They may stay in the legs during the flight and cause no symptoms at the time. But hours later, when the person is more active and there is more blood circulating through the legs, these clots can be swept up into other parts of the body. If one of these clots lodges in a vital organ, it can result in a medical emergency. If it goes to the lung, for instance, it can cause a pulmonary embolism, which can be fatal. Nobody knows how many people suffer from such blood clots after flying, but aviation medical experts have long called for more warnings to passengers about the dangers.
Some airlines suggest exercises during long flights to reduce the risk.
Airline officials say passengers need to understand that flying creates unique physical stresses. The organization has a tipsheet for travelers that includes exercises to improve circulation in the legs during flights. Airlines around the world are stepping up their warnings. British Airways said Tuesday that it will now give a written warning to every passenger who buys an overseas ticket. And Japan Airlines is studying the health risk more closely and considering new ways to help passengers protect themselves.