A consultation of international experts, convened by
the World Health Organization (WHO) on 12-13 March 2001, reviewed the existing state of evidence on air
travel and venous thrombosis and concluded that further research is
needed into the matter.
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
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 H˘pital Pasteur,
France; Dr Roderik A. Kraaijenhagen of University of Amsterdam, The
Netherlands, and Mr John Scurr of Middlesex & University College
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
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
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.