From Nature:
A study of the effects of low oxygen levels on ‘economy class syndrome’ has re-opened the debate over how long-haul flights increase passengers’ risks of deep vein thrombosis (DVT), the formation of potentially fatal blood clots.
Although it is well known that restricting blood flow by sitting immobile for long periods can increase the risk of such clots, some researchers have proposed that there may be other factors on flights that contribute to the risk. Studies comparing people on long-haul flights to those sitting still on the ground have indicated that there is a difference between the two groups, although it isn’t clear exactly why. Researchers have suggested that the explanation could lie with passenger stress, poor air quality, low humidity, low air pressure, or exposure to cosmic radiation.
More here.