Are you fit to go the distance? People who undertake long journeys by car, bus, train and boat - as well as on flights - can be affected by "economy-class syndrome", research shows.

Vascular surgeon George Geroulakos at Imperial College examined 810 patients with leg pain and swelling: 101 were found to have blood clots. Of these, 15 had travelled recently - nine by air and six who had undertaken long journeys by other transport.

The good news is that a new blood test has been developed to identify those whose blood has a predisposition to clot - thrombophilia. Currently the test, available on the NHS, is only requested for those who have had a deep vein thrombosis (DVT) or whose relatives have developed it. It is not routinely carried out on everyone going on a long-haul flight because of the expense. However, the test is available privately at several London clinics and costs from £275 to £543. British Airways travel clinics do not offer the service. A spokesman says: "It is an issue for a traveller's GP and the NHS. If someone is concerned about their fitness to fly, they should discuss this with their doctor."

The clinics no longer stock elasticated stockings and exercise audiotapes "because it is a question of space. The tapes are available on board, along with advice on best practice in the inflight magazine."

The items were withdrawn after a high-court ruling in December that victims of DVT could not sue airlines for causing their condition.

Meanwhile, the all-party parliamentary group looking into DVT is pressing for the regulations governing aircraft seating to be changed so that all new aircraft will have seats with a pitch of at least 34 inches, giving passengers more leg-room. The airlines' old argument that if you want more leg-room you should pay more and travel in business or first class no longer holds sway. It is like telling a car driver he will only be safe on the roads if he buys a Rolls Royce.

It is difficult to get a true figure of flight-related DVT deaths because victims do not all drop dead in the arrivals hall, but die some days later, so the link is not always apparent and DVT is not recorded on the death certificate.

One in every 100 people who develops DVT dies. The cause of death is usually pulmonary embolism, where a blood clot travels from the legs to the lungs and blocks an artery. Geroulakos points out that there does not have to be a postmortem after every death in Britain, and so more deaths could be caused by DVT. Others, where there has been a history of heart disease, may be attributed to a heart attack.

He says: "What I can tell you is that a report of 61 deaths occurring on arrival at Heathrow over a three-year period showed that 11 were from pulmonary embolism, which is a very high incidence. The problem can arise from long trips by car, train or bus, but in a plane other factors kick in to make matters worse."

Geroulakos believes that in the low-pressure environment of airliners the factors that activate blood clotting increase two to eightfold. The consumption of alcohol, coffee and salty snacks, coupled with the dry air and the inadequate intake of hydrating fluids, thickens the blood and increases its tendency to clot.

According to John Scurr, vascular surgeon at University College Hospital, London, who carried out the first clinical study into DVT in 1998, 10% of the population have an inherited tendency for their blood to clot anyway. "The body has to keep a delicate balance between letting the blood flow as liquid or allowing it to clot," he says. "For most people, flying is a very safe way to travel. The secret is for us to identify those at risk. The old and immobile with heart and lung problems are vulnerable, but with the right advice and preventative measures they can fly, too.

"In most cases preventative measures means wearing fitted elastic stockings. It is important to be measured for them, otherwise they may not be effective. Travellers should measure their ankle circumference in the morning and use this along with their height measurement when ordering their stockings, which are available online or from a chemist."

Scurr, who is also based at the Lister Hospital, is a member of the World Health Organisation's scientific committee looking into DVT. He has been carrying out research into blood clots in the legs for 25 years. It was when a pop star, an airline president and a senior politician all went to see him on the same day in 1992 with a blood clot after flying across the Atlantic that he began campaigning for funding to carry out the first clinical study into flying and DVT.

Geroulakos finds the airlines' attitude to DVT is starting to change thanks to pressure from the media. In the past they have argued that immobility is not specific to flying, but where else, he asks, do you sit for 18 hours at a time?

"Three years ago when a passenger developed a pulmonary embolism after he had left the plane, [the airlines'] attitude was that it was nothing to do with them. Now they can see that its development is linked to long-distance travel and are taking the problem more seriously, and starting to give passengers advice on exercises to do in their seats.

"The airlines have to take an interest in what their customers want and keep them happy because they don't want to end up in a situation where they have to pay out compensation."

Geroulakos says: "This shows economy-class syndrome can affect anyone on long journeys, and that a significant percentage of blood clots are associated with it. It is vital more research is carried out to highlight the full extent of the problem and to identify those at risk."

Now, he is trying to raise £200,000 to conduct a larger study that will involve checking passengers before and after flights. Sadly, due to a lack of funding a large-scale study by Who of 200,000 frequent fliers (costing £8.5m) has had to be shelved. But a smaller study looking at the incidence of DVT among frequent fliers is going ahead at Leiden University in the Netherlands, while researchers at Amsterdam University are looking at the effects of altitude on blood clotting.

The lack of funding also means Scurr's rerun of his first trial has had to be postponed for 18 months. Not surprisingly, he feels that the vast sums of money being spent going through the courts in a bid to seek compensation from the airlines would be better spent on research into what can be done to help prevent DVT.

Labour MP John Smith, the all-party chairman, would like all airlines to give out free compression stockings and allow passengers enough leg-room. He is calling on other countries to follow the UK's lead - it has contributed £1.2m - and help fund Who's proposed large-scale study.

Smith's interest in DVT was sparked off when one of his constituents, a 30-year-old policeman who had just passed his police medical, died as a result of it on his return from his honeymoon in Hawaii.

He says: "Without the large-scale study we will not be able to identify if there is a link between flying and DVT, and so I am backing Who in its campaign to get other countries to join the UK in funding the research.

"Ideally, after they have gone through the instructions of how to inflate your life jacket I would like airline stewards to advise passengers on how to exercise in order to reduce the risk of DVT. I would also like airlines to give out free compression stockings and give their passengers enough leg-room, with seats with a pitch of at least 36 inches."

Research by John Scurr shows that wearing elastic stockings that squeeze the leg and promote blood flow prevents clots from developing in the deep veins of the leg. In a study of 100 long-haul passengers who wore such stockings, Scurr was unable to detect any clots in their deep veins. But 12 clots were found in the parallel group not wearing stockings.

Four passengers who wore stockings complained of pain as a result of their stockings rubbing on their varicose veins causing inflammation.

He says: "If you have bad varicose veins, and cannot wear these kind of stockings, then you should have your veins sorted out before you travel."

Scurr also advises long-haul flyers to exercise their feet using an airogym or airwalker, because research has shown these to be effective.

Those at risk include anyone with a family history of thrombosis; cancer patients; women on the Pill or HRT; anyone overweight; smokers; a nyone with coronary artery disease; pregnant women; the over-60s; anyone who has undergone recent surgery, especially on the hips or knees, and those with Leiden Factor V - an inherited blood-clotting tendency.

People at risk should discuss their travel plans with the GP, who may recommend wearing support stock ings and give them an Heparin injection as additional protection.

Geroulakos says: "It is important for people to get individual advice. It is wrong to tell everyone to take aspirin, for example, because for some it can have risks."

You can reduce risk by moving around at least once every two hours; taking a bottle of mineral water on board and avoiding alcoholic drinks and by getting yourself some compression stockings and an airwalker.

What causes the clots?

DVT is a result of immobility and was first reported during the second world war among people who sat in cramped conditions for long periods in air-raid shelters. The blood pools at the bottom of the legs and then clots. A clot will then attach itself to the wall of a vein. If you are unlucky, when you do start to move around the clot will break off and travel up to your lungs, resulting in pulmonary embolism, or, if you have a hole in the heart it will pass through to the brain causing a stroke. According to the Office for National Statistics, 7,207 people in the UK died last year after developing DVT. John Smith, Labour MP for the Vale of Glamorgan and chairman of the all-party parliamentary group looking into DVT, says: "We believe that as many as one-fifth of these deaths could be flight-related."

· For more information see www.airwalker.tv. See www.mediuk.co.uk for elastic stockings.