NEW YORK -- Deep-vein thrombosis, a blood clot that forms in the lower leg, can develop during long-distance flights when passengers sit in the same position for a long time without movement.

Most clots are harmless, but they can become loose and travel to the lungs. People over age 50 are the most susceptible.

The new issue of Travel Smart Newsletter -- http://www.travelsmartnewsletter.com -- out Feb. 15-20, offers tips for preventing the condition whether you're on a plane, train, bus, or just sitting at your computer researching your next trip.

Most of the tips involve ways to increase circulation, such as avoiding clothing that's tight around the upper thighs; wearing compression hosiery; walking up and down the aisle once an hour, and doing six to 10 deep-knee bends each time you get up. The newsletter also suggests doing simple exercises from your seat at least once an hour, such as ankle circles (using your feet to draw circles, clockwise and counterclockwise).

The World Health Organization also has information online on deep-vein thrombosis and other medical issues; search for "Travel by air: health considerations" at http://www.who.int/ith/en. According to WHO, risk factors for developing deep-vein thrombosis include a history of the disease or of pulmonary embolisms; use of estrogen-therapy or oral contraceptives; pregnancy; recent surgery or trauma. WHO also said that some researchers believe there is a risk from smoking, obesity and varicose veins.