Deep Vein Thrombosis in Air Travellers
by Maurice McKeown, BDS, PhD
(our New Zealand correspondent)
The problem of deep vein thrombosis (DVT) after airline travel rises to public consciousness from time to time as
the popular press report stories about healthy young travellers succumbing to the condition, or provide details of
high profile cases going to court.
What is deep vein thrombosis?
In deep vein thrombosis a blood clot forms in one of the veins deep inside the leg. It can occur at any age but is
much more common in older people. It can be caused by a wide variety of factors other than air travel. The clot
may break away from its position and travel through the heart to the lung where it may cause severe symptoms
which can result in death.
What are the risks of developing DVT after air travel?
Are we all becoming more and more unjustifiably fearful in a stressful world? My own perception is that the
average airline passenger is much more concerned that the plane will crash than the possibility that they could
develop a life-threatening medical condition. Yet the chances of the former are miniscule compared to the latter.
The airlines have done little to help. Many have now put general flight-health advice on their websites and
placed information pamphlets on in-flight health in the pocket in front of our seats, the ones with the exercise
advice that almost never mention the words 'deep vein thrombosis'. One suspects that their motive is to forestall
litigation rather than help their customers avoid the condition. The airlines are clearly in denial but a growing
body of research is pointing to a major health problem.
There are a number of important questions to ask ourselves. How common is the problem? How susceptible are
we to developing the condition and what can be done to minimise the risk?
We now have a rough estimate of the frequency of DVT and some indication of which groups of individuals are
in a higher risk category. Unfortunately, the categories of individuals who are at greater risk are quite long. One
is left with the impression that the majority of travellers on any particular flight fall into a high-risk category. High-
risk groups include older people (probably those over 60 years), travellers with a variety of known medical
conditions, pregnant women, those who have recently had surgery and women taking oral contraceptives or
hormone replacement therapy.
The risk of developing DVT, which can of course occur quite independently from air travel, is very strongly linked
to age. It is very uncommon in young people and very common in the elderly. If we consider air travel in
isolation, studies have shown that 3-5% of travellers develop clots in veins. Some are, of course, asymptomatic
or occur with mild symptoms. Thus a precise statistical analysis of the incidence of the condition is very difficult.
In other words the traveller is unaware that they have suffered a clot. In 2001 The Lancet published an
analysis estimating that 1 million cases of DVT related to air travel occur in the US every year and that 100,000
of these cases result in death (Lancet, September 8, 2001, p. 838).
However imprecise these figures may prove to be; they do suggest that the risks posed by air travel may be
dramatically greater than commonly perceived. One analysis has concluded that frequent business travellers
have a 5% risk of contracting DVT in any one year; data which ought to bring the matter to the attention of their
companies' medical department, not to mention their health insurers.
What can be done to minimise risk?
The popular press have dubbed the condition – "economy class syndrome", in the belief that the cramped
seating arrangements, particularly with respect to legroom, are the prime cause. But is this really true? Recent
analysis of the frequency of the condition suggests that it is equally prevalent in all classes of passengers and
even aircrew. (The possible exception being cabin attendants who move around all the time - if on duty.) It
seems that cramped conditions may not be the only precipitating cause. It appears more likely that lack of
movement is much more important. On a recent round trip from New Zealand to Europe I formed the distinct
impression that diligent exercisers were in a very distinct minority on board my flight.
People at risk
It is best to consider risk in the context of those who are apparently healthy and in those who have a known
medical problem. If the healthy individual seeks advice from their doctor they may be told that they have little to
fear, particularly if they are young and healthy. They may be advised that if they are really concerned they
should take plenty of in-flight exercise, drink lots of water and even take an aspirin before takeoff.
The exercise advice appears to be excellent and it should of course include a warning not to go to sleep!
(Difficult on flights of 8 -14 hours or more.) Perhaps the doctor might prescribe an amphetamine!! Advise on
regularly drinking of plain water may be misplaced.
Is it of value to drink large amounts of water?
There appears to be no evidence that it is. In fact a study in Japan by Hamada et al, published in the Journal
of the American Medical Association found that subjects who drank one cup of water per hour during a nine-
hour flight experienced increased blood viscosity. Interestingly the study found that those who drank an
electrolyte fluid (similar to a good quality sports drink) in the same manner, had no increase in blood viscosity
and no increase in urinary output. Hamada used an electrolyte drink containing 110 mg (per 8 oz cup) of sodium
and 30 mg of potassium (JAMA, February 20, 2002, pp. 844-45).
Advice is also commonly given to avoid caffeinated beverages because of their diuretic effect. Despite the fact
that research has shown that coffee and other caffeinated beverages do not increase dehydration. (Armstrong:
International Journal of Sport Nutrition and Exercise Metabolism June 2002)
Travellers are also advised to avoid alcohol because of its dehydrating effects. Yet red wine consumption is not
particularly diuretic and is known to be beneficial to the health of blood vessels and has the effect, in moderation
at least, of reducing the stickiness of blood platelets. A Polish research group last year found that the resveratrol
present in the human diet (red wine carries significant amounts) may be an important compound responsible for
the reduction of platelet adhesion and changed reactivity of blood platelets in the inflammatory process. (Olas et.
al. Thrombosis Research 15 August 2002)
It remains to be determined what effect a moderate amount of red wine might have for the flying public. It is an
impending research study which ought to have no lack of volunteers.
Does aspirin prevent blood clots?
I have been unable to unearth any definitive research that says it does. Medical opinion is divided. There is no
doubt that it confers certain benefits on the arterial side of the cardiovascular system since it reduces the
incidence of first heart attacks if taken daily for long periods. It should be pointed out that the arterial side of the
cardiovascular system can also be affected by thrombosis, although to a much lesser extent than the venous
part of the system. Arterial clotting has been closely linked to platelet adhesion. Aspirin and other natural
substances can help reduce this. Some medical researchers feel that aspirin's beneficial influence in the venous
system is minimal. A study of 300 high-risk passengers recorded a 4.8% incidence in the control group and a
3.6% incidence in those taking aspirin. Clearly aspirin had some value in this instance. (Belcaro et al:
Angiology Vol 230, 2002).
Are only long haul passengers at risk?
A study by the UK-based Aviation Health Institute found that 17% of flight-related DVT cases occurred in
association with short flights. It has also been demonstrated that the duration of travel is not linked to the
severity of the thrombosis suffered. (Parsi et. al. Australian and New Zealand Journal of Phlebology June
The British Independent newspaper has just published interim findings on the incidence of DVT in high-risk
passengers as a result of a short-haul flight (London-Rome), which is of less than 3 hours duration. The authors
released the preliminary results prior to journal publication because of their potential significance. The authors
found that 4.3% of 568 passengers developed clots, which were detected by ultra sound. Two of the victims
went on to suffer a pulmonary embolism. The lead researcher Professor Gianni Belcaro, of G d'Annunzio
University in Italy said that their research suggested that most blood clots develop in the first two to three hours
of a journey and grow larger and more dangerous with time. Unfortunately, we shall have to wait for the
completion of the project and publication of the final report in order to find out full details, such as who was
deemed to be at high risk.
What groups of healthy people are at risk?
It is now clear that aircraft cabin altitude, determined by the pressure within, appears to be the key-precipitating
factor for an increased risk of blood clotting. Altitude, not immobility, is the primary problem. The information
below suggests that we are all at risk. The degree of risk is determined primarily by our own
A Norwegian study published by Bendz et al in The Lancet put 20 young men in a hypobaric chamber,
which simulated usual aircraft cabin altitude.
Cabin pressures simulated an altitude of 5000-8000 feet in various aircraft types; reducing oxygen pressure from
98 to 79 mmHg as calculated for a Boeing 747. It has also been calculated that this can lead to 90% saturation
of haemoglobin with oxygen; a figure that may be reduced even further by sleep and the effects of cramped
conditions on respiratory mobility. Other environmental factors, notably humidity levels are also being
investigated. Cabin humidity falls rapidly after take off. Its potential effect on factors like dehydration is currently
controversial. In individuals with other respiratory problems, reduced oxygen saturation can lead to a chain of
events in their blood which favours clotting.
It was found in the Norwegian study that a substantial hour-by-hour increase in blood clotting factors occurred in
all of the healthy subjects. There was a 2-8 fold increase in clotting factors. The implication is that all flyers are
subject to this increased risk, suggesting that those who succumbed to DVT have a variety of risk factors
deriving from their own genetic and physiological make-up, and their environmental circumstances (Lancet,
November 11, 2000, pp. 1657-58).
Enhanced likelihood of coagulation has also been demonstrated by Wolfgang Schobersberger et. al. in a study
measuring coagulation factors on an actual long-haul flight. The effects were observed in all test subjects. He
concluded, "Long-haul flights induce a certain activation of the coagulation system. This activated coagulation
could be a risk factor for VTE during long-haul flights mainly when other risk factors are present."
(Thrombosis Research October 2002).
It has been established that people carrying the Factor V Leiden variant are much more likely to suffer DVT than
those without the variation. Caucasians populations can have a 5% incidence of the gene variant. The mutation
does not appear to be present in Black or Asian populations, although it is present to a limited extent in Afro-
Americans. Factor V Leiden increases the risk of venous thrombosis 3-8 fold for heterozygous (one bad gene
inherited) and by 30-140 fold, for homozygous individuals (two bad genes inherited). Risk is dramatically
increased beyond that if the individual is also suffering high blood homocysteine levels. The Wellman clinic in
London (UK) has developed a series of tests to identify the Factor V Leiden variant and a number of other
genetic variants related to increased clotting risk. It is estimated that the Factor V Leiden variation is responsible
for 40% of all cases of thrombosis. Interestingly, it increases the risk of DVT for men by 8-fold and in women by
80-fold. Women therefore appear to be at much greater risk.
Women taking oral contraceptives are also much more vulnerable to DVT, although the risk for those on
estrogen replacement therapy is higher because they are older and face a greater base-line risk. Women who
have recently given birth are also at increased risk. There are those who believe that pregnant women are at
such increased risk that they should not fly at all, since preventative anticoagulant therapy can have serious
consequences for the foetus. If they do fly it is advisable that they scrupulously follow the preventative advise at
the end of this article.
Conventional wisdom says that the young, fit and healthy have little to be concerned about. Unfortunately, this
may not be the case. In fact they may be at much greater risk than the young and unfit! This applies particularly
to athletes, especially endurance athletes, who show a high incidence of the condition according to some
researchers. This may be partly due to their very efficient cardiovascular systems pumping blood around more
It has recently been reported that at least two international soccer teams wear compression stockings on long
haul flights, as a number of studies have shown that wearers of compression stockings are dramatically less
susceptible to DVT than those not wearing the hose. One study has shown a complete elimination of the risk in
stocking wearers (Lancet, May 12, 2001, pp. 1485-88). It has been claimed that 85% of flight DVT victims
fall into the athletic category.
Airhealth.org, an organization dedicated to the dissemination of information concerning DVT estimates that
100,000 deaths due to air travel related DVT occur in the US every year (Lancet, September 8, 2001, pp.
838). This would make the condition the 5th leading cause of death overall after heart disease, cancer,
stroke, and respiratory disease.
According to the US National Center for Health Statistics at the Center for Disease Control and Prevention the
following four are the most common causes of death in the 20 to 44 year age group (National Vital Statistics
Report, Vol. 49, No. 11, Deaths: Leading Causes for 1999):
- Accidents - 34,540
- Cancer - 21,404
- Heart disease - 16,767
- Suicide - 13,727
Air travel related DVT, however, may well be the most important cause of death among people in the 20 to 44
year age group. Airhealth.org reports that 47% of the victims in their registry were in this age group
corresponding to 47,000 deaths out of the total 100,000 deaths.
A substantial number of people, commonly in older age groups, have a variety of medical problems which
require special attention e.g. those who have recently had surgery*, those already taking anticoagulant
medication, people with cancer, heart disease or diabetes or a family history of DVT. They all need medical
advice about preventative measures, which might include anti-coagulant therapy.
* Some authorities recommend that patients who have undergone surgery – particularly orthopaedic surgery,
should not fly for 90 days after their operations.
What are the symptoms of DVT?
It should be pointed out that symptoms may arise many days after the end of a flight. One symposium concluded
that a two-week post-flight risk period is likely. You may not therefore immediately associate any ill effects with
your trip. Most patients report symptoms within one week of the end of their trip however.
- Sudden swelling in one lower leg. The left is much more commonly affected. (A little swelling in both legs is
very common in flight.)
- Cramp or tenderness in one lower leg. (Healthy athletes may mistake this for cramp)
- A bruise or swelling behind the knee.
- Shortness of breath
- Rapid breathing
- Cramp in your side, painful breathing
- Chest pain, sometimes accompanied by shoulder pain
- Coughing up blood
- Fainting (often the first sign, especially in older people)
How can a diagnosis be made?
Leg clots can be readily diagnosed with ultrasound. A clot, which has moved to the lung, is more difficult to
diagnose. A measure of your blood oxygen concentration usually taken with a simple attachment to your finger,
can help. If it is low for no obvious reason further testing is required. It is sobering to note that one US research
study has shown that 50% of DVT cases have no symptoms and 50% of those will progress to pulmonary
embolism. In the study forty seven percent of fatal pulmonary embolisms were not diagnosed before death.
(Zamula: FDA Consumer Nov. 1989).
If you have long legs you may want to choose an airline with the largest seat pitch possible. (The seat pitch is
the horizontal distance between similar points on two seats situated one behind the other.) Comparative seat
pitch information for major airlines is available on the Internet on various websites.
It is probable, but not proven, that DVT is much more likely to occur in people with specific risk factors, notably
inherited genetic clotting abnormalities. It has been estimated that up to one third of the population have some
degree of thrombophelia – an enhanced tendency to form blood clots. Since most of us are unaware of these
latent tendencies it would seem prudent to take all reasonable precautions. Little is usually said about nutritional
supplements but there is good theoretical reason to believe that they may help.
Likely helpful substances are:
Vitamin E (400-800 IU) daily for some days on either side of the trip. Consult your doctor if already
taking anti-coagulant medication.
Pycnogenol or Grape Seed Extract Both contain the active bioflavonoid called proanthocyanidins
(OPC's), which helps strengthen capillaries, and enhances blood flow.
Vitamin B6 (in the form of pyridoxine hydrochloride), which has been shown to reduce platelet
Pinokinase This is a new commercially available pill, which has recently become available. It is an
oral pro-fibrinolytic anticoagulant. It is made from fermented extracts of pine bark and soybeans. A study just
published in Angiology journal shows that it was 100% effective in preventing clots.
A small amount of aspirin (junior size) may help, if tolerated. People eating diets very high in fruit and
vegetables may have as much salicylates in their blood as that provided by a pill according to a study of
Buddhist monks in Scotland. (Blacklock et.al. J of Clinical Pathology 2001 Vol 54)
Resveratrol Available in the form of supplement capsules for those who do not drink red wine.
Other nutrients of possible value are ginkgo biloba, niacin, vitamin C, and vitamin B12.
Other important measures
- Wear properly fitting flight hose designed to be tight around the ankle, pressure gradually reducing
further up the calf. This is the single most effective preventive measure.
- Flight stockings differ from ordinary hose. There have special elastic threads that are utilized to apply a
precise amount of pressure to specific areas of the leg. They are available in a number of different strengths
that are prescribed for a variety of medical conditions. Compression stockings exert maximum pressure at
the ankles and gradually reduce that pressure up the length of the stocking. The pressure exerted at the
ankle reduces by approximately 50% at the top of the stocking. This compression is scientifically designed to
force blood from the secondary venous system near the skin into the primary deep vein system. The value of
the stocking has been demonstrated by a study of flyers who wore flight hose on only one leg. This resulted
in negligible swelling in the stocking clad leg, while the leg without the stocking experienced significant
increases in volume. Thus flight hose can also relieve simple swelling of the lower extremity.
- If you have varicose veins it may be necessary to wear stockings above the knee. Please seek
- Do leg flexing exercises while seated as studies have demonstrated that a variety of foot flexion
movements result in substantial increases in blood flow in the leg.
- One authority recommends flexing of foot and calf muscles, while seated, for two minutes every half
hour. If you are an avid armchair exerciser you might like to visit the Qantas airlines website. In the in-flight
health section you can look at pictures of recommended exercises which ought not to injure the passenger
seated next to you!
- Avoid sleeping if at all possible. If your jet lag is worse as a result try melatonin. (A prescription drug in
many countries) It works miracles for me!
- Keep hydrated but do not drink pure water. Drink an electrolyte-balanced solution, available in a good
quality sports drink. You may have to take some powder on board and add it to the water provided, if your
baggage limit is in jeopardy.
- Do not drink large amounts of dehydrating beverages.
- Wear loose comfortable clothes.
- Take suggested supplements before, during and after your trip.
- If you are in a high risk category talk to your doctor and above all familiarise yourself with the symptoms
of DVT, the diagnostic procedures necessary and possibly learn a little about what treatments might be
- If the worst happens and you suspect that you have a clot seek immediate medical attention.
NOTE: Flight hose (socks) specifically designed for air travel are available at major airports and pharmacies.