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Experts call it "economy class syndrome" — but it can happen to anyone in any class of an airplane. Deep vein thrombosis, or DVT as it's known, occurs when blood clots in one or more veins. It can happen at any time, but people who travel on long-haul flights are at a higher risk because they sit for long periods of time, said Dr. Pinakin V. Parekh, a consultant cardiologist at Singapore's Harley Street Heart and Vascular Centre. "Theoretically people who are in better classes of air travel — business class, first class — have more space to move about, stretch their legs," he told CNBC Travel. "So it's all about real estate in the plane." But Parekh is quick to point out that so-called "economy class syndrome" doesn't discriminate as to where you're sitting or how often a traveler flies, he said. "I've had patients who travel even in business class also develop DVT," he said. Plus "it can be the only flight of your life, if you decide not to move."

What increases the risks

Deep vein thrombosis typically forms in the legs. Symptoms include pain, swelling, changes in skin color, and a feeling of warmth, but some people have no signs at all, according to Mayo Clinic. Those who are obese, over the age of 60, take birth control pills or hormone replacement therapies, or who smoke have a higher risk of developing deep vein thrombosis, according to the medical center. But flying carries its own risks, said Parekh. "For one in every 5,000 flights that are flown, one patient ... can develop DVT just by the risk of air travel," he said.

Those who fly long-haul flights are at greater risk, but deep vein thrombosis can develop on shorter flights, he said. "People used to define long haul flights as eight hours," he said, "but there's some data to suggest that even four hours now is considered long enough." Alok Tapadia, a 52-year-old former businessman in the banking industry, said he developed deep vein thrombosis as a result of four-hour flight from Singapore to Hong Kong. He said he plays badminton three times a week, so he knew something was wrong when he became breathless while walking up a flight of stairs after arriving in Hong Kong. "One of the escalators had stopped working," he said, so he took the stairs. "I had to stop for a while, and I was wondering what's happened to me." It wasn't until he returned to Singapore, still breathless and with an elevated heart rate, that he finally went for a check-up. He said doctors told him a clot had traveled to the pulmonary artery connecting his heart and lungs.

This kind of an episode really shakes you up. Alok Tapadia

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