A study at the hospital identified 30 deaths in three years from deep-vein thrombosis arising from air travel. And even though deaths are relatively rare, the condition is causing growing concern.

Ashford Hospital's Dr. John Belstead said emergency nursing staff there started noticing long-haul travelers dying of massive blood clots as early as eight years ago.

"It's the red-eye flights from the west of the United States and the Far East, and also from Australia," he said. "It's mostly people who sleep in their seats."

The condition often has been dubbed "economy-class syndrome," but Belstead said a French study suggests that term is a misnomer.

"They found two people (with the condition) who traveled in business class and one person in first class, so it's not just economy class," he said.

Fatalities rare

Fatal cases of so-called traveler's thrombosis are very rare, Ashford physicians stress. For every victim, there are a million other long-haul passengers who arrive safely at Heathrow, they say.

But according to some doctors, as many as one in 10 air travelers could be affected by some form of the condition.

At Heathrow, concern is tempered by realism.

"I think the planes should offer more space, I really do, in the economy section," one traveler said. "People have to take the choice: Do they want a cheap flight or do they want to pay more money for their flight and have more leg room?"

Passenger warning

British Airways takes the problem seriously, and has begun issuing new warning leaflets to passengers, plus is showing a video on its flights advising travelers to exercise while the airplane is aloft.

Australia's two biggest airlines also said they will print health warnings onto their tickets about the danger of blood clots.

Even though deaths from traveler's thrombosis aren't common, the message from doctors is that the risk can be minimized by in-flight exercise and other preventive measures.

Some doctors recommend travelers take aspirin, which thins the blood, before flying, or wear elastic knee-length stockings to help prevent clots.

"The passenger has to take responsibility for their own health and make sure they are fit to travel, that they've thought about the issues surrounding travel, and that they take heed of the advice," said Dr. David Flower of British Airways.

Snacking, moving can decrease risk

Take your next airplane flight with a little food and drink, doctors advise. A snack or small meal and nonalcoholic beverage may decrease the risk.

Japanese researchers have found that in an airplane cabin environment, food and nonalcoholic beverages improved oxygen levels in the body by 21 percent and oxygen to the brain by 48 percent. After eating and drinking, blood volume may be increased.

Increased blood volume helps to keep circulation going, lessening the chance that blood can pool in the legs to form clots. Such clots become a danger when they travel to the brain or heart, causing stroke or heart attack.

Sitting too long in cramped seats -- whether on a plane or in a car -- can lead to what is being called "economy class syndrome," or clot formation in the deep veins of the legs.

"Sometimes people get blood clots that go from their legs to their lungs -- pulmonary emboli," said Dr. Gerald Fletcher, a cardiologist at the Mayo Clinic in Jacksonville, Florida. "This is a risk, and it's been fairly well documented as a slight problem in the literature."

Dutch scientists recently concluded that there is no increased risk for most travelers on trips averaging 8 hours. However, they said more research is needed to determine the effects of traveling more than 15 hours, especially for people who are already at increased risk for thrombosis.

Eating too much before a flight may be less effective than eating just a bit, however, doctors said. This is because a big meal could divert blood flow to the digestive tract.

"The fact that you're sitting on an airplane for maybe 2, 3, 4, 6 hours -- that's enough to really impede some of the blood flow and maybe increase your risk for having a deep vein thrombosis," said Frid.

Fletcher and other physicians recommend that travelers get up for a walk periodically to keep circulation humming. Some even wear support stockings when flying.

If you can't move easily, at least flex your feet and ankles and shift your position in your seat now and again, doctors said.

The Air Transport Association agrees that passengers should move around periodically during flight, eat lightly and drink plenty of fluids, but limit alcohol and caffeine intake.