I've heard conflicting reports about the risks of blood clots forming during long flights.  My wife and I fly frequently...
D000401:Aircraft; D006495:Heparin, Low-Molecular-Weight / Q000627:therapeutic use; D006801:Humans; D009043:Motor Activity; D010100:Oxygen; D012307:Risk Factors; D013997:Time Factors; D014195:Travel; D020246:Venous Thrombosis / Q000209:etiology* / Q000517:prevention & control
D006495:Heparin, Low-Molecular-Weight; D010100:Oxygen