Heading off to Gatwick Airport for a holiday is very exciting and, for some people, the further they travel the better.

The growth of long-distance air travel means many hours of relative immobility and possible cramped seating, which some health experts believe can help trigger off deep vein thrombosis (DVT).

The Aviation Health Institute (AHI), a medical charity dedicated to keeping airline passengers safe, has found that out of 544 cases of DVT reported by British passengers since 1999, 85 were fatal.

The charity has advice and information for passengers aimed at reducing the risk of developing a clot.

DVT occurs as a result of blood thickening and clotting in the veins. The clot normally dissolves and disappears without serious complications but, if it travels to the heart, lungs or brain, it can be fatal.

The main symptom is aching or swollen legs but, in some cases, there may not be any symptoms until a blockage occurs, causing pain in the arms and chest, plus breathlessness.

DVT can sometimes take two weeks to develop into a potentially fatal problem.

Although conditions on a plane are believed to increase the risk of DVT, other factors which affect certain groups of people are also linked to the condition.

These include women on a contraceptive pill or on hormone replacement therapy, people who smoke, have varicose veins, have a blood disorder or are dehydrated.

Those who have had major surgery or a period of immobilisation should not take a flight longer than three hours for up to 90 days.

Anyone who is uncertain should contact their GP before flying.

One of the most effective ways to reduce the risk of DVT is to take aspirin before a flight. The drug thins the blood, reducing the risk of a clot.

The AHI advises taking about half a tablet on the morning of the flight, then one tablet daily for the following three days. This should lower the risk of DVT by one third.

The recycled and airconditioned atmosphere in a plane can cause dehydration, which makes blood thicken, so people are advised to drink as much water as possible.

Having a glass of water once an hour while flying will "dilute" the blood, improving circulation.

Bodies can't store water so people should drink regularly, especially those returning from a hot country who may already be dehydrated.

Alcohol should be limited to one or two drinks during a flight and experts recommend people avoid drinking too much for a couple of days before a long flight.

People are also advised to wear loose clothes and loosen shoelaces during a flight to help blood circulate to the lower limbs.

The Cardiac Patients Association recommends passengers get out of their seat every 30 minutes and walk up and down the aisle a couple of times to get the heart pumping and the circulation going.

Don't sit with your legs crossed on a flight because this can restrict the flow of blood. Exercising in the seat will help to increase circulation.

The AHI has a series of simple exercises which it recommends are done every half-hour for a couple of minutes: Move your feet up and down to simulate walking, open and clench your fists, rotate your ankle joints in large circles, fully stretch your arms up and down and reach up to the panel above your head.

If someone develops pains, swelling or redness in their lower legs during or after a flight, they should get medical advice immediately.

Chichester-based GP David Smith said: "If people take all the necessary precautions they should have no problems.

"Some people are more susceptible than others but everyone is at risk, so it is important people are aware of DVT and what to do."

British Airways and Virgin both have a leaflet and a video on board about how to minimise the risk of DVT.

For a leaflet on healthy flying from the Aviation Health Institute, write to: 17c Between Towns Road, Oxford OX4 3LX.