A recent study from Paris show that air travel over 5000 km increase risks of deep vein thrombosis, DVT, in the legs. DVT may be dangerous because the blood clot can loosen and obstruct blood vessels in the heart and lungs.
Prevention of DVT
Drink abundantly (not alcohol).
Take a walk in the aisle at least with a few hours interval.
Do exercises with your legs by contracting and relaxing the lower leg muscles 5 to 10 times each hour. Previous DVT and compromised circulation increase the risk for DVT during air travel, which makes prevention even more important.