Passengers on long-haul flights are at a risk of developing DVT because they are often immobile for long periods, which can allow blood clots to form in the pelvic, leg or upper-extremity veins.
If that happens, the blood clot may break down naturally, however it poses a risk if the clot travels through the heart and into the lungs, where the result is potentially fatal.
DVT is also known as "economy class syndrome" because it occurs more often where movement is restricted.
Compression stockings and a pre-flight dose of asprin have both been suggested as offering some protection against DVT but according to Professor David Gradwell of the Centre for Human and Aerospace Physiological Sciences at King's College London, there is no conclusive evidence that either strategy protects otherwise healthy people from the formation of blood clots. Only travellers with a predisposition to blood clotting might benefit from either measure.
The most effective way to stop DVT from occurring is to maintain good circulation. Moving around the cabin and drinking plenty of water are crucial, and for economy passengers especially, seat choice is a contributing factor.
Passengers in an aisle seat are likely to move around the cabin more than those seated by the window, because it's easier for them to do so.