Q. I've heard there's a risk of getting blood clots when you travel on airplanes. Is there anything you can do to prevent them?

A. Blood clots, also known as thrombophlebitis, or deep vein thrombosis (DVT), can occur when people are immobilized during flights of five hours or more. DVT can be life-threatening if a blood clot breaks loose from the vein and lodges in the lungs or pulmonary artery (this is known as a pulmonary embolism).

Get medical help immediately if you experience any of these symptoms after a long flight: feelings of apprehension, shortness of breath, sharp chest pain, rapid pulse, sweating, a cough with bloody sputum, or fainting.

In the general population, DVT affects about 1 in 2,000 people. You are at increased risk if you have cardiovascular disease, large varicose veins, severe obesity, a personal or family history of blood clots, a coagulation disorder, a recent history of surgery on your hips or knees, or you've had cancer within the past two years.

Blood clots can occur more frequently in pregnant women, women who have recently given birth and women on birth control pills or estrogen replacement therapy.

If you are at high risk, your doctor may prescribe blood-thinning medication such as warfarin to help prevent clots.

Regardless of your risk, these tips can help prevent a clot while flying: Stand up and walk every hour; exercise your calf muscles; drink one liter of water every five hours; avoid alcohol; avoid crossing your legs; and wear loose-fitting clothing.

Pycnogenol, an extract from pine bark, may also help prevent blood clots because it can improve microcirculation. A double-blind, placebo-controlled, randomized trial published in the journal of Clinical and Applied Thrombosis/Hemostasis in 2004 reported that after 198 passengers at moderate-to-high risk for DVT or superficial vein thrombosis (SVT) took 200 milligrams of Pycnogenol two to three hours before an eight-hour flight, followed by the same dose six hours later and 100 milligrams the next day, they had no DVT events and significantly fewer SVT events than the placebo group.

"This study," the authors conclude, "indicates that Pycnogenol treatment was effective in decreasing the number of thrombotic events (DVT and SVT) in moderate to high risk subjects, during long-haul flights."

Since people at low risk are also susceptible to blood clots, it was suggested that Pycnogenol be considered as a preventive supplement in this population as well.

Laurie Steelsmith is a naturopathic physician and licensed acupuncturist in Honolulu. You can contact her and look at her past columns at www.drlauriesteelsmith.com. This column is for information only. Consult your health provider for medical advice.