If you are a frequent long distance air traveller, you need to know about deep vein thrombosis (DVT) and take some precautions.

DVT is the formation of blood clots in the legs. Prolonged inactivity can be a factor and it has been linked to cramped conditions during air travel. The US government's Aviation Health Unit offers this advice:

• All of us should exercise the calf muscles regularly, drink enough water, but avoid alcohol and coffee.
  
• Minor risk (If you have any of these factors: age over 40; very tall, short or obese; history of leg swelling; recent minor surgery or extensive varicose veins.) Do not sleep for long periods or take sleeping pills.
  
• Moderate risk (Recent heart disease; pregnant; on the Pill or HRT family history of DVT.) Take a doctor's advice about travel. You may need special stockings known as "graduated compression stockings" to prevent DVT.
  
• High risk (Recent major surgery, stroke or leg injury, known clotting tendency.) Take medical advice. You may need blood-thinning medication.
  

And here's a video by NewYouLifestyle on DVT.

You can buy graduated compression stockings to prevent DVT here or here.

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• Tips for long distance flights
  
• Flying long distance with kids