The contraction of muscles is an important factor in helping to keep blood flowing through the veins, particularly in the legs. Prolonged immobility, especially when seated, can lead to pooling of blood in the legs, which in turn may cause swelling, stiffness and discomfort.

It is known that immobility is one of the factors that may lead to the development of a blood clot in a deep vein, known as deep vein thrombosis (DVT). Research has shown that DVT can occur as a result of prolonged immobility, for instance during long-distance travel, whether by car, bus, train or air. A WHO study showed that the risk of DVT increased 2- to 3-fold after long-haul flights of more than 4 hours and similar forms of travel.

In most cases of DVT, the body is able to gradually break down the clots and there are no long-term effects. Larger clots may cause symptoms such as swelling of the leg, tenderness, soreness and pain. Occasionally a piece of a clot may break off and travel with the bloodstream, to become lodged in the lungs. This is known as pulmonary embolism and may cause chest pain, shortness of breath and, in severe cases, sudden death. This can occur many hours or even days after the formation of the clot in the leg.

The risk of developing DVT when travelling is increased in the presence of other risk factors, including:

previous DVT or pulmonary embolism

history of DVT or pulmonary embolism in a close family member

use of oestrogen therapy – oral contraceptives (known as the pill) or hormone replacement therapy (HRT)

pregnancy

recent surgery or trauma, particularly to the abdomen, pelvic region or legs

cancer

obesity

some inherited blood-clotting abnormalities.

DVT occurs more commonly in older people. Some researchers have suggested that there may be a risk from smoking and from varicose veins.

It is advisable for people with one or more of these risk factors to seek specific medical advice from their doctor or a travel medicine clinic in good time before embarking on a flight of 4 or more hours.

WHO suggests moving around the cabin during long flights to reduce prolonged immobility, although this may not always be possible. A regular trip to the bathroom, for example every 2–3 hours, is a reasonable measure. Many airlines provide helpful advice on exercises that can be carried out in the seat during flight. Exercise of the calf muscles can stimulate the circulation, alleviate discomfort, fatigue and stiffness, and may reduce the risk of developing DVT. WHO also suggests that hand luggage should not be placed where it restricts movement of the legs and feet, and clothing should be loose and comfortable.

In view of the clear risk of significant side effects and absence of clear evidence of benefit, passengers are advised not to use aspirin specifically for the prevention of travel related DVT. Travellers at greatest risk of developing DVT may be prescribed specific treatments and should consult their doctor for further advice.