It has been called "economy-class syndrome," but that's a misnomer as even people who fly first-class have been killed by it. A better name is "travellers' thrombosis," although the correct name is deep-vein thrombosis or DVT.

It's caused by sitting rigidly for too long – usually on lengthy intercontinental flights – so that a blood clot forms and makes a lethal journey to the lungs or heart. It was first noted more than 50 years ago in London, when people crowded into air-raid shelters to escape bomb attacks during the Second World War.

It became front-page news in September 2000 after the death of Emma Christoffersen, a 28-year-old Briton who died after returning to London from Australia, where she had attended the Olympic Games. Christoffersen collapsed at Heathrow airport following the 19,000-kilometre, 20-hour trip. She died before she reached hospital. She had developed a clot in her leg on the Qantas flight; the clot dislodged and made its way to her heart.

Estimates of the number of deaths from travellers' thrombosis vary widely. A conservative estimate is about 100 a year, though David Derbyshire, medical writer for The Daily Telegraph, says that doctors who carried out a study at Ashford Hospital in Surrey believe more than 2,000 people a year die from the condition in Britain. What made Christoffersen's case more noteworthy was that she was young, in good health and about to be married.

Travellers' thrombosis is regarded as such a new condition that it has not properly been tracked and many deaths may not be recorded as caused by long-haul flights. The Science and Technology Committee of Britain's House of Lords issued a report demanding that the air travel industry do "urgent" research into deep-vein thrombosis among fliers. (The committee also recommended an end to the phrase "economy-class syndrome" on the basis that it is not accurate.)

Blood clots, thrombosis and embolisms are not new, but there appears to be a sharp increase in their occurrence among long-haul air travellers during the past 10 years. Many speculate one of the contributing factors is the decrease in legroom on passenger planes.

To cite an example, British Airways has decreased the distance between seats from 91 centimetres to 79. Seats on Qantas flights are about 81 cm apart. For Air Canada, the seats are spaced about 83 cm apart. The distance between seats is not identical even on the same plane. Exit and bulkhead seats have much more legroom, making them safer for potential travellers' thrombosis victims.

A hospital study in England found that at least 30 people died in a three-year period in the late 1990s of massive blood clots after arriving at Heathrow airport on long-haul flights. An Australian surgeon has been quoted as saying 400 people a year landing at Sydney airport suffer blood clots.

The condition is not always fatal. A story in The New York Times in October 2000 estimated that five million Americans a year experience a blood-clotting thrombosis caused by prolonged immobility. The story said this results in some 800,000 hospitalizations a year. DuPont Pharmaceuticals Co. has responded to the situation by selling the blood-thinning drug Innopehn, designed to prevent and treat the condition.

Remedies suggested include taking Aspirin as a blood-thinner before long flights, maintaining liquids (but avoiding alcoholic beverages), wearing knee-length elastic stockings and getting up from the seat at regular intervals. Critics have also drawn attention to increasingly smaller, cramped seating arrangements in economy sections, especially on charter flights.

The problem has more to do with immobility than with cramped conditions on planes. Travellers' thrombosis has been known to affect passengers in buses, trains, cars, trucks – even theatres. Deadly blood clots also tend to be more of a risk to the elderly, smokers, pregnant women, diabetics, cancer patients and overweight people.

The risk is exacerbated when these people are confined in a cramped, immobile position for long periods. Something as innocent as a kick in the leg a week before a flight can add to the risk of an in-flight blood clot.

The World Health Organization launched a major study in the summer of 2005 aimed at determining how best to prevent travellers' thrombosis. It involves 30,000 travellers on long flights.

The study's participants will be divided into three groups. One third will be asked to use a special machine that keeps the calf muscles stimulated. A second group will be given special exercises to keep the blood flowing. The last group will do nothing. They will be monitored for eight weeks.

It's believed the risk of developing travellers' thrombosis is about one in 1,000 for people on long-haul flights. Results of the study are expected by the end of 2006.