Blood clots (also known as deep vein thrombosis), can be a serious risk to some air travelers. Most blood clots dissolve on their own, but when they don’t they can cause fatal blockages. For instance, if part of a blood clot travels to the lungs, it can cause a pulmonary embolism.

The most important information to know about blood clots are the symptoms, so you are able to get medical attention if you need it. Seek help if you notice unusual or prolonged swelling in an arm or leg, unexplained pain or tenderness, skin that is warmer or redder than the rest of your body, as these are the most common symptoms of deep vein thrombosis.

Similarly, get medical attention immediately if you notice any difficult breathing, a fast or irregular heartbeat, chest pain or discomfort, or lightheadedness. Travelers with life-threatening blood clots may also feint, or cough up blood: symptoms of a pulmonary embolism.

What causes blood clots?

Blood clots are rare, but each person’s level of risk varies depending on their health and the duration of travel. Blood clots form when you sit still for too long. The longer a body is immobile, the greater the risk.

Certain travelers, however, are more likely to develop blood clots than others.

For example, elderly travelers, people who are overweight, individuals who have recently had surgery or suffered an injury, people who use estrogen-based birth control or are on estrogen-based hormone therapy, women who are pregnant or recently gave birth, people with a personal or family history of blood clots, people currently undergoing or who recently underwent cancer treatment, people with limited mobility, people who have catheters, and people with varicose veins have a significantly greater risk for developing blood clots.

Though the likelihood of developing a blood clot is very low, it's possible for a number of these factors to compound and increase your risk.

How to avoid blood clots

The best way to avoid a blood clot is to move around as much as possible during your flight. Get up out of your seat at least once an hour, and move your limbs frequently while sitting — flexing and bending your feet, ankles, and knees. If you are worried about blood clots, talk to your doctor before flying. He or she may also recommend wearing compression socks, or prescribe a blood thinner.