An elevated risk of developing deep vein blood clots, known medically as deep venous thrombosis or DVT, has long been associated with air travel, but new guidelines from the American College of Chest Physicians (ACCP) conclude that there's no definitive evidence to support the idea that flying - specifically, flying economy class - increases the risk. DVT is a potentially life-threatening condition that occurs when blood clots form in deep veins, typically those in the legs. The danger is that these clots can detach and travel to the lungs (pulmonary embolism).
The new guidelines, published in the February 2012 issue of the journal Chest, debunk the existence of an "economy class syndrome" - the increased danger of developing DVTs while flying coach - and state that the very small risk of contracting DVTs while flying that does exist is no greater in economy class than it is in business class or first class. However, the panel of experts who wrote the guidelines warned that on flights of six hours or more, passengers sitting in window seats are at greater risk of DVT than those seated elsewhere, because in that location you're more likely to remain immobile for long stretches of time than you would if you were on the aisle.
The panel found no evidence to support the idea that dehydration or drinking alcohol increases the risk of DVT.
The authors emphasized that while DVTs are rare in passengers subjected to taking long flights, those that do occur usually affect individuals who have one or more risk factors and have been on flights longer than eight to 10 hours. Risk factors include previous DVT or pulmonary embolism, certain types of cancer, recent surgery or trauma, immobility, advanced age, taking birth control pills or other forms of estrogen, pregnancy, and obesity.
If you're at risk of DVT, the guidelines recommend getting up often on long flights to walk around and stretch your calf muscles. Book an aisle seat whenever possible so you can get up easily to move about the plane. Another option for those at risk is the use of below-knee compression stocking. (However, the guidelines specify that these stockings should not be used by anyone who is not at increased risk.)
All told, the average risk for a DVT is about one per 1,000 persons per year. That risk doubles for long flights (or sitting for long hours on buses, trains or driving a car). Unless you know for sure you're at risk, I wouldn't worry too much about developing a DVT during air travel. But do get up occasionally and walk the aisles.
Andrew Weil, M.D.