Select Committee on Science and Technology Fifth Report


6.1 Four main medical concerns have arisen from our Inquiry. Of these, we and many of those who submitted evidence consider the most significant to be deep vein (or venous) thrombosis (DVT). This is discussed in this Chapter[77], together with associated issues of seating, comfort and stress. The three other main medical concerns (the transmission of infection, the effects of the cabin environment on vulnerable individuals and the handling of in-flight medical emergencies) are considered in Chapter 7.

Deep vein thrombosis


6.2 Blood provides the principal means of transporting metabolic necessities round the body and collecting wastes. Considering for simplicity only the respiratory cycle mentioned in paragraphs 4.2ff, oxygen-rich blood from the lungs is pumped by the heart through a network of arteries. When the blood reaches the organ or tissue served by a particular artery, it is diffused through a mass of small blood vessels ("capillaries") through which the blood supplies oxygen to the tissues and carries away the waste carbon dioxide. Through a reverse network of veins, the blood then returns to the heart and lungs for re-oxygenation and elimination of carbon dioxide.

6.3 The pressure at which the heart pumps blood through the arteries is substantially dissipated by the capillaries, making the return through the veins less vigorous. The heart is located in the chest, and blood thus has a long way to return from the feet - from which the flow is also hampered by the effects of gravity. To assist that return blood flow, the main leg veins are deep inside the leg muscles and contain a series of non-return valves enabling muscle action to augment the pumping action of the heart.


6.4 DVT is a condition in which a small blood clot or thrombus (thrombi in the plural) forms mainly in the deep veins of the legs[78]. Such clots can be present without symptoms or signs, but may give rise to swelling of the affected leg, sometimes accompanied by pain (particularly when the foot is flexed hard upwards) and local tenderness. Such swelling is not to be confused with the commonly experienced swelling of both lower legs during and after a long flight, which is due to inactivity and soon disappears after leaving the aircraft.

6.5 DVT is not dangerous in itself but complications arising from it may occasionally be life-threatening. Complications occur when a thrombus breaks away from the wall of the vein to which it is attached and is carried along with the flow of the blood as what is termed an embolus. If the embolus reaches a blood vessel through which it cannot pass, it blocks the vessel. The consequence of such a blockage is called an embolism. The most serious of these occurs in the lungs (a pulmonary embolism) giving rise to chest pain and breathing difficulties and, in the worst cases, death from respiratory failure.

6.6 The full syndrome is properly known as venous thrombo-embolism (VTE) as used in some of our evidence. However, we follow the more common usage of DVT throughout this Report as it is the initial deep vein thrombosis which is the main concern in relation to the aircraft cabin environment, VTE being a complication occasionally arising from it.


6.7 It has been known for many years that clotting in blood vessels (thrombosis) is associated with:

  • poor circulation or stagnation of the blood;
  • excessive coagulability (thickening leading to increased tendency to clot) of the blood; and
  • abnormalities in, or damage to, the walls of the blood vessels.

6.8 The presence of one or more of these factors (known medically as Virchow's Triad) leads to an increased possibility of blood clotting in an otherwise healthy person. DVT is a frequent complication of major surgical procedures, occurring in about a third of such patients, and much of the present knowledge of the condition has been obtained from studies on post-surgical patients. As pointed out by Dr Kesteven (p 246), that may not necessarily reflect the natural history of travel-related DVT (see paragraph 6.14).

6.9 It is thought that anaesthesia and enforced immobility may induce the formation of small thrombi around the venous valves in the deep veins of the calf. If there are additional factors present tending to augment the potential of the blood to clot (thrombotic tendency), a thrombus may grow and extend to other veins, may amalgamate with other clots, and may spread into the larger deep veins of the upper leg. When this occurs the risk of a large thrombus breaking away and giving rise to a serious pulmonary embolism is much higher than if the thrombi remain small and limited to the calves. The time interval between the development of the initial small thrombi and such an embolism may be anything from days to weeks.

6.10 Pulmonary embolism is a relatively rare complication of DVT, Professor Kakkar and Dr De Lorenzo (p 181) giving an incidence of about 1 in 100 post-surgical patients who have suffered a DVT. However, many more patients with DVT, about 30% according to the Royal College of Physicians of Edinburgh (p 280), will develop local complications in the affected leg (known as post-thrombotic leg or post-phlebitic syndrome). This may lead to skin deterioration, venous ulceration, and physical disability, and particularly to a substantially greater risk of further DVT in future adverse circumstances. Mr Scurr (p 283) estimated the cost to the United Kingdom of dressings alone for these complications at in excess of £300 million per year.

6.11 To keep this in proportion, it has been concluded from post-mortem studies that up to one half of all people with DVT and/or pulmonary embolism will show no signs or symptoms at all (pp 181 & 246), and only one in three cases of suspected DVT is subsequently confirmed (p 181).

6.12 Possible links between DVT and air travel have been suggested in published case reports with increasing frequency during the past 50 years or so. Professor Mohler's evidence (p 251) included his own paper referring to many of these reports. However, as noted by Dr Janvrin of CAA (Q 23) and Dr Morgan Williams (Appendix 4 - who kindly sent us a copy of the original paper), the first strong evidence that pulmonary embolism was causally linked with spatial confinement, immobility and constrained seating conditions was published 60 years ago by the late Professor Keith Simpson, an expert in forensic medicine[79]. He noted a sharp increase in deaths from pulmonary embolism among people who had spent long periods in air-raid shelters, and found that such deaths were six times higher among those who had sat in hard-edged deckchairs than among those who had not. Many of the subsequent reports cite this important paper in their linking of DVT with enforced immobility and unsuitable seating in cars, coaches, trains and, in particular, aircraft.


6.13 From studies on surgical patients, various factors have been recognised as contributing to a raised risk of DVT - see Box 2. As discussed in paragraph 6.17, there are also factors in the aircraft cabin environment which may be risk factors for DVT in themselves, or which may augment the risk from the predisposing factors. Most of the factors shown in Box 2 have been identified only by retrospective studies, though some would be expected from the known natural history of the condition.

6.14 Views differ as to the relative significance of individual factors, and some are disputed, particularly in relation to air or other travel (as discussed below), but the overall contribution of known risk factors to the incidence of DVT is high. In studies cited by the Royal College of Physicians of Edinburgh (p 280) and Professor Mohler (p 251), it was found that only 11% of cases of post-surgical DVT had no risk factors identifiable by non-laboratory tests, and that 40% of new acute DVT cases were associated with inheritable clot-enhancing conditions.

6.15 The evidence relating to these risk factors is generally derived from surgical studies. We note that, in his 1940 report (see paragraph 6.12), Professor Simpson commented upon clear pathological differences between clots found in the lungs of post-surgical cases and of the air-raid shelter cases. When considering any possible contributions of predisposing or environmental risk factors to the development of DVT in travellers (see paragraphs 6.17ff), it must be borne in mind that the natural history of the latter - about which we have no specific knowledge - may differ significantly from that of post-surgical DVT.

Box 2

Predisposing factors for DVT

Generally agreed by witnesses

•  Increasing age above 40 years

•  Pregnancy

•  Former or current malignant disease

•  Blood disorders leading to increased clotting tendency

•  Inherited or acquired impairment of blood clotting mechanisms

•  Some types of cardiovascular disease or insufficiency

•  Personal or family history of DVT

•  Recent major surgery or injury, especially to lower limbs or abdomen

•  Oestrogen hormone therapy, including oral contraception

•  Immobilisation for a day or more

•  Depletion of body fluids causing increased blood viscosity

Witnesses' views variable

•  Varicose veins

•  Obesity

•  Current tobacco smoking

Sources: Dr Giangrande (p 234), Professor Kakkar & Dr De Lorenzo (p 181), Dr Kesteven (p 246), Professor Meade (p 174), Professor Mohler (p 251), Royal College of Physicians of Edinburgh (p 280), Mr Scurr (p 283)


6.16 Up to 20% of the total population is thought to have some degree of increased clotting tendency, largely from inherited or acquired biochemical, metabolic or blood abnormalities, and this indicates that there might be a "natural" underlying incidence of DVT in the general population associated with those factors alone. There are no published prospective (forward-looking) studies of the development of DVT in normal populations, but retrospective studies (finding cases which have already occurred in a defined population) indicate that the "natural" incidence increases rapidly with age, averaging overall about one case per thousand people per year. Thus, in those who have recently travelled by air, we might expect DVT to occur at an average rate of about one per thousand per year, with a lower rate in the young and a higher rate in the old - although, as noted in paragraph 6.11, in many travellers the DVT may not be apparent.


6.17 From the incidence among the general population, it is certain that a number of people will develop DVT while travelling or soon after they have travelled by air. The fundamental question yet to be answered is whether this number is greater than the number from an equivalent population who would have developed DVT if they had not recently flown. The time-scale between flight and the development of DVT is important because the anecdotal evidence indicates that flight-related DVT may develop, or be diagnosed, at any time from in-flight to many days post-flight. As discussed in paragraph 6.12, prolonged immobility and cramped seating are known to be causative factors for DVT in susceptible individuals. It is a matter of fact that these conditions are readily experienced in the aircraft cabin.

6.18 The actual incidence of DVT in those who have recently travelled by air is not known, because the appropriate scientifically rigorous epidemiological studies have not yet been carried out. We note that Mr Scurr (p 283) and Dr Kesteven (p 246) are both carrying out research in relation to this, and we look forward with much interest to seeing their reports. However, there are currently no authoritative data to show any clear difference in the incidence of DVT between those who have recently travelled and those who have not. Thus it is also not yet known whether air travel increases the incidence of DVT above that found in the general population or above that found in other travellers. Indeed, as we were finalising this Report, a study was reported[80] in which the authors concluded that there was no increased risk of DVT among travellers.

6.19 Nevertheless, a wealth of anecdotal evidence, a few case-collection reports, and a case-control study - all cited widely by our witnesses - have led Professor Mohler, the Consumers' Association and others to conclude that there is a clear causal relationship between air travel itself and an increased incidence of DVT (pp 59 & 251). However, Mr Scurr, Professor Meade and Professor Kakkar think that any such relationship has not yet been established (pp 283, 174 & 181), and Dr Kesteven and CAA (pp 246 & 16 ) as well as the airlines think that any increased DVT incidence is related to long-distance travel itself, rather than with the means of transport.

6.20 There are various circumstances associated with long-distance travel by air, road, or rail, which are postulated as risk factors for an increased incidence of DVT in travellers. There are factors related primarily to the aircraft cabin environment itself which might act similarly; and there are also predisposing personal factors which could contribute adversely in relation to DVT in air and other travellers: see Box 3. When considering the possible contributions of any of these factors to a given case of DVT, it is the multiple contribution of a number of them acting together that is important rather than the impact of a single entity. However, it must be reiterated here that there are no data currently available by which the contribution of air travel to the overall risk of DVT from any of these factors, singly or in combination, can be estimated.

6.21 Views on some of the factors given in Box 2 differ. Obesity, for instance, is considered as a well-recognised risk factor for DVT by Professor Meade (p 174), but not by Dr Giangrande (p 234) except in terms of restricting mobility in aircraft. More controversially, perhaps, smoking is viewed by Professor Kakkar and Dr De Lorenzo (p 181) as predisposing to DVT, while Dr Giangrande (p 234) considers that it may actually be protective. We note in Box 2 that, while age over 40 is a risk factor for DVT in general, travel-related DVT seems to strike the young as well as the old. We consider that, in women, this may be related in part to the widespread use of oestrogen hormones in oral contraception, and we draw attention to this and hormone replacement therapy (HRT) in Box 4.

6.22 The postulated travel-associated factors given in Box 3 are largely theoretical, being deduced from consideration of what is known about the natural history of DVT itself, and how travel environments might impact upon it. We have already excluded low cabin humidity as a potential contributor to central dehydration in our discussion about the significance to health of cabin RH in paragraphs 5.30-5.36. When considering the possible contribution of any of these factors to the risk of DVT in travellers, it is vital to be aware that interactions between them are likely to be much more important than any individual factors, as we conclude more generally in paragraphs 6.58ff.

6.23 The term "economy-class syndrome" first appeared in 1977 in a paper by Symington and Stack[81] and is widely used to refer to flight-related DVT. It is misconceived in suggesting that the possibility of DVT need not concern business[82] and first class air travellers - or those using other forms of long-distance transport. We recommend that health professionals and others stop using the seriously misleading term "economy-class syndrome". "Flight-related DVT" or "traveller's thrombosis" would be more appropriate.

6.24 If there is an increased risk of DVT solely from flying, it is small. Anecdotal and case-series reports (see paragraph 6.16) indicate that anything from 0% to 50% of all cases of DVT/VTE may have an association with recent travel, and the case-control study (Ferrari et al, 1999), cited by Professor Meade (p 174) and others, centres this at about 25%. However in this study, only one quarter of the cases were associated with air travel, most of the rest being linked with road travel. Thus, the current estimates of the added contribution of recent long-distance travel to the 1:1000 per year risk of DVT in the general population vary from zero to 0.4:1000. Again taking the centred figure of some 0.2:1000, this implies that, amongst every million people taking a long journey by any mode once per year, at least one thousand cases of clinically detectable DVT will be found, because that is the general population incidence, plus possibly another 200 because of the additional risk of travelling. Many of the latter will have additional risk factors (see paragraph 6.13), so for healthy individuals, the risk of getting a clinically significant DVT solely because they are taking a flight seems to be exceedingly small. For those who are already at risk because they are subject to predisposing factors, there may be an additional risk from flying, but it is not currently quantifiable.


6.25 We are very conscious that the remarks above on incidence can only be tentative. Firm conclusions cannot be reached from the available evidence. This lack of knowledge is particularly important in terms of the ability of people to make informed judgements on their own position in relation to any risk of incurring DVT through flying or other activities. It also affects the soundness and quality of advice that can be given to them by their medical advisers regarding the risk and any precautions or preventive measures they should take to alleviate it. It is imperative that the current paucity of data on DVT be remedied and we recommend that an epidemiological research programme of the case-control type[83] outlined by Professor Meade (p 174 and Q 496) be commissioned by DoH as soon as practicable.

6.26 We also agree with Professor Meade that other research into any relationship between flying and DVT, as suggested by a number of witnesses, seems premature until the statistical facts of any link have been established, in which the outcome of current studies by Mr Scurr and Dr Kesteven, referred to in paragraph 6.18, may be very helpful. As there is a public health issue here that is wider than air travel, Professor Kakkar's suggestion of a National Register for travel-associated DVT (Q 498) has considerable merit but, to avoid what might prove to be unnecessary expenditure of scarce resources, we see no case for pursuing this in advance of the outcome of the recommended research by case-control studies.


6.27 From the predisposing factors given in Box 2 it is clear that certain groups of people are at increased risk of developing DVT, and it is possible that their susceptibility may be increased further by association with some or all of the postulated travel risk factors given in Box 3. Some would-be travellers will have more than one of the predisposing factors, and each may be compounded by association with relevant travel risk factors. It is known from post-surgical studies that the greater the number of predisposing factors present, the greater the risk of developing DVT (p 251). This is likely to be true also of the travel risk factors: the risk of getting DVT as a result of travelling probably increases as more factors are present.

Box 3

Postulated risk factors for DVT associated specifically with long-distance travel

Travel by air, road and rail

•  Increasing age above childhood

•  Increasing duration of travel

•  Increasing frequency of long-distance travel

•  Immobility, elected or enforced

•  Seating constraints, particularly leg-room

•  Seated posture, including when asleep

•  Wearing of tight undergarments or movement-restricting clothing

Within the aircraft cabin

•  Reduced pressure leading to abdominal distension acting against venous return from the legs

•  Reduced oxygen and/or pressure leading to increased blood clotting tendency

•  Low humidity affecting body fluid content (not unique to the aircraft cabin)

•  Excessive consumption of alcohol and coffee leading to dehydration (not unique to the aircraft cabin)

•  Safety procedures compounding immobility

•  Cabin crew activities discouraging mobility

•  Increasing duration of non-stop flight sectors

Personal risk factors compounded by the travel environment

•  Altered physical environment affecting predisposing factors (unique to the aircraft cabin)

•  Obesity compounding immobility and seating comfort

•  Height compounding mobility, seating and posture constraints

•  For smokers, enforced non-smoking altering physiology and psychology

Sources: The Consumers' Association (p 59), Dr Giangrande (p 234), Professor Kakkar & Dr De Lorenzo (p 181), Dr Kesteven (p 246), Professor Meade (p 174), Professor Mohler (p 251), Royal College of Physicians of Edinburgh (p 280), Mr Scurr (p 283), Bendz B (et al) 2000, Lancet 356: 1657-8

6.28 Intending travellers who know they have, or suspect they might have, any of the predisposing factors noted in Box 2 should take advice from knowledgeable authorities to enable them to decide firstly, whether and how to travel, and secondly, what precautionary measures they should take. We make general proposals in paragraphs 9.5-9.8 to improve the availability of sound advice. In the meantime, the immediately following paragraphs make proposals to bridge the gap with regard to DVT. We should, however, emphasise that intending passengers who are on medication which might influence susceptibility to DVT (such as anti-coagulants, steroids and cholesterol-reducing agents) must take specialist advice before travelling.

Box 4

Interim precautionary and preventive advice concerning air travel and DVT

Those with no known predisposing factors:

•  Move around in seat and in cabin as much as practicable

•  Exercise calf muscles whilst seated by half-hourly flexing and rotating of ankles for a few minutes

•  Avoid excess of alcohol and caffeine-containing drinks, both before and during flight

•  Drink only water or non-caffeinated soft drinks or juices when thirsty or feeling dry

•  Observe and act on advice given in in-flight media

Those at minor risk - i.e. meeting one or more of the following conditions

                    aged over 40

                    very tall, very short, or obese

                    previous or current leg swelling from any cause

                    recent minor leg injury or minor body surgery

                    extensive varicose veins

•  As above plus the following:

•  Avoid leg discomfort whilst seated

•  Avoid alcohol and caffeine-containing drinks, both before and during flight

•  Take only short periods of sleep, unless normal sleeping position can be attained

•  Do not take sleeping pills

•  Consider the need to wear support stockings

Those at moderate risk - i.e. meeting one or more of the following conditions

                    recent heart disease

                    pregnant or on any hormone medication - particularly the
                     contraceptive pill and HRT

                    recent major leg injury or leg surgery

                    family history of DVT

•  All the above plus the following:

•  Take professional medical advice about the risks involved

•  Take pre-flight low dose aspirin as advised by doctor unless contra-indicated

•  Take professional advice about the need to wear compression stockings

Those at substantial risk - i.e. meeting one or more of the following conditions

                    previous or current DVT

                    known clotting tendency

                    recent major surgery or stroke

                    current malignant disease or chemotherapy

                    paralysed lower limb(s)

•  Consider avoiding or postponing flight, taking medical advice if unsure

•  If travelling, all the above but have low molecular weight heparin prescribed instead of aspirin

Sources: Dr Dawood (p 220), Dr Giangrande (p 234), Dr Kesteven (p 246), Professor Mohler (p 251), Virgin Atlantic (p 107)


6.29 The current lack of sound information makes it difficult for individuals to make reasoned judgements about their personal DVT risk and, consequently, the precautions to take. To help meet those highly understandable needs, we have summarised the relevant material submitted by our witnesses during the Inquiry. As an interim measure pending the development of more authoritative guidance[84], we recommend airlines, their agents and others with consumer interests to repackage the summary indicative and precautionary advice on DVT in Box 4, together with the summary information on predisposing and risk factors in Boxes 2 and 3, and make it widely available to the general public. This will enable those who have no access to other advice to make preliminary decisions about their travel and the risk of DVT.

6.30 We can understand the airlines' reluctance to accept suggestions that there might be factors specific to the aircraft cabin environment that lead to an increase in the overall risk of DVT. Although, as noted in paragraph 6.24, any additional risk is likely to be small, it is not in doubt that the risk factors of prolonged immobility and cramped seating are present in aircraft. As noted in Q 315, however, these circumstances are not limited to aircraft, and we recommend the Government to consider tackling the issues on a wider travel-related front or, indeed, as a general public health matter.

6.31 In relation to air travel alone, however, and applying the precautionary principle used in other fields where health risks are considered possible but are not well defined or quantified, there are measures which could be taken to improve information and alleviate concerns about flying and DVT, and to encourage preventive activities. We recommend that airlines and their associates reappraise their current practices in relation to not only the provision of information for passengers but also the design of the cabin[85] and cabin service procedures. The following are the main points:

    (a)  easily available authoritative information on DVT and travel;

    (b)  high-profile pre- and in-flight preventive advice;

    (c)  active encouragement of in-flight mobility and preventive leg exercises;

    (d)  restraints and constraints on seating and eating, and on sleeping comfort;

    (e)  improved seat space and leg-room to a healthy minimum, together with increased availability of pre-bookable more spacious seating at reasonable additional cost;

    (f)  freedom and space to move around, with minimum limitations on aisle access due to cabin service activities; and

    (g)  reduced availability of alcohol and other dehydrating beverages with increased availability of re-hydrating drinks.

6.32 We also recommend the Government, aviation regulators, trade groups and consumer representatives to consider what action they should take in relation to the above points.

77   In our discussion on DVT, we have drawn heavily on the written evidence submitted by our expert witnesses identified in Box 2 on page 46. We do not cite them individually in the text except where there are specific attributions to be made. Back

78   The condition commonly known as varicose veins is not causally linked to DVT (Q 483), although it is considered by some as a risk factor (pp 181 & 251).  Back

79   Shelter deaths from pulmonary embolism, Lancet, December 1940. Back

80   Kraaijenhagen R A et al (2000), Travel and risk of venous thrombosis, Lancet 356: 1492, 28 October 2000 Back

81   Symington I S and Stack B H R, Pulmonary thromboembolism after travel, Br J Chest, 1977. Back

82   Lord Graham of Edmonton suffered his DVT following a business class flight (Q 84). Back

83   A study which involves identifying people ("cases") who have DVT and comparing their recent travel histories and medical backgrounds with a closely matched series of "control" subjects without DVT. Back

84   As a starting point for which we offer the material in Boxes 2-4. Back

85   See also paragraph 5.40 about cabin design implications for the provision of air nozzles under individual passenger control. Back

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