell, get red, or cramp up. Or the clot may go unnoticed, eventually dissolving on its own. But if a piece breaks off and travels to a lung, then it can be lethal.

People with certain clotting disorders, those who have had recent surgery or a major injury, smokers, and cancer or cardiovascular patients are more likely to develop clots. Being dehydrated, obese, pregnant, or on birth control pills raises the likelihood, too. All those factors add up.

So don’t just sit there. Stretch or walk every couple of hours. If you must stay in your seat, flex those toes and ankles frequently. Drink lots of water. Avoid tight clothing, alcohol, and sleeping pills. And if you have extra risks, check preflight with your doctor, who can fit you for compression stockings and in some cases prescribe medication.