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As if there's not enough to worry about when you're flying the friendly skies, here's one more thing: Recent studies have linked long-distance air travel with an increased risk of developing blood clots in the legs.

The studies themselves are controversial. Although some researchers have found a strong association between flying and the development of blood clots, others have found the risk to be minimal.

Some airlines are attaching health warnings to passengers' tickets for long-distance flights.

Doctors refer to a blood clot in the leg as a deep venous thrombosis, or DVT for short. As its name implies, this type of blood clot affects mainly the deep veins in the circulatory system.

The deep veins of the lower extremities are most frequently involved. In approximately 80 percent of cases, the process begins in the calf, although it can begin in the veins of the thigh or pelvis.

The clot causes problems when it interferes with circulation in the area, but the real danger results when it breaks loose and travels through the bloodstream. The clot can end up lodging in the blood vessels of the brain, lungs or heart, causing severe damage to the organ involved.

Long-distance flying isn't the only activity that boosts your chances of developing blood clots. Any prolonged sitting, bed rest or other type of immobilization can put you in danger of developing a DVT.

Surgery can increase your risk of forming the clots, especially operations involving the hips or knees, or gynecological surgery. Some medications, including drugs used for hormone replacement therapy or birth control can increase your risk, especially if you smoke cigarettes. Roughly half of all patients with DVTs have no signs or symptoms at all. If symptoms are present, they may include a dull ache, a sensation of tightness, or pain in the involved calf or the entire leg, especially during walking. Some victims may notice swelling or redness in the affected limb, a slight fever and a rapid heart rate.

Because of the difficulty in making a precise diagnosis based on physical examination alone, doctors usually rely on high-tech studies to help them out. One of the most common studies used is the Doppler ultrasound blood flow detector. The test is painless and noninvasive, and allows doctors to evaluate circulation in the lower extremities.

If you're unlucky enough to develop a DVT, prompt medical attention is critical to prevent the clot from traveling to other organs in the body, an event that could result in serious illness, or even death.

Treatment of DVT involves the administration of anticoagulation drugs, like heparin and warfarin, to reduce the clotting ability of the blood. In most cases, patients with DVTs need to continue anticoagulation therapy for a period of about six months.

Since an ounce of prevention is worth a pound of cure, your best bet is to avoid getting a blood clot in the first place. On long plane trips, it's helpful to get out of your seat and move around from time to time. Even if you don't have to use it, walking to the lavatory every hour or so is a good idea.

For extra protection, you might want to invest in a pair of compression stockings. Although they're not exactly stylish, the elastic material helps prevent blood from pooling in your legs.

If you know you're going to be facing a long stretch of immobility, you may want to consider aspirin therapy to reduce the clotting ability of your blood. For most folks, taking 81 to 325 milligrams of aspirin a day is an acceptable way to keep blood clots away.

Dr. Rallie McAllister is a family physician in Kingsport, Tenn. Her column appears three times a month on this page. Her Web site is

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