The jury is still out and the airline industry is as keen to know the answer as anybody else

Dr Michael Bagshaw, British Airways

Department of Health statistics estimate DVT occurs in one in 2,000 people. One in every hundred people who develop DVT dies.

A possible link between DVT and long haul air travel was first suggested by reports in medical journals in the 1950s.

Dr Bagshaw, head of occupational and aviational medicine for BA told BBC News Online: "The jury is still out and the airline industry is as keen to know the answer as anybody else.

"The airline industry is not in denial about DVT, despite what has been suggested."

'Traveller's thrombosis'

Dr Bagshaw added he believed immobility was the main concern, whether people were sitting still on a plane, a coach or in the theatre.

He believes calling the condition economy class syndrome is inaccurate because there is evidence it has affected people in business or first class - or in people using other forms of long-distance transport.

He would like it to be renamed "traveller's thrombosis".

He said the evidence of the link between flight and DVT was conflicting. "On the one hand, a research paper or report that purports to show a definite link.

"Then that's countermanded by the next piece of research which shows there isn't such a strong link."

He pointed to two recent pieces of research. One, from France, showed people on long haul flights were at greater risk of developing fatal blood clots.

But a second piece of research looked at 14 fatal cases and found that in five there was evidence of pre-existing pulmonary-embolic disease, and in four the fatal clot had formed before the flight.

Dr Bagshaw will point to Swiss research published last year which found professional pilots were less likely to suffer DVT than the general population, and 16 out of 27 pilots who did had well recognised, pre-disposing factors.

It is hoped the debate will be resolved by research being carried out under the auspices of the World Health Organization (WHO) and the International Civil Aviation Organisation.

The study will examine whether the risk of blood clots is increased by flying, how high that risk could be, any other factors which might contribute and the effect of interventions.

Exercise

In the light of the existing concerns, BA developed a leg exercise regime, developed in the light of WHO advice.

Dr Gerard Panting, head of policy for the Medical Protection Society, told BBC News Online it was up to airlines to provide conclusive evidence to disprove the link.

"If the airline industry want to take issue with it, then it's up to them to come up with robust data."

Dr Panting recommended patients follow guidance published by the Department of Health on DVT and flying, which he said was "eminently sensible"

The guidance says people must consult their GP before their trip if they:

• Have ever had DVT or pulmonary embolism
• Have blood clots already
• Have a family history of clotting conditions or an inherited tendency to clot
• Have cancer, or have been treated for cancer
• Have undergone major surgery, a hip or knee replacement in the past three months
• Have ever suffered from a stroke