The group of experts was charged with examining whether there was a link between air travel and venous thrombosis; identifying existing gaps in knowledge; and identifying priorities for further research.

Presentations made by some of the experts examined the possibilities of a link between air travel and venous thromboembolism (or deep vein thrombosis and pulmonary embolism). Each presentation used different methodologies. The results varied from no clear association to a strong relationship between air travel and venous thrombosis. The studies were very different in design and therefore were not strictly comparable because of variations in duration of flight or mixed passenger groups.

Presentations were made by Dr Paul Giangrande of Oxford Radcliffe Hospital, UK; Dr Emile Ferrari of Hôpital Pasteur, France; Dr Roderik A. Kraaijenhagen of University of Amsterdam, The Netherlands, and Mr John Scurr of Middlesex & University College Hospital, UK.

Thorough research of available literature published over the last 20 years was also presented indicating that the existing evidence consisted mainly of case reports with two clinical case control studies. Dr Patrick Kesteven of Freeman Hospital, UK, presented a view indicating that rather than outlining the "gaps" in knowledge, it was in fact difficult to obtain consensus on any points.

Based on the weight of the evidence reviewed, the summary position of the experts was that a link probably exists between air travel and venous thrombosis. Such a link is likely to be small, and mainly affects passengers with additional risk factors for venous thromboembolism. Such additional risk factors include, among other factors, history of venous thrombosis, obesity, women on oral contraceptives or hormone replacement therapy, people who have had recent surgery or trauma, cancer or genetic clotting abnormalities.

Similar associations may exist for other forms of travel. The experts said that the available evidence does not permit an estimation of actual risk. There are insufficient scientific data on which to make specific recommendations, they said, except for leg exercises during travel. In particular, in view of the recognised side effects, the experts said that the indiscriminate use of pharmacological agents cannot be recommended.

Representatives of the airline industry were also asked for their views. They said that there probably exists an association between venous thrombosis and travel in general but there is insufficient data on which to make recommendations. Consequently, airline companies and the International Air Transport Association (IATA) said they were committed to support further study into the matter.

Based on their discussion, the group of experts identified priorities for further research, which included:

1. A set of multi-centre international epidemiological studies including a large prospective cohort study examining hard clinical end points to answer the questions: Is there a link and if so what is the absolute risk? What is the size of the problem? This cohort study will also provide clues to other causative factors.

2. Special studies seeking intermediate end points in groups of volunteers examining isolated independent environmental risk factors such as cabin air pressure, cabin oxygenation and behavioural risk factors such as excessive use of alcohol or lack of exercise.

3. An interventional study involving passengers prospectively using objective diagnostic methods and examining various interventional modes.

It was the unanimous view of the group that these studies should be undertaken as soon as possible under the auspices of WHO and the International Civil Aviation Organisation (ICAO), supported by an independent scientific committee and in collaboration with IATA and airline companies.

Finally, the issue of common sense measures taken to increase the comfort of airline passengers was raised. They included reducing the intake of alcohol, adequate hydration, wearing loose clothing, or performing exercises while seated. Many airlines are implementing such measures and the group concluded that while there is little scientific basis for such measures at present, they support these common sense measures taken in the interest of passenger comfort.