CHICAGO (AP) -- Good news for budget-minded travelers: There's no proof that flying economy-class increases your chances of dangerous blood clots, according to new guidelines from medical specialists.

Travelers' blood clots have been nicknamed "economy class syndrome" but the new advice suggests this is a misnomer.

The real risk is not getting up and moving during long flights, whether flying coach or first-class. Sitting by the window seems to play a role, because it makes people less likely to leave their seats, the guidelines say.

Still, even on long flights, lasting at least four hours, the risk for most people is extremely low and not something to be alarmed about, said Dr. Gordon Guyatt, chairman of an American College of Chest Physicians' committee that wrote the new guidelines.

The group, based in Northbrook, Ill., represents more than 18,000 physicians whose specialties include lung disease and critical care. The guidelines were released online Tuesday in the group's journal, Chest. They're based on a review of recent research and other medical evidence on deep vein thrombosis, blood clots that form deep in leg veins.

Flights lasting at least eight hours are riskiest, the guidelines say.

Muscles in the lower legs help push blood in the legs and feet back to the heart. Sitting still for extended periods of time without using these muscles puts pressure on leg veins and blood "tends to sit there," which can increase chance for clots to form, said Guyatt, a researcher at McMaster University in Hamilton, Ontario. These clots can cause leg pain, swelling and redness, and can be life-threatening if they travel to the lungs. They can be treated with blood-thinning drugs, but may cause permanent damage to leg veins.

Most people who develop these clots have risk factors, including obesity, older age, recent surgery, a history of previous blood clots or use of birth control pills.

The average risk for a deep vein blood clot in the general population is about 1 per 1,000 each year. Long-haul travel doubles the chance, but still, the small risk should reassure healthy travelers that they're unlikely to develop clots, said Dr. Susan Kahn, a co-author of the new guidelines and a professor of medicine at McGill University in Montreal.

Traveling by bus, train and car may also increase the risks, the guidelines say.

Besides taking a stroll down the aisle during flights, doing calf exercises including flexing and extending the ankles while seated can help prevent clots, Kahn said.

The guidelines recommend these precautions and use of special compression stockings only for people at increased risk for clots. They advise long-distance travelers against using aspirin or other blood thinners to prevent blood clots.

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