Select Committee on Science and Technology Fifth Report


5.1 Chapter 4 looked at the body's need for an adequate supply of oxygen, removal of carbon dioxide and protection from harmful atmospheric contaminants. Against that background, this Chapter deals with the practical provision of healthy cabin air - the topic about which we received most complaints.


5.2 Ventilation of the aircraft cabin is essential for four main purposes:

  • to meet the occupants' respiratory needs;
  • to clear contaminants and odours from the cabin air;
  • to control the temperature of the cabin environment; and
  • to maintain cabin pressure when at altitude.

5.3 During our consideration of respiratory needs in Chapter 4, we found that these were very substantially more than met in the aircraft cabin by the typical air supply of 20 cfm per occupant[63]. Before discussing the other purposes of ventilation, we must first describe how it is provided.

5.4 Until around 1980, aircraft cabins were ventilated entirely with fresh air. One of the ways aircraft manufacturers found to meet the commercial and environmental pressures to reduce oil consumption which arose at this time was to reduce the amount of outside air taken from the engines, maintaining overall air supply by re-circulating some of the air already present in the cabin. As Boeing noted, the resulting air quality was more than adequate for respiratory needs, but the air needed to be filtered for satisfactory contaminant control (p 204).

5.5 All large modern airliners now use re-circulation of up to 50% of cabin air in their environmental control systems. Boeing indicated that the typical cabin air flow of 20 cfm of air per occupant is equivalent to a full change of cabin air every 2 to 3 minutes, i.e. 20 to 30 times per hour (p 204). As half of the air being changed is re-circulated cabin air, this is equivalent to an entire exchange of cabin air with fresh air 10 to 15 times per hour - although, as noted in paragraph 4(g) of Appendix 5, the mixing arrangements mean that the actual replacement is a form of progressive dilution.


5.6 Aircraft environmental control systems work[64] by taking hot compressed air from the engine compressor stages and passing it through heat exchangers (pre-coolers) to provide bacteriologically-sterile air at the appropriate temperatures and pressures for the aircraft ventilation, air-conditioning, and pressurisation systems. Air is also provided for de-icing, cargo heating, pneumatic and hydraulic systems.

5.7 Air for the cabin is then passed, through ozone converters (see paragraph 4.47) if fitted, to up to three air-conditioning packs, to produce temperature-controlled dry sterile air to the cabin air mixing chamber. Here, the fresh-air flow is combined with up to 50% of air taken from the cabin by re-circulation fans. The re-circulated air is filtered prior to passing into the mixing chamber (see paragraphs 5.18ff), and the rest of the cabin air is exhausted through pressure control valves.

5.8 The mixed air, at a temperature of about 18ºC, is ducted through to the cabin overhead ventilation system. From there it is distributed to each of up to six seating zones in the aircraft, with a separate supply to the flight-deck zone. Prior to distribution, it may be heated by the addition of hot air to match the temperature requirements of the individual cabins. In modern aircraft, as indicated by Airbus Industrie, the conditioned air flows downward over the cabin occupants, in carefully designed flow patterns to avoid areas of stagnant air and to minimise draughts and flow along the cabin (Q 428), to the cabin floor where it is vented through return-air grilles and either exhausted or re-circulated. (Air vented from galleys and lavatories is exhausted directly.)

5.9 The exhaustion overboard is through valves which, by setting the rate of release against rate at which air is drawn in, control cabin pressurisation including the rates of pressurisation change on ascent and descent. In modern aircraft, the environmental control system (including the pressurisation system) is entirely automatic, being controlled by appropriate sensors and valves - although some aspects may be manually controlled from the flight deck, in particular the fresh and re-circulated air flows, the number of air-conditioning packs in operation and zone temperatures. There are also parameter level and system warning indicators on the flight deck, together with manual regulators which may be needed for emergency purposes.

5.10 A common allegation is that, to save fuel, flight crew shut down some of the air-conditioning packs and thereby reduce air quality below the intended standard. If true, that would be inexcusable. However, we find BALPA's rebuttal conclusive (p 213). The regulatory emergency requirement noted in paragraph 3.40 means that there is an inherent over-capacity in a fully serviceable environmental control system. The automatic control may well result in packs running at less than full flow rates while still delivering the required output.


5.11 Many of our witnesses expressed serious concerns that re-circulatory systems provide lower quality cabin air than systems using only fresh air (see, for example, Annex 4). The concerns are centred on reduced oxygen availability, increased carbon dioxide and other gaseous contaminants, increased risks of cross-infection, and generally increased "staleness of air". The root of the concerns would seem to be that, with the introduction of re-circulation, the fresh air component of the standard 20 cfm of air per person has been halved to about 10 cfm (see paragraph 3.33)[65].

5.12 The addition of re-circulated air does not affect the fact that, as noted in paragraph 4.7, the design level of 10 cfm of fresh air per cabin occupant provides more than ample oxygen for respiration.

5.13 The picture regarding build-up of gaseous contaminants is less clear-cut. With regard to ozone, cabin air supply at 20 cfm using 50% re-circulation potentially improves the situation because only half the quantity of ozone is brought into the cabin compared with a 20 cfm full fresh-air system. With carbon dioxide and all other contaminants whose source is inside the aircraft, re-circulation potentially increases build-up, with the equilibrium level for each contaminant being reached when the rate of its production is equal to the rate of its removal. For contaminant control, the important factor is the number of complete changes of cabin air per unit time (see paragraph 5.5).

5.14 The level of carbon dioxide in cabin air is important both in its own right (as discussed in paragraphs 4.13ff) but also as a proxy for satisfactory ventilation. Until 1996, the FAA/JAA regulatory limit for carbon dioxide in the aircraft cabin was 30,000 ppm (3%), based on the safety level used by the National Aeronautics and Space Administration (NASA). In 1981, ASHRAE set a maximum level of 2,500 ppm as the standard for ground building environments as a measure of satisfactory ventilation. In 1989, in the light of concerns about "sick building syndrome" as described by Mr Gurney (p 234), ASHRAE reduced this to 1,000 ppm to ensure adequate ventilation for odour and contaminant control in buildings which by then were using up to 90% re-circulation of air to conserve energy. Many aircrew and air passenger groups misinterpreted this move as a reflection of increased knowledge of carbon dioxide toxicity rather than as a surrogate measure of adequate ventilation, and demanded the application of the ASHRAE standard to aircraft cabins (p 204).

5.15 When ASHRAE became aware of this misinterpretation in 1995, it set up special committees and working groups specifically to examine all the standards for contaminants, including carbon dioxide, that should be applied in the aircraft cabin environment. A September 1999 article[66] by Dr J N Janczkewski described how ASHRAE was tackling the work. At the time of writing, the work was still in progress, but the outcome will be published in ASHRAE Standard 161. The US authorities already set a workplace limit of 5,000 ppm (0.5%), and FAA and JAA have adopted the same level (0.5%) as the aircraft cabin standard (see paragraph 3.33). As discussed in the section on carbon dioxide as a respiratory gas (paragraphs 4.13ff), cabin levels under normal re-circulation ventilation conditions with full passenger loads vary between 0.05 and 0.15%, averaging 0.1%. Cabin air ventilation standards would therefore seem to be entirely acceptable.

5.16 Because the volume of air supplied to the cabin continues to provide ample quantities of oxygen, and because the rate at which cabin air is exchanged keeps carbon dioxide and other internal-source contaminant levels to well below those of significance to health, we do not accept the widely held view that the introduction of re-circulatory ventilation systems has resulted in any harmful change in the quality of cabin air[67]. Nevertheless, the industry should pay attention to these common perceptions of the effects of re-circulation - for example, by publicising the results of monitoring as discussed in paragraphs 5.48ff.

5.17 We see no case for the re-introduction of fresh air ventilation to alleviate these perceptions. The environmental and economic pressures[68] which led to the introduction of re-circulating systems remain and, as noted above, we do not find any consequent harmful change. However, JAA's requirement for only fresh air to be supplied to the flight deck reinforces the perception that there is something intrinsically "bad" about re-circulated air (Q 363). We understand that FAA does not have this requirement. We recommend the Government to urge JAA to reconsider its requirement for ventilation of the flight deck with only fresh air.


5.18 The proper functioning of ventilation systems using re-circulated air depends on the effectiveness of the filtration arrangements. There are, however, no FAA/JAA/CAA regulatory requirements for the use of filters in aircraft ventilation systems, and thus no filtration standards have been set. This appears surprising, but reflects the regulatory authorities' remits which are limited to secure arrangements for physically safe flights and safe landings: as noted by CAA, those are not known to be under threat from the nature of cabin air filtration (p 39). Accordingly, where ventilation system filters are used (as they are in the great majority of aircraft), their design, specification and maintenance criteria are determined by agreement between aircraft constructors, airline operators and filter manufacturers and suppliers. Nevertheless, as BATA noted, where filters are fitted, CAA requires airlines to implement reliability monitoring programmes which ensure that filters are used in accordance with the manufacturers' recommendations, and that their performance does not fall below manufacturers' standards (p 124).

5.19 When re-circulatory environmental control systems became the norm in the 1980s, particle/dust filters became integral components of the systems. As particle filters did not remove gases or vapours, activated charcoal filters were sometimes added for ozone control which, as noted in the supplementary material submitted by Boeing (p 204), were later replaced by ozone converters[69]. It was soon realised that passenger and crew complaints about reduced quality of breathing air could be due to inadequate filtration of the re-circulated air, and that re-circulation brought a greater risk of transmission of infection within the cabin. The efficacy of the filtration systems being used in removing particulates was also questioned, and improvements were sought and made.

5.20 Most environmental control systems today use High Efficiency Particulate Air (HEPA) filters. These were developed for hospital infection control settings where it was vital to prevent cross-infection and air contamination, and it seemed that they would be eminently suitable for transfer to the aircraft cabin. A major advantage of HEPA filters is that they have very little negative impact on airflow velocity and throughput, and their efficacy improves with use between changes (pp 165, 204 & 259).

5.21 The efficiency rating of HEPA filters is based on their ability to remove a defined proportion of particles of a given mean diameter set according to the test method used. Commercially available HEPA filters are rated from 85% to 99.995% removal efficiency based on liquid droplets of mean size 0.3 microns (DOP test), or solid particles of mean size 0.65 microns (Salt Flame test) (p 259)[70].

5.22 As CAA has noted, different grades of HEPA filter have been, and are still being, used in different aircraft types (p 39). The current HEPA filter efficiency standards set for the commercial aviation industry are 99.97% by DOP test in the USA (ASTM D2986-95), and 99.99% by Salt Flame test in Europe (BS 3928). All new Boeing and Airbus aircraft are being fitted with these standards of filter (p 259). Many older aircraft can be fitted with the latest standard filters, although there seems to be no easy way of establishing the extent to which this has been done (QQ 457 & 458).

5.23 Diamond Scientific Ltd made worrying statements about poor standards in the installation and use of HEPA filters in ground-based environments (p 225). Pall Aerospace (p 259) and BATA (p 293) robustly and persuasively refuted such poor practice in the tightly-controlled aviation industry.

5.24 HEPA filters are changed at intervals based on the filter manufacturer's recommendations[71], according to the filter specification requested by the aircraft manufacturer. The changes are generally made at fixed aircraft maintenance intervals, typically every 15 months (p 104) or 6,000 flight hours (p 124). The change intervals are agreed with the regulating authorities (see paragraph 5.18), but operators may choose to change them more frequently if aircraft usage and occupancy are higher than average (p 124). It was alleged by Mr Kahn (p 44, QQ 136-144) that some airlines did not change filters at the required frequencies or intervals, but this was strongly refuted by BATA (p 293) and, in the event, Mr Kahn was unable to provide material to support his allegations (footnote to Q 144).

5.25 A key question about filtration concerns the efficiency in removing microbiological particles from re-circulated air. Without efficient filtration, moving from single-pass fresh-air ventilation systems to re-circulating systems could lead to a rise in transmission of infectious microbiological particles. We consider transmission of infection in Chapter 7, and conclude our consideration of filtration in paragraphs 7.23ff.

Cabin humidity

5.26 The low humidity of cabin air is widely felt to be bad for the health of the occupants, particularly in relation to risks of deep vein thrombosis, the transmission of viral infections[72] and, by AsMA and Professors Moyle and Muir of Cranfield University, as a significant factor in general malaise which some crew and passengers ascribe to exposure to the cabin environment (pp 198 & 218). Low humidity was also cited as contributory to jet-lag by the Research Institute for Sport and Exerciser Science (RISES - p 269) and to eye, nose and respiratory problems by Delta Air Lines, the Building Research Establishment (BRE) and Boeing (pp 204, 211 & 224).

5.27 "Humidity" is used loosely to refer to the amount of water vapour in the air. Human comfort generally depends on "relative humidity" (RH), expressed as a percentage (% RH) of the maximum water vapour that air at that temperature can hold. (Above 100% RH, water vapour is precipitated as mist or, on surfaces, as condensation.)

5.28 At cruising altitudes, external air is very dry. After pressurising and conditioning, fresh air is delivered to the cabin at less than 1% RH. The now standard re-circulation of cabin air means that some water vapour is added to the cabin atmosphere by cabin occupants and some cabin activities. Depending on aircraft type, cabin configuration and passenger load, the relative humidity in the cabin averages around 10-15% within a range from 5%-35% (p 204).

5.29 There are no specific regulatory limits for cabin relative humidity. As noted by BRE, the levels normally found in aircraft cabins are well below those recommended as comfort levels for buildings of 30-70% (p 211). Nevertheless, many millions of people live healthily in climates as dry as aircraft cabins. Such dry atmospheres can also be found outdoors in summer, and indoors in winter (p 204). In addition, as pointed out by BAE Systems, IAPA and Varig, a low relative humidity is positively beneficial to the aircraft structure and equipment in reducing moisture and condensation, thus limiting corrosion and opportunities for bacterial and fungal growth (pp 200, 243 & 288).

5.30 The dry cabin atmosphere commonly gives rise to sensations of dryness to the eyes, nose, mouth and skin. Such "peripheral dehydration" may be uncomfortable for some. However, it can be easily dealt with by local application of moisture and it is not itself a threat to health (Q 215). The key question about low relative humidity in the cabin is whether it can lead to the loss of so much water ("central dehydration") that the body's normal water balance is significantly disturbed. There has been some suggestion that this could lead to potentially adverse conditions such as abnormal distribution of water around the body and increased viscosity (thickening) of the blood.

5.31 The body has in-built systems for the maintenance of water balance, controlled by hormones circulating in the blood, and urine output through the kidneys. Evaporation of water from the skin (sweating) and lungs is a major element in the body's temperature control, the amount being lost varying according to body temperature, the degree of physical activity being carried out, and the temperature and RH of the ambient air. In addition, water is excreted in faeces and urine. When central dehydration threatens, the body will respond by reducing sweat loss, reducing urine output, and increasing the sensation of thirst.

5.32 Even in the absence of sweating, the body loses about 1.5 litres of water a day, one litre through the skin, lungs and bowel and a minimum of half a litre through the kidneys as urine. The intake of water from food and drink is normally about 2.5 litres per day. Water intake would, therefore, have to be reduced by at least a litre per day to begin to produce central dehydration. Thus, central dehydration would not arise during a flight, assuming normal eating and drinking, unless the body had lost at least a litre of water over and above its normal losses[73].

5.33 The definitive experimental work on this topic was carried out at the then RAF Institute of Aviation Medicine (now DERA) at Farnborough, Hampshire. This showed clearly that exposure to 5% RH for 24 hours did not lead to changes in overall water balance amounting to central dehydration (p 72). Professor Nicholson also showed that the maximum increase in water loss for a person spending 8 hours at 0% RH was about 0.1 litre, well below even the thirst sensation level[74]. He, Dr Sowood (p 72), and Professor Denison (p 94, Q 214) were of the firm view that any extra water loss due to the dry cabin environment is of no significance to health, and that central dehydration of passengers in low humidity aircraft cabins is a myth. As Dr Giangrande noted (p 234), the assertion in Q 108 that breathing dry cabin air means that passengers are not replenishing their blood plasma is nonsense.

5.34 We received no evidence to indicate that artificially raising cabin RH levels might be beneficial to passengers in general, but we were given details by Le Bozec of their vapour (steam) air humidifier which they claimed would raise cabin RH to 30%. They said this was fitted widely on long-range business aircraft and by some airlines, and further large aircraft tests were planned, but we received no evidence from others about aircraft humidifiers. From the engineering point of view (see paragraph 5.29) increased RH would not be beneficial.

5.35 Against this background, we are satisfied that low cabin humidity is not harmful. Any uncomfortable dryness of skin, mouth, nose and throat can be alleviated simply by a sip of water or other local application of moisture and is not a threat to health. On a long flight, assuming normal fluid intake, one glass of water can more than offset any additional loss due to low cabin humidity. The common advice to drink a little more water than usual is thus sound.

5.36 There may be a tendency towards central dehydration in those passengers who, before or during a flight, drink sufficient alcoholic or caffeinated beverages (such as coffee or cola) to cause excessive production of urine (diuresis). If the peripheral dryness caused by low cabin humidity leads such passengers to further consumption of inappropriate drinks, they may expose themselves to the possibility of central dehydration. This is a risk factor in DVT, as discussed further in Chapter 6. Responding to our questions, Professor Denison (Q 220) and Professor Kakkar (Q 506) were both of the opinion that, in relation to travel-related DVT, neither excessive water consumption nor its possible relationship with the swelling of the lower legs commonly seen during long flights were of significance.

Cabin temperature

5.37 As noted by Airbus Industrie, the heat given off by passengers in a fully occupied cabin is considerable (Q 445). Incoming air needs to be at or below the required cabin temperature if that temperature is to be maintained. The cabin temperature is set according to seating zone from the flight-deck with a range of control from 18-27ºC, and is normally maintained in the range 22-24ºC, the same as that found in many office environments (p 211).

5.38 There are no regulatory standards for cabin temperature. As noted in the supplementary material submitted by Boeing (p 204), flight crew normally change cabin temperatures in response to cabin crew requests based on passenger representations about their personal comfort. Because passengers are normally in repose and cabin crew are working, their perceptions of thermal comfort are likely to be different. Cabin crew may feel uncomfortably hot and change dress accordingly.

5.39 We received a number of complaints about cabin temperatures, including inappropriate settings by crew to encourage passengers to sleep after meals. Temperature is one of the most quickly sensed aspects of the aircraft cabin environment. Being too hot or too cold is likely to affect a passenger's general perception of the whole flight experience. We endorse ASHRAE's suggestion that further work should be done to establish guidelines for cabin thermal conditions (as drawn to our attention by Boeing, p204) and we look to the industry to carry this forward.

5.40 We have also received representations from Inflight Research Services (p 240) and Mr Baker (Appendix 4) about the general lack of personally controlled air nozzles in current aircraft cabins. Both Airbus Industrie and Boeing confirmed that such nozzles were available on many aircraft, but the general absence of air nozzles under personal control in newer aircraft reflects airlines' preferred cabin layouts (QQ 463-468, p 204). While we understand (Appendix 5) that, where fitted, such nozzles deliver the same air as otherwise available, the directed movement of air can provide personal refreshment. The absence of individual air nozzles reduces the personal control passengers have over their flight experience, and we recommend airlines to review and modify their cabin design considerations to include such nozzles.


5.41 The main purpose of cabin pressurisation is to provide passengers and crew with sufficient oxygen for respiration. As noted in paragraphs 4.7 and 4.8, we agree that this purpose is met for all those in reasonable health. There are, however, some other health issues connected with cabin pressurisation. For the most part these arise from changes in pressure on ascent and descent.

5.42 In the absence of specific regulation, the rates of pressurisation change are set at the design stage to minimise any passenger or crew discomfort within the requirements for safe aircraft operation. According to the supplementary material from Airbus Industrie (Q 427) and Boeing (p 204), the reduction in pressure after take-off is normally limited to the equivalent of increasing altitude by 500 feet per minute. Human anatomy and physiology mean that more problems are likely to be experienced with increasing pressure on descent, and the rate of change for that is normally limited to the equivalent of 300 feet per minute.

5.43 If changes in cabin pressure are too rapid, there is insufficient time for the body to adjust to the substantial changes in the volume of air normally present, or abnormally trapped, in various body cavities. Pain in (or damage to) parts of the body caused by such changes is termed "barotrauma". Barotrauma of the middle ear and nasal sinuses is experienced by many passengers, particularly those with current or recent common colds, as discomfort or pain in the ears, face, nose or head. The symptoms can be relieved by allowing the pressure of the trapped air to equalise with the cabin pressure which, Professor Denison noted, is normally achievable by swallowing, yawning, jaw-moving and nose-blowing (p 94).

5.44 Pain in the head from pressure changes may be particularly severe for those with active upper respiratory problems and, quite apart from the infection risk they may present for others, they may need to take medical advice about flying. Before take-off, cabin crew should alert all passengers to the potential for head pain from pressure changes, and of the simple manoeuvres to prevent or alleviate them[75]. All cabin crew should be trained in the use of the Valsalva technique (generating oral pressure against pinched nostrils and closed pharynx) in order to help passengers to clear ear-block if simpler measures have failed.

5.45 Abdominal discomfort or pain sometimes occurs during ascent or descent, but this is usually temporary and relieved by gas re-distribution in the bowels.

5.46 As noted in paragraphs 7.42ff on vulnerable individuals, there are some medical conditions of the lungs (such as previously existing partial collapse of lung tissue or fixed-wall lung cavities) and of the bowel (particularly after recent surgery), which can be seriously affected by pressure changes, mainly on ascent. Passengers subject to these conditions would normally be under medical care and should have sought advice about air travel in advance of flying, including informing their airline if considered appropriate. If passengers are unaware of having such conditions, however, they may present in flight as medical emergencies, when descent may be the only practicable remedial measure.

5.47 People who have recently experienced high-pressure atmospheres, such as scuba and deep-sea divers or caisson and tunnelling workers, are at risk from the reduced pressure itself rather than the transitions. They may have excessive amounts of nitrogen (or other gases such as helium, depending on the gas mixtures used in their high-pressure environments) dissolved in their tissues for many hours after being exposed to high ambient pressures. If they travel by air too soon after returning safely to sea level, the further reduction in atmospheric pressure may lead to decompression sickness ("the bends"). Because of the risks of decompression sickness, sub-aqua divers (particularly occasional leisure divers) should ensure that the effects of any recent diving will not create an additional hazard when they fly. If in doubt, they should take professional advice. These points could usefully be drawn clearly to passengers' attention at least at the time of booking.

Monitoring of air quality

5.48 As will be seen from Appendix 4, cabin air quality was one of the main concerns among the individuals who made representations to us. Various studies of cabin air quality have been widely quoted in the evidence received from airlines, manufacturers, and aircrew and passenger representatives. However, the studies are mainly of the "snapshot" variety, leaving questions about whether they are typical, or are used selectively in support of the witnesses' main contentions. British Airways (p 99) and BATA (p 124) told us that a number of basic cabin environment parameters are continuously monitored in-flight, and recorded in the flight data recorders, including air-conditioning pack status, airflow rates, cabin pressure and zone temperatures.

5.49 Passengers' perception of general cabin air quality is one of the key factors in their assessment of the flight experience as a whole. We recommend that airlines collect, record and use at least some of the basic cabin environment data being continuously monitored, not only to give authoritative substance to their refutation of the common allegations, but also to provide a better basis for public confidence in these matters. Indeed, we are surprised that they do not already do so.

5.50 We noted previously (paragraph 3.33) that there are regulatory limits for carbon dioxide, carbon monoxide and ozone. We are satisfied that, under normal operating conditions, environmental control systems keep cabin atmosphere levels of these and other contaminants well under control (see paragraphs 5.14-5.16). We also note that British Airways is undertaking independent studies of cabin air quality (p 99), but we have seen no evidence that cabin air is monitored or sampled either routinely or even under abnormal or unusual conditions when passengers or crew feel that conditions are not right. We recommend airlines to carry out simple and inexpensive cabin atmosphere sampling programmes from time to time, and to make provision for spot-sample collection in the case of unusual circumstances. This would be helpful for passengers and staff, and also benefit airlines themselves. We also suggest that this might form part of Government-sponsored research, as discussed further in paragraph 9.3, and note that the Australian Senate Inquiry Report[76] makes similar recommendations

5.51 A key point in such monitoring is the basis for comparison. We noted earlier (paragraph 5.15) that ASHRAE is working to set such standards for cabin atmospheres. Airbus Industrie (Q 469), at least, would welcome this, because there are no international standards for air quality; and only building regulation, public place and workplace standards are currently available for use by the aviation industry. We welcome the ASHRAE work on cabin air quality standards and recommend the industry to support and encourage its timely completion and promulgation. We recommend that, in the light of the outcome, regulators consider extending cabin air quality standards beyond those for carbon dioxide, carbon monoxide, and ozone for which they already provide.

63   As noted by Airbus Industrie (Q 445), this volume of air is needed for temperature control (discussed further in paragraphs 5.37ff). Back

64   See also Appendix 5. Back

65   There are some complicated inter-related issues in all this which seem frequently to be misunderstood. Dr Murray Wilson stated that such had found their way into the British Medical Journal and into a transport paper for the European Parliament (p 255). Back

66   Air Quality on Passenger Planes, ASHRAE Journal, September 1999. Back

67   Indeed, we note in paragraph 5.28 that re-circulation is beneficial in significantly increasing relative humidity in the aircraft cabin. Back

68   For example, the savings on the total fuel costs of aircraft operation were indicated by Airbus Industrie to be 1-2% (Q 441), and by Professor Hocking to be 1-3% (p 236). Back

69   As noted in paragraph 4.37, filters which can absorb gaseous contamination are again available. Back

70   As discussed further in paragraph 7.23, these tests do not necessarily deal with particles of the most penetrating size - mean diameter 0.1 microns. Back

71   We noted during our visit to British Airways Maintenance, Cardiff (see Appendix 5) that filter efficacy was not checked between replacements. Back

72   Discussed further in Chapters 6 and 7 respectively. Back

73   Basic physiology and data from Samson Wright's Applied Physiology, 10th edition, 1961. Back

74   Low Humidity: Dehydration, dipsosis or just dryness? RAF SAM Report 01/96 and Dehydration and long-haul flights in Travel Medicine International 1998 Vol 16 No 5. Back

75   As discussed in paragraph 7.61, the solution for very young babies is to encourage them to feed.  Back

76   See paragraph 2.14. Back

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