During the holidays, many Americans travel long distances to be with family and friends, and this can mean prolonged sitting in cars and buses, trains or planes. This type of extensive travel can put you at risk for developing a particular type of blood clot known as a deep vein thrombosis, or DVT.

Deep vein thrombosis is a condition in which a blood clot forms within deep-circulation blood vessels, typically in leg veins. These veins drain blood back to the heart so it can be oxygenated in the lungs. Blood clots often form when blood is allowed to pool at extreme levels for long periods of time, which can occur during prolonged sitting or lying in bed for extended periods of time, such as after surgery.

Sitting still for more than four hours at one time can increase the risk for DVT, and after a period of prolonged sitting, your risk for forming a DVT may persist for several weeks.

The American Heart Association (AHA) estimates that one in 1,000 people develop DVT each year. Symptoms of DVT include swelling, warmth or redness in the leg or calf. In some cases, patients develop pain in the calf that is worse when standing or walking. This is particularly alarming if it is associated with redness and swelling. DVT may not be apparent right after travel, but the condition often appears after the trip is over and in the weeks following the trip.

There are risk factors for DVT including prior blood clots, recent surgery, pregnancy, cancer and certain medicines such as oral contraceptives and hormone replacement medications.

Deep vein thrombosis can be very serious. The most serious complications of DVT can be a pulmonary embolism, or PE. In the case of a PE, a piece of the blood clot in the vein can break off and travel to the lungs. If the clot lodges in the lungs, it can prevent circulation through the lungs and even result in death if it's not treated quickly.

If you think you may have developed a DVT, you should seek medical care immediately. If you develop shortness or breath or chest pain with breathing, call 911 and seek emergency help, as this could indicate that the blood clot has traveled to the lungs and a pulmonary embolism has occurred.

There are a handful of things you can do to prevent DVT while traveling this holiday season .If you're traveling by car, stop every hour and get out and walk around. If you're on a plane, try to get up and walk every 30 minutes, if possible. Also, avoid alcohol, caffeine and other substances that may cause dehydration. Drink plenty of fluids, preferably water. While sitting on the plane or in the car, try to raise your toes and flex your calves every 20 minutes— this motion causes your calf muscles to contract and promotes the flow of blood in the leg veins, which can help keep blood from pooling and forming a clot.

If you have risk factors for DVT, you may want to consider talking with your physician before traveling to obtain compression stockings for your legs. If you have had a previous blood clot, talk to your doctor about the option of taking blood thinners.

Dr. Kevin Campbell is the author of  "Losing Our Way in Healthcare: The Impact of Reform", and an assistant professor of medicine in the division of cardiology at University of North Carolina at Chapel Hill. For more from Dr. Campbell, visit his website DrKevinCampbellMD.com.