Select Committee on Science and Technology Fifth Report


7.1 This Chapter deals in detail with each of the three other main medical concerns that have arisen from our Inquiry, namely: the transmission of infection; the effects of the cabin environment on vulnerable individuals; and the handling of in-flight medical emergencies. The main non-medical issues are then dealt with in Chapter 8.

Transmission of infection

7.2 There is a widespread general perception that the transmission of infection between aircraft cabin occupants, both passengers and crew, is common, and that this is facilitated by the nature of the cabin environment. The general perception is reinforced by the personal anecdotes we received in evidence (Appendix 4). As we note in paragraph 7.14, there are very few published reports of such cross-infection in the scientific literature - although, of course, only serious incident reports would achieve publication.

7.3 The clear view of all the health professionals who submitted evidence to us is that, while cross-infection can and does occur in the aircraft cabin, it does so because of the crowding together of large heterogeneous groups of people. Although, as pointed out by Professor Pavsol (p 264), there are good reasons at first sight why the aircraft cabin environment should increase the risk of cross-infection between its occupants, the reality is that (apart from crowding) environmental factors in the aircraft cabin appear to make little or no contribution to the transmission of infectious agents. As noted in paragraph 7.16, most passengers will mix with many more people - in crowded circumstances and probably poorer environmental conditions - both before and after their actual flights.


7.4 There are three main types of infection.

    (a)  VIRAL. Viruses are sub-microscopic bodies which can multiply only in living host cells. Common viral infections include: upper respiratory tract infections (colds); influenza; measles; mumps; and chicken pox. Viral infections are fought by the body's immune system, antibiotics being of no use in dealing with viruses themselves. Once the immune system has fought an infection for the first time, it is usually able to deal more rapidly with any re-infection. However, certain viruses, particularly those causing upper respiratory tract infections and influenza, can mutate rapidly into strains which the body does not recognise and thus has to fight again as new infections.

    (b)  BACTERIAL. Bacteria are larger than viruses but still microscopically small. Unlike viruses, they can survive and multiply outside the body: they are widespread throughout nature. Most bacteria are harmless, and many are beneficial to humans: food digestion, for instance, is very dependent on appropriate bacteria living in the bowel. Harmful bacterial infections include: tuberculosis, pneumonia, meningitis, and gastro-enteritis. The body defends itself against bacteria and their toxic products by its immune and reparative systems. Serious infections may be treated with antibiotics[92].

    (c)  PARASITIC. Various parasites may use the human as a host, either permanently or as part of their life-cycle, causing illness. Examples are amoebic and other dysenteries; intestinal, other organ, and skin worms; fungus infections of body and skin; malaria; and schistosomiasis.

7.5 Whether people exposed to infective material then develop disease depends on various factors: the dose they receive; whether they have any immunity; and their general state of health. If the body's defences are weakened by infection or disease, it is more susceptible to further infection. For example, many of the complications of "colds" are due to secondary bacterial infections of the affected membranes following their viral infection.

7.6 Infections are spread through the air, by direct or indirect contact (including through water, food and other ingested material), and by the bites of insect "vectors". These are considered in turn below.


7.7 In airborne transmission, the infective particles are discharged into the air in aerosols produced by the sneezing, coughing and spluttering of those already infected. The infections concerned are usually viral, although bacterial diseases, notably tuberculosis, meningitis and pneumonia, can also be spread in this way.

7.8 Human pathogenic viruses generally remain infective for only a few minutes outside the body; bacteria in suitable environments for up to a few days. The duration of viability in both cases is dependent on the nature and type of the organism, and on the physical conditions in the environment into which it is released. Particularly relevant to the aircraft cabin environment is that, as noted by Pall Aerospace, dry air is thought to increase virus, and reduce bacterial, survival times, but conditions in aircraft ventilation re-circulating systems are generally not conducive to extended survival of micro-organisms (pp 259). Whether people exposed to infective material actually develop disease depends not only on the viability of the organism, but also on the dose of the organism they receive. The latter is related directly to the proximity of the source and, as Dr Bagshaw of British Airways pointed out, the density of the cloud of infective particles reduces rapidly with distance from the source (Q 264).


7.9 The primary means of spreading viral, bacterial and some parasitic infections is by contact. There are three main contact routes:

    (a)  skin to skin;

    (b)  accidental or purposeful exchange of body fluids (blood, sexual fluids, saliva);

    (c)  absorption through points of entry to the body, primarily mouth and nose, from direct contact with infectious agents, or through intermediaries such as water and other beverages, food, cutlery, sanitary and washing facilities, toilet articles, clothing, bed-linen, and body wastes.

7.10 All of these are strongly linked to personal and environmental hygiene. This gives rise to concern because the aircraft cabin environment inevitably brings into close physical contact peoples with different customs and practices in relation to personal and general hygiene, together with major constraints on sanitary, culinary, and waste disposal facilities.

7.11 We received no evidence or complaints about possible food-poisoning from in-flight catering. Other than commending airlines and their agents for their high standards of hygiene in relation to catering and encouraging their maintenance, we make no further comment on the control and management of food hygiene.


7.12 Some very serious infections (such as malaria, yellow fever, dengue fevers and plague) are spread by insect bites, including mosquitoes and fleas. There have been reported cases of insect-spread disease near airports in areas where the disease is not endemic[93] where the assumption is that insects have been inadvertently transported by aircraft. However, perhaps largely due to WHO's disinsection procedures discussed in paragraph 4.24ff, transmission of infection by insect bites has not been reported within the aircraft cabin itself, and we consider this matter no further. Some fleas and lice can be problematic as parasitic infestations in their own right and, as noted briefly by Inflight Research Services (p 240) there may be some cross-infestation in the cabin environment due to proximity.


7.13 Bringing a mix of people together in large numbers and in limited space would appear to present ideal circumstances for spreading infections between them. The more people present, the greater the risk that one or more may be harbouring potentially infective organisms. Given the role of proximity in airborne and contact transmission discussed above, the more that people are crowded together, the greater the risk of cross-infection. As we also note above, the higher the dose of infectious organism to which people are exposed, the greater the risk of contracting infection: flight sector length is thus also important in defining the duration of exposure. A further factor is that, with the continuing growth of air travel and a wider range of national airlines on many routes, people from many different countries are increasingly sharing flights and potentially exposing themselves to new infectious agents against which they have no immunity.

7.14 Against that background, and bearing in mind that about 5 million passengers fly every day, the number of reports of confirmed cases of on-board transmission of infection between cabin occupants is amazingly small. In the evidence we have received, the only widely-reported instances from the professional literature appear to be two influenza outbreaks, one of measles, three of tuberculosis (although WHO investigated seven reported cases arising between 1989 and 1994) and a small number of outbreaks of gastro-intestinal infections (pp 264 & 280). In addition, we received evidence from Anne Lewis (Appendix 4) about an outbreak of viral gastro-enteritis in an aircraft setting, together with other personal reports about air-travel related (mainly respiratory) infections. (Appendix 4).

7.15 There is also some serious misreporting. For example, our attention was drawn to a newspaper article[94] reporting a case of two people infected with TB during a flight. In fact, testing carried out by Dr Perry had shown that they were not so infected (p 267).

7.16 The incidence of infectious illness among those millions of passengers is, of course, bound to be much higher than indicated by the above figures, but it is obviously wrong to attribute all air travel-related illnesses to cross-infection in the aircraft cabin. Many of the cross-infection risks in the cabin during flight are not unique to that environment. Similar close-contact situations arise elsewhere, not least at the margins of air travel such as airport lounges and buses, in other forms of mass transport, and in restaurants, bars and places of public entertainment.

7.17 On the evidence we have been given, it seems to us that the risk of transmission of infection due specifically to being in the aircraft cabin environment is no greater than elsewhere, at least in relation to major illnesses, provided circulation and filtration systems are working properly. For lower level illnesses such as coughs and colds, it seems very unlikely that conclusive evidence could be obtained to point one way or the other. Given the evident success in avoiding transmission of major infections, we do not agree with those who seem keen to attribute any travel-related infectious ailment to augmented transmission within the aircraft cabin environment.

7.18 The small number of confirmed cases of flight-related cross-infection is a call for continued vigilance rather than an excuse for complacency. As discussed in the immediately following section, aircraft manufacturers and operators go to very great lengths to reduce the chances of cross-infection between cabin occupants to as low a level as practicable with current technology. We also consider in paragraphs 7.34ff the control of intending passengers infected with transmissible conditions, and of post-flight monitoring, with the aim of understanding better, and thus reducing further, the possible risk of cross-infection in the aircraft cabin.


7.19 A common allegation among those summarised in Appendix 4, and echoed in other evidence and the media, is that modern cabin ventilation systems appear to facilitate - indeed encourage - cross-infection. As discussed above, this is not supported by the facts. Features which could promote cross-infection are longitudinal flows of air within the cabin and ineffective filtration of the re-circulated air.

7.20 In older aircraft, the general flow of ventilating air was from the front to the back of the cabin. Both Airbus Industrie and Boeing stated unequivocally that, in modern aircraft, the airflow is from the top of the cabin downwards to the floor (Q 428, p 204). Airbus told us that they use the latest mathematical modelling design techniques to ensure this, and that they test within their aircraft that the design flows are achieved and that "dead zones" are avoided. They also said that this would be unaffected by seating configurations (QQ 428 & 429). Any transmission of an infectious agent from an affected passenger would therefore seem to be physically limited to those passengers within the immediate vicinity. This is at least similar to - and probably better than - conditions in any ground environment where numbers of people are gathered closely together.

7.21 As discussed in Chapter 5, half the cabin air is re-circulated through filters, and then mixed with an equal volume of fresh air (which, as noted in paragraph 5.6, is bacteriologically sterile - again better that most ground level "fresh" air) before the next pass. If there were no filtration, infectious organisms given off continuously by a cabin occupant would be dispersed throughout the cabin atmosphere, on the face of it presenting a significant risk of infection to all cabin occupants. However, progressive dilution with fresh and re-circulated air and removal by air extraction overboard would lead to a general cabin level of infectious organism greatly lower than that in the immediate vicinity of the infected person. Thus, even in the absence of filtration, the level of infectious agent to which most cabin occupants would be exposed is very small though they would be so exposed for the duration of the flight.

7.22 The Air Transport Users Council (AUC) expressed concern about cabin conditions on the ground when the aircraft ventilation system was not operating. They would like to see enacted a US National Academy of Science recommendation that full auxiliary ventilation should be used, or passengers removed after 30 minutes without it (QQ 155 & 161). Dr Dawood was of a similar view (p 220). The AUCs' concern was associated with the risk of cross-infection between cabin occupants as occurred with influenza in the Alaska Airlines incident in 1977 (Q 162). ATA also considered hold-ups on the ground with no aircraft ventilation to be a risk factor for transmission of communicable diseases (p 110). To reduce cross-infection risks (as well as for general comfort), we recommend airlines to ensure that they have suitable policies for occasions when aircraft with passengers on board have to be held on the ground for extended periods without full ventilation. Such events are rare, so it is all the more important to have in place clear guidelines for action.

7.23 Efficient filtration (a topic introduced in paragraphs 5.18ff) is essential to minimise the potential for cross-infection in aircraft using re-circulatory ventilation. Evidence from airlines was that the HEPA filters they used remove bacteria and virus particles with an efficiency of over 99% (pp 99, 107 & 104) although they appeared only to be quoting filter manufacturers' claims. Airbus Industrie and Boeing reported only the filter specifications (pp 165 & 204). Because the standard particle size tests noted in paragraph 5.21 may not accurately reflect microbiological effectiveness, Pall Aerospace tests its own HEPA filters with bacteria and viruses. These have been found to remove microbiological organisms with an efficiency greater than 99.999%, which Pall have stated is the "microbial equivalent of a 100% outside air single-pass system" (p 259). Furthermore, Pall calculates that, even with a significant rupture (of 10mm by 1mm), such a filter would still provide for 99.9% efficiency (p 259).

7.24 Although we do not dispute the design claims for HEPA filters, we fail to see how the industry can effectively rebut charges that such filters do not perform as designed when so little attention is given to their performance. For example:

  • they are replaced "on time" rather than "on condition"[95];
  • they are not subject to regular between-replacement or post-replacement inspections;
  • they have been shown to remove viruses only in laboratory tests; and
  • no routine monitoring of air contamination by sampling is carried out.

We note the point made by Airbus Industrie and Pall Aerospace that newer systems are fitted with differential pressure gauges which indicate damaged or blocked filters (pp 165 & 259), but this may not be enough and we recommend the industry as a whole to review and substantially improve overall in-service performance monitoring of filters.

7.25 The theory, the facts and the vast majority of our specialist witnesses all support the conclusion that HEPA filtration, if properly installed and maintained, should remove the possibility of cross-infection that would otherwise exist in re-circulatory cabin air systems. We agree that, on the evidence presented to us, the modern aircraft cabin environment generally poses no greater risk of transmission of infection between its occupants than crowded situations elsewhere - and may, indeed, be safer than most of them.

7.26 As noted in paragraph 5.22, however, HEPA filtration is not yet standard. To minimise the risk of cross-infection, we are clear that it should be, and we recommend the Government and regulators to make filtration to best HEPA standards mandatory in re-circulatory systems. In the meantime, we recommend airlines to upgrade all filtration to the best HEPA standards.

7.27 We noted[96] that, feeding on fears about poor cabin air quality, Mr Kahn's Aviation Health Institute was selling face-masks which were stated to "screen out 98% of bugs including TB". We cannot see how these would be of any practical use unless the wearer happened to be sitting directly next to someone with an active bacteriological infection (and there was also a good fit of the mask to face). Additionally, Professor Denison warned of the dangers to certain vulnerable passengers of the wearing of face masks (Q 215).

7.28 It came to our notice that there were other technologies under development that might supplement or replace HEPA filtration for aircraft cabin ventilation. AEA Technology provided us with an outline of their work on plasma (ultra-violet, electron, and free-radical bombardment) techniques for air sterilisation (p 197). The technology is interesting and, when more fully developed, may be found to have advantages over present systems of filtration.


7.29 The only way of eliminating any risk of cross-infection in the aircraft cabin would be to prevent intending passengers carrying transmissible infections from flying. This is simply not achievable. The incubation period (the time between initial infection and the appearance of symptoms) for most viral and bacterial infectious diseases is typically several days or even weeks. A person may be highly infectious during this period, and remain infectious even when the disease is obviously present. Moreover, those who have been in contact with infectious diseases against which they have immunity may nevertheless be capable of infecting others.

7.30 It would clearly benefit fellow passengers if people did not fly in such circumstances. Ideally, individuals who thought that they might be infectious would either identify themselves and postpone their flight or request advice from their medical advisers or the airline about their fitness to fly. Such actions place great demands on individuals' honesty.

7.31 Flying for either business or pleasure tends to be part of significant journeys or events which may be difficult to disrupt. In addition, many people will have non-refundable tickets, leaving costs to be reclaimed through any insurance arrangements. There must be an understandable tendency for intending passengers to "feel well enough to fly", and not to consider any potential hazard to others. It would be a counsel of perfection to expect passengers with minor coughs and colds to defer their flights. The test for individuals must be whether, if they themselves were well, they would be happy to sit next to someone in their own present state of ill health.

7.32 Airlines can, of course, refuse to fly passengers they consider may be a risk to others. As Delta Air Lines pointed out, pre-boarding ground staff or cabin crew are likely to be alert to this, and will refuse to board a passenger who may obviously be a risk to others boarding the flight concerned (p 224). At most major airports, pre-boarding staff are able to refer to airline or airport health professionals for advice. As in all cases where an intending passenger may present risks to the safety of the aircraft, its crew, or its passengers, the final decision whether to carry the person rests with the aircraft captain.

7.33 As part of improved health information for intending passengers discussed in Chapters 8 and 9, we recommend the Government and airlines to do more to dissuade intending passengers from flying while they are likely to infect others. This could be further reinforced by a reminder that boarding may be denied to those who are obviously infectious.


7.34 WHO's International Health Regulations (1969 with subsequent updates)[97] are incorporated into the national public health regulations of individual countries. The Regulations seek to minimise the international spread of disease, and set out various requirements for handling outbreaks of specific infectious disease, currently plague, cholera, and yellow fever, cases of which have to be notified to international and local public health authorities when discovered. Those pertaining to aircraft are incorporated into national civil airline regulations. (From time to time, WHO also issues non-mandatory recommendations concerning other transmissible diseases, those currently in force being concerned with TB, malaria and AIDS/HIV.)

7.35 The Regulations and recommendations detail preventive and hygiene control measures to be taken by national health administrations, and they can impact directly on airline passengers; for example, if a specified infectious disease is notified on or before the landing of a flight, the aircraft may be impounded so that passengers can be medically inspected and contact details taken. National authorities may also apply the regulatory measures locally for other infectious diseases if they are concerned about incoming or outgoing spread of potentially epidemic diseases.

7.36 One of the main purposes of notification is to enable local authorities to trace contacts of the infected case. Given that incubation periods are frequently several days or even weeks, the initial notification is usually made by a health professional some time after the flight concerned. The airline or its agents may already have disposed of any useful information about individual passengers by then. Such late notification may still lead to the initiation of contact tracing by the local authorities, but airlines have no direct responsibilities in this regard. They may, however, be able to assist the authorities by providing tracing information that they may hold on crew and passengers.

7.37 In their recent investigations into TB cross-infection in aircraft, WHO found that the only information generally available from airlines seemed to be passengers' names, usually contact telephone numbers (through ticket sellers), and allocated (although not necessarily occupied) seats. Ticket agents may have better contact details, but this information may remain available for only a short time after a flight. Some information may also be available from immigration and customs declaration landing cards in the case of some passengers on incoming international flights, but this has not been a useful source for follow-up purposes.

7.38 Given those difficulties, it is unsurprising that WHO finds that the financial and labour costs of contact tracing for epidemiological purposes are out of all proportion to the value of the results achieved. The primary purpose of the notification and tracing arrangements remains simply to enable health authorities to inform people that they may have been exposed to an infected person during the flight and to advise them that they should seek medical advice.

7.39 Despite this rather pessimistic view of the value of seeking to trace contacts after passengers have dispersed post-flight, we are of the view that such procedures should not be dismissed out of hand. Although the need for contact tracing might be infrequent, there seems little necessity (given modern data-storage systems) for airlines and their agents to dispose quickly of potentially important passenger details. We were pleased to note the comment from Mr Nafzger of DETR (Q 34) that there probably should be some sort of recall system, and that his Department was discussing this with DoH and the airline industry.

7.40 To help with post-flight contact tracing when exposure to serious infections may have occurred, we recommend the Government to consider requiring UK airlines and their agents to retain all aircraft passenger information which could be useful in tracing contacts for a minimum of three months after all flights, and that the Government should seek to extend this requirement internationally[98]. We also recommend airlines and their agents to move towards this immediately.

7.41 From time to time, airlines and their representative bodies review the passenger data collected for marketing and other analytical purposes. In doing so, we recommend they also consider improving such data (or at least ensuring greater standardisation) to help meet the potential needs of post-flight contact tracing.

92   As discussed in our Report Resistance to Antibiotics and Other Antimicrobial Agents (7th Report Session 1997-98, HL Paper 81-I), care must be taken in the use of antibiotics to reduce the pressure on bacteria to mutate into resistant forms. Back

93   Such as airport malaria as reported in the August 2000 issue of the Bulletin of the World Health Organisation. Back

94   It's high-flyers who may be laid low, Daily Telegraph, 22 June 2000. Back

95   That is, by reference to some pre-set period rather than when a specified condition is reached. Back

96   From the article Breathe Easy, Style magazine, Sunday Times, 25 June 2000 - which repeated the serious misreporting already noted in paragraph 7.15. Back

97   Throughout this section, we have drawn on the provisions of these Regulations and on WHO's Tuberculosis and Air Travel: guidelines for prevention and control, WHO/TB/98.256. Back

98   Although the risk of infection generally increases with the duration of the flight, the evidence shows that virus infections have been transmitted while an aircraft is on the ground before a relatively short flight. Passenger data therefore need to be kept for all flights.  Back

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