Those who smoke, women who are pregnant or taking the contraceptive pill or hormone replacement therapy (HRT), those who have recently had leg or pelvic region surgery and those who are dehydrated are all in extra danger of suffering a deep vein thrombosis (DVT) on a long-haul flight.

This happens when a blood clot forms in a deep vein in the leg, a more common occurrence the older you are, and can be fatal if a piece breaks away and reaches the heart or lungs. (embolism) A warning sign, not always given, is a painful swelling in the leg and at this point the clot will need to be treated, usually with anti-coagulant drugs and occasionally with surgery.

Travellers can reduce their risk by giving up smoking, by avoiding dehydration by drinking lots of water and not drinking alcohol, by not taking sleeping pills, by wearing compression stockings, walking around as much as possible and by flexing the lower legs every half hour or more. This is done while seated by keeping the toes on the floor and raising and lowering the heels ten times, and then keeping the heels on the floor and raising and lowering the toes ten times. Having plenty of leg-room to stretch legs can also help.

It could save your life .