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.

He said his first scan showed an enlarged heart, which occurs when the heart is pumping too hard. A second scan resulted in Tapadia being immediately admitted to the hospital’s intensive care unit, he said.

The doctors found that he had been “massively” short on oxygen and his lungs were completely blocked, he said.

Doctors said the situation was “critical, because there was so much pressure on the heart that it could have, you know, gone into an arrest or gone into a more critical stage any moment,” Tapadia said.

Blood thinners didn’t dissolve the clot fast enough, he said.

So his doctors eventually resorted to using cardiac catheterization, which involved threading a catheter through his blood vessels near his heart to dissolve the blood clot from the inside, Tapadia said.