House of Lords - Science and Technology

Select Committee on Science and Technology Fifth Report

CHAPTER 4: ELEMENTS OF HEALTHY CABIN AIR

4.1 As noted in paragraph 3.37, there is no settled position on the cabin atmosphere to be provided for passengers. To help clarify the present muddle, this Chapter considers the various components of a healthy atmosphere for the aircraft cabin, dealing first with respiratory needs and then with provisions for the minimisation of contamination. The actual delivery of these requirements is then discussed in Chapter 5.

Respiratory gases

4.2 To maintain life, the body needs oxygen. The process of taking in air (inhaling), extracting oxygen from it and breathing out (exhaling) waste gases, especially carbon dioxide, is known as respiration. Drawing particularly on the written evidence from the Ministry of Defence's Royal Air Force Centre for Aviation Medicine (p 73) and Professor Denison (p 94) as well as the authoritative medical publications referred to in paragraph 8.42, this section deals with human respiratory needs in the context of the aircraft cabin environment.

OXYGEN

4.3 Air at sea level contains about 21% oxygen, and the remainder consists of physiologically inert[39] gases (78% nitrogen and nearly 1% others), some water vapour, and a very small amount of carbon dioxide (about 0.03%). At each inhalation, the blood passing through the lung tissue absorbs about a quarter of the available oxygen. The oxygen-rich blood is driven by the pumping action of the heart to all body tissues to provide, along with food, energy for the body's activities. The main gaseous waste product of this energy production, carbon dioxide, is carried back to the lungs by the circulating blood and exhaled together with the unused oxygen and other components of the inhaled air.

4.4 The constituent of blood which transports oxygen is called haemoglobin, the properties of which mean that the amount of oxygen taken up by the blood falls only slightly even if the amount in the inhaled air is considerably lower than normal. Such conditions are experienced at altitude where, although oxygen continues to form 21% of the atmosphere, the lower pressure or density means that each lungful of air contains fewer molecules of all atmospheric gases, including oxygen. At the regulatory maximum effective cabin altitude of 8,000 feet, the atmospheric pressure - and thus the amount of oxygen per lungful of air - is about 70% of the sea level value. Nonetheless, the amount of oxygen available in the blood stream for the body to carry out its activities is still more than 90% of that available at sea level. At 6,000 feet, a common effective cabin altitude[40], where the amount of oxygen per lungful of air is about 80% of sea level values, the blood oxygen level is about 97% of that at sea level.

4.5 For general economy and efficiency of operation, modern aircraft need to be flown at altitudes of up to around 40,000 feet. To avoid the possibility of any effects upon the cabin occupants, the ideal would be for cabin pressure to be maintained at the equivalent of ground level throughout a flight. This would require substantially stronger aircraft structures, which would be impracticably heavy. As a compromise, aircraft cabins are pressurised so that, whatever the cruising altitude, the pressure in the cabin is maintained at the equivalent of around 6,000-8,000 feet altitude depending on the aircraft type[41].

4.6 Professor Mohler of Wright State University, Ohio noted that the minimum cabin pressure[42] requirement has remained unchanged since the concept of pressurisation of commercial aircraft cabins was realised about 60 years ago (p 251). As Professor Denison explained, research had shown that, primarily due to the characteristics of haemoglobin discussed in paragraph 4.4, the human body could maintain blood oxygen at levels sufficient for its needs up to about 10,000 feet altitude (Q 236). At 8,000 feet, any lowering of blood oxygen levels was found to be physiologically insignificant and undetectable by normally healthy people. Professors Muir and Moyle of Cranfield University (p 218) pointed out that, as much of this early work was carried out on healthy young adults, it is not necessarily directly applicable to some of today's air passengers. While we have received no detailed evidence to suggest that the findings are invalid[43], there may be an argument for updating the research in view of not only the wider spectrum of passengers' ages and health but also improvements in medical technology, such as the non-invasive blood oxygen sensor noted by Mr Brundrett (p 216).

4.7 As noted in the Airbus Industrie and Boeing supplementary evidence on aircraft ventilation systems[44], the body needs to breathe in only about 0.3 cfm of air (at normal cabin altitude) for respiratory purposes. This is far exceeded by the supply of fresh air per person[45] as noted in paragraph 3.37. The very great majority of passengers can easily accommodate the reduced availability of oxygen in cabin air. Being generally at rest with consequently reduced need for oxygen, very few passengers are likely even to be aware that they are in a reduced oxygen environment.

4.8 Even those whose respiration is already compromised by a congenital, chronic or acute condition - particularly of the heart and lungs - may still fly with no special additional provision. A crude test of a passenger's fitness to fly is, as suggested by some staff from the National Heart and Lung Institute at the Imperial College School of Medicine (p 239), the ability to walk 50 or so metres without shortness of breath.

4.9 Those with more significantly compromised respiration will, subject to medical advice about their fitness to fly at all, need pre- and in-flight medication such as asthma inhalers and other bronchodilators and, as discussed in paragraphs 7.46ff, pre-ordered supplementary oxygen

4.10 With regard to aircrew, there would be potential safety as well as health concerns over any adverse effects of reduced oxygen. Congenital and chronic medical conditions which affect the ability to perform satisfactorily in reduced-oxygen environments would preclude individuals from acceptance for flight crew training. If they later developed a medical condition significantly adverse to their health in such circumstances, this would be discovered during medical examinations needed for their repeated certification as fit to carry out their duties (see paragraph 3.43).

4.11 Although cabin crew are not subject to regular medical re-certification, the employment process will involve a health assessment, and those who might suffer adversely from lowered oxygen availability would not be employed as cabin crew. Cabin crew work may sometimes be sufficiently heavy for cabin oxygen levels to have a noticeable impact on metabolism particularly in increasing respiration, pulse rate, and fatigue. However, we consider that these are normal physiological responses, and that repeated exposure to the environment would be unlikely to have an adverse effect on health unless an individual were already unwell from another cause.

4.12 All aircrew have personal responsibilities to ensure that they are fit for their work, and are thus likely to seek early medical attention if they think their work performance or health may be jeopardised by a reduced-oxygen environment. The pre-employment and other medical arrangements mean that the slightly reduced oxygen availability in the aircraft cabin should not be significant for aircrews' immediate or longer term health and wellbeing.

CARBON DIOXIDE

4.13 Carbon dioxide is the natural waste product of respiration. The quantity of carbon dioxide exhaled into the cabin air is about 80% of the quantity of oxygen that is removed from it during inhalation. Because (as noted in paragraph 4.7) the cabin ventilation system supplies more than enough oxygen for the cabin occupants, it is more than adequate to remove the exhaled carbon dioxide.

4.14 There are, however, other sources of carbon dioxide in the cabin, notably the dry ice (solid carbon dioxide) used to chill food and drinks. The total amount of carbon dioxide produced in the cabin from all sources may reach about 0.5 cfm per occupant. However, aircraft ventilation systems generally supply and remove at least 10 cfm of air per occupant, and the average carbon dioxide level in a fully occupied cabin under normal operating conditions is some 0.1%. While this is about four times the normal level in fresh air, it is only one fifth of the airworthiness standard noted in paragraph 3.27. Professor Denison (Q 238) and Boeing (p 204) noted that there are no known ill effects or biochemical effects from being exposed continuously for many weeks to carbon dioxide levels of 0.5 to 1%, i.e. five to ten times the normal aircraft cabin levels[46]. We therefore conclude that the carbon dioxide levels normally found in the aircraft cabin atmosphere are of no adverse consequence to health.

4.15 There is some controversy about the maximum acceptable level of carbon dioxide in the cabin atmosphere. This has arisen from some confused comparisons with standards in buildings, as discussed in paragraphs 5.11-5.15 in the context of ventilating the aircraft cabin.

Cabin atmosphere contamination

4.16 Aircraft cabins are no different from many air-conditioned ground environments such as office and hotel buildings in providing circumstances in which particulates[47], gases and vapours can build up in the air. These "atmosphere contaminants" may arise from:

the occupants themselves;
their clothing;
the structural, decorative and furnishing materials within the environment (a process known as "outgassing");
introduced chemicals or other materials;
sanitary and cooking activities; and
accident conditions such as fire and unplanned chemical reactions.

Ventilation systems must be designed to remove them or at least keep their equilibrium levels below those which may be hazardous to the health or safety of the occupants. Those atmosphere contaminants of significant concern are discussed in this section.

4.17 A number of national authorities exist which set standards in their own countries for the maximum levels of chemical compounds which are permitted in living and working environments. Many, although not all, of these standards are agreed internationally. Workplace or occupational standards are defined on the basis that continuous exposure of workers for eight hours a day, five days a week, for a working lifetime, at or below those levels[48] will not cause detriment to health. They are set by organisations such the US Occupational Safety and Health Administration (OSHA), the American Conference of Governmental Industrial Hygienists (ACGIH), and the United Kingdom's HSE. The standards are referred to in this Report as "workplace limits".

4.18 Some standards have also been set for general public exposure in domestic or public environments. They are recommended by such organisations as the American Society of Refrigerating, Heating and Air-Conditioning Engineers (ASHRAE) and the United Kingdom's Building Research Establishment (BRE), and some may be given legal force under appropriate legislation. Such standards are usually considerably lower than limits for workplaces because the occupants may be exposed to them for 24 hours a day, and the people concerned include the very old, the very young, and those who may already be sick or infirm. These limits are generally known as "building standards" but, to avoid possible confusion, we refer to them in this Report as "public exposure limits".

4.19 The validity of applying such limits to the aircraft cabin at cruising altitudes could be questioned. Most limits are published as volume ratios at "25ºC and 760 mmHg pressure". As no significant adjustment is required for cabin temperature, the question is what, if any, adjustment is required for reduced cabin pressure. As with oxygen levels (see paragraph 4.4), reduced pressure means that, without changing the volumetric ratio for a given chemical, the mass of that chemical present per unit volume will be reduced. As toxicity depends on the amount present rather than its proportion, applying "ground" limits to the cabin enhances the safety margins. Nevertheless, as discussed in paragraphs 6.58ff, the complex interplay of many factors must be borne in mind when considering the overall impact of the aircraft cabin environment on health and, when using published maximum exposure limits of this type, consideration must also be given to whether the combination of environmental factors in the cabin has any impact on the way the body deals with the chemicals in question.

PARTICULATE COMPOUNDS

4.20 Particulates are materials present in cabin air as dusts and contained within aerosols. They emanate from on-board activities such as:

cleaning, food preparation and cooking;
disinsection procedures (see paragraphs 4.24ff);
paper usage, baggage handling, and personal grooming and other bodily activities such as breathing, sneezing and coughing; and
where still permitted, tobacco-smoking (see paragraphs 4.29-4.31).

Microbiological organisms including bacteria, viruses, fungi and moulds are often associated with dusts and aerosols and, in the aircraft cabin, present the greatest theoretical risk to health from particulates.

4.21 Particulates may be measured as:

(b) respirable (or breathable) suspended particulates (RSP), which are of greater importance to health; and

(c) more recently, particulates of diameter range 2.5 to 10 microns[49] (PM10) which are the most easily retained in the lungs, and hence potentially the most hazardous, of the respirable particulates.

The unit of measurement used for all is micrograms[50] per cubic metre (µg/m³) i.e. the mass of particulates in a given volume of air[51].

4.22 Generally, the measured levels of particulates are not in themselves helpful in assessing any potential hazard to health, but are helpful for comparing different environments. The evidence put to us by Airbus Industrie, Boeing and others shows that particulate levels in the cabin atmosphere are generally very similar to those in typical houses and office buildings, with mid-flight microbiological organism levels usually substantially lower than comparable ground environments, including airports.

4.23 Whatever the nature of particulates in the cabin atmosphere, comfort and health considerations dictate that their levels are kept low, and in any case below published public exposure limits such as the US National Academy of Science's recommendation for PM10 particles of 150 µg/m³. This is achieved by ventilation[52] and the filtration of re-circulated air, as discussed further in paragraphs 5.2ff and 5.18ff respectively.

INSECT CONTROL

4.24 Two types of treatment are used for the WHO/ICAO procedures (known as "disinsection") noted in paragraph 3.10:

(b) residual (surface cover) treatment for food preparation areas, closed spaces such as cabin lockers, and areas within the aircraft that are not accessible for aerosol treatment[53].

4.25 As Professor Sir Colin Berry noted, the aerosols may be used in three ways (p 71) dependent on the national regulatory requirements of the destination country:

(b) "blocks-away" - sprayed into holds, flight-deck and cabin prior to departure with doors closed and air-conditioning off; and

4.26 The concern is whether the disinsection procedures are potentially harmful to the health of cabin occupants. The ICAO rules require that the procedures used are not injurious to the health of, and cause the minimum of discomfort to, passengers and crew - nor must they have any deleterious effect on the aircraft structure or equipment.

4.27 The ICAO rules permit the use of different insecticides, all of which are WHO-approved, based on their efficacy and minimal human toxicity. The most widely used are pyrethroids, but none of the approved chemicals produces cabin atmosphere levels which might cause toxic effects in man (Q 229, pp 71 & 78).

4.28 Allegations of harm from pesticide exposure in aircraft are the subject of litigation in the USA; some health authorities have queried the wisdom of aircraft disinsection (p 39); and WHO is undertaking further research on the matter (p 1). However, we received no specific allegations or queries on this during our Inquiry. Our two main witnesses on the health effects of insect control procedures, Professor Sir Colin Berry and Dr Virginia Murray, said that they were unaware of any UK cases of ill health from exposure to pesticides in aircraft, and that they had received surprisingly few inquiries about such exposures (pp 71 & 78, QQ 227-234). We share their view that insect control procedures are not a significant health issue.

TOBACCO SMOKE

4.29 Had our Inquiry been conducted a few years ago, we are sure we would have received many complaints about the health hazards of environmental tobacco smoke (ETS). The general silence on this topic presumably reflects Mrs Bish's welcome (Appendix 4) for the now widespread ban on smoking in the aircraft cabin. As the Academy of Medical Sciences noted, the ban is not yet universal and the "dangers of passive smoking" cannot be ignored (p 197).

4.30 As noted in paragraph 3.8, the ban is not a binding requirement from ICAO, but a recommendation - and only in relation to international flights. It has not yet been universally achieved, but we note that the USA has just banned smoking on all scheduled flights between the USA and other countries[54]. DETR indicated that, like many other countries, the United Kingdom has chosen not to enact legislation (p 1). It seems, however, that consumer demand is achieving the desired outcome.

4.31 Nevertheless, we recommend the Government to urge ICAO to upgrade the smoking ban recommendation to a formal requirement on its Member States in relation to all flights. Pending a formal ban, we recommend those airlines which still permit in-flight smoking to complete the ban on a voluntary basis. Where in-flight smoking may still be permitted, we recommend that airlines and their agents should actively make this clear to intending passengers prior to ticket purchase.

VOLATILE ORGANIC COMPOUNDS

4.32 Vapours from volatile organic compounds accumulate in the cabin atmosphere by evaporation from the parent substances such as oils, paints, adhesives, furnishings, pesticides, disinfectants, cleaning agents, foods and alcoholic drinks. (We deal specifically with tri-ortho-cresyl phosphate - TOCP - in paragraphs 4.35ff.) Being gaseous, they are not removable from cabin air by normal ventilation filters. Atmospheric levels of volatile organic compounds must therefore be controlled both by limiting their presence in the cabin environment and by adequate air exchange through the ventilation system.

4.33 The levels of volatile organic compounds in atmospheres are usually measured collectively unless specific hydrocarbons are being investigated. The studies that have been drawn to our attention by Airbus Industrie, Boeing and others have shown that, under normal operating conditions, volatile organic compounds in cabin air were found to be either undetectable or at very low levels of up to 3 ppm - of which the majority (80%) were alcohols from alcoholic drinks. These levels are far below the 1,000 ppm workplace limit recommended by ACGIH (a public exposure limit for volatile organic compounds was not found in our evidence), and below the workplace limits for any single component. We thus conclude that cabin atmosphere levels of volatile organic compounds present no risk to cabin occupants under normal operating conditions.

4.34 Pall Aerospace drew our attention to a filter they had developed in conjunction with Airbus Industrie that combined odour removal with normal filtration and could be used in existing systems without modification (p 259). In addition to odour removal, the filter should also absorb some volatile organic compounds. (It may be that other manufacturers have similar products.) As noted in paragraph 5.7, only re-circulated air is filtered, so such filters would reduce rather than eliminate any contamination of the incoming fresh air. Notwithstanding the availability and use of filters that can remove volatile organic compounds, every effort must be made to avoid such contamination in the first place.

TRI-ORTHO-CRESYL PHOSPHATE (TOCP)

4.35 We received evidence from a number of witnesses, particularly the Organophosphate Information Network (OPIN - p 257), BALPA (p 213), and the International Association of Flight Attendants (AFA - p245), expressing concerns about the risk of TOCP poisoning for cabin occupants, particularly for crew who might be subjected to repeated exposure in some aircraft types[55], as a result of oil leaking into the cabin air supply. Although Dr Murray's examination of the London National Poisons Information Centre records and a literature search revealed no cases or enquiries about TOCP in relation to aircraft (p 96), the concerns are serious, and we pursued them further.

4.36 Tri-cresyl phosphate (TCP [56]) has, as noted by Dr Howse of Rolls-Royce, been used for many years as an anti-wear additive in aviation lubricants, at levels varying between 1-3% in currently available oils (p 271). Mr Fogarty of ExxonMobil commented that no other additive had been found to match TCP's anti-wear and load carrying performance (p 232).

4.37 TCP exists in three different forms or isomers, of which the "ortho" form (tri-ortho-cresyl phosphate - TOCP) is highly toxic. As noted by the Medical Toxicology Unit and OPIN, the most significant adverse effect of overexposure to TOCP, which might arise from improper use of the parent material such as swallowing or prolonged or repeated inhalation or skin contact, is peripheral neurotoxicity (nerve damage). This can lead to pain and serious paralysis of limbs, and bowel and lung disorders. After exposure ceases, some recovery usually ensues but a degree of permanent disability is not uncommon (pp 96 & 257).

4.38 Rolls-Royce stated that all current TCP used in the formulation of aviation lubricants contained far less than 0.1% TOCP and that, in fully formulated oils, TOCP was at practically undetectable parts-per-billion levels (p 271). ExxonMobil noted that jet engine oils formulated with TCP[57] are not classified as dangerous according to the toxicological criteria defined in the Dangerous Substances Directive (p 232).

4.39 Calculations by Airbus Industrie (Q 461 and refined in subsequent correspondence) showed that the worst-case scenario of the total discharge of an engine's lubricant into the engine would result in about 0.4 kg of oil passing into the cabin ventilation systems. Assuming that the oil contained 3% TCP, of which 0.1% was TOCP, the peak cabin atmosphere TOCP level would be about 0.025 mg/m³, reducing as a result of normal ventilation thereafter. The peak level would be a quarter of the workplace limit of 0.1 mg/m³ (and less than a tenth of the emergency workplace limit of 0.3 mg/m³). Contamination at much lower levels would result in visible smoke and odour which would normally result in the crew switching off the ventilation feed from the affected engine.

4.40 This question - including the potential effects on aircrew from any long-term exposure - has been looked at in much greater detail by a Committee of the Australian Senate inquiring into particular allegations of such contamination in the BAe 146. Although its Report[58] referred extensively to cabin air quality and chemical contamination in the aircraft, and recommended that the engine lubricating oil used (a Mobil product) be subjected to a further hazardous chemical review, it made no specific points about TCP or TOCP that have given us additional concerns[59].

4.41 The absence of confirmed cases of TOCP poisoning from cabin air and the very low levels of TOCP that would be found in even the highly unlikely worst case of contamination from oil leaking into the air supply lead us to conclude that the concerns about significant risk to the health of airline passengers and crew are not substantiated.

CARBON MONOXIDE

4.42 Carbon monoxide is a colourless and odourless gas which, if present in inhaled air in sufficient amounts, can substantially reduce the oxygen-transport capabilities of the blood and thus the amount of oxygen available to the tissues. It is always present to some extent in any closed or semi-closed human environment, its sources being people[60], the drying out of some paints and glues, and cooking and other forms of incomplete combustion of carbon-containing compounds. Before the now almost complete ban (see paragraphs 4.29ff), tobacco smoking was the main source of the gas in aircraft cabins. Introduction of carbon monoxide can also arise accidentally from internal fires and leaks of exhaust gases from hydrocarbon-fuelled engines.

4.43 The airworthiness requirement (a maximum concentration of carbon monoxide in cabin air of 50 ppm[61], as noted in paragraph 3.33) and exposure limits are set to ensure no hazard to health. The supplementary material submitted by Boeing (p 204) showed that, as a result of normal ventilation, typical levels of carbon monoxide found in non-smoking aircraft cabins are around 1 ppm, and maximum levels detected, even in smoking-permitted cabins, range only up to 7 ppm. These levels are well below the airworthiness requirement and the workplace limit of 25 ppm, and below public exposure limits which range from 9 ppm to 35 ppm. The levels found cannot be considered as of consequence to the health of cabin occupants.

OZONE

4.44 Ozone is a molecule of three oxygen atoms. It is colourless but, unlike normal atmospheric oxygen (which is a molecule of two oxygen atoms), has a distinctive pungent smell and is potentially harmful. It is a natural component of the Earth's upper atmosphere, normally existing in significant quantity only well above 60,000 feet. However, the amount of ozone present varies with latitude (being higher in polar regions), season and varying wind patterns. Moreover, ozone plumes sometimes extend downwards to about 25,000 feet containing concentrations of around 1 ppm (up to 4 ppm at Concorde cruise altitudes). Aircraft pass through these plumes from time to time, and ozone will then be drawn into the ventilation systems. The high temperatures resulting from air compression in the aircraft engines will convert some[62] ozone into oxygen, and Boeing (p 204) and Airbus Industrie (p 165) told us that some long-haul aircraft flying polar routes were now fitted with equipment to remove the remainder. For aircraft not equipped for ozone removal, however, a DERA/British Airways joint study showed that ozone is frequently detectable in the cabin during and after flying through plumes (p 250).

4.45 FAA has for some years had a cabin ozone standard of not more than 0.1 ppm for three hours with a maximum of 0.25 ppm at any time. This was adopted recently by JAA. The workplace limit is 0.1 ppm with a ceiling of 0.3 ppm for 10 minutes (0.2 ppm for 15 minutes in the United Kingdom), and public exposure limits vary between 0.05 and 0.12 ppm.

4.46 During and shortly after passing through an ozone plume, ozone levels in the cabins of aircraft where ozone removal equipment is not fitted typically average 0.01-0.02 ppm (but sometimes up to ten times higher). As noted in supplementary material from Boeing (p 204) and Airbus Industrie (Q 427), ozone at the higher concentrations will be detected by cabin occupants and occasionally give rise to minor eye, nose and chest symptoms in some.

4.47 The possibility of problems with ozone levels has been known since the 1970s. Small lightweight catalytic ozone converters have now been developed for both new aircraft and for fitting to existing aircraft. These have been shown to be very effective in reducing cabin ozone to levels below the regulatory standards (pp 99, 165 & 204). To minimise potential health problems when aircraft fly through ozone plumes, we recommend airlines to fit ozone converters to their aircraft used on routes where they may come into contact with such plumes.

OTHER CABIN ATMOSPHERE CONTAMINANTS

4.48 JAA noted that, while an aircraft is taxiing, fumes of kerosene (the aviation fuel) from preceding aircraft may enter the cabin, although concentrations were found to be below toxic levels (p 130). Mr Koplin of JAA found this personally uncomfortable. Shutting off the air intake - needed for ventilation - did not seem to be the answer, but he thought the matter merited further investigation (Q 373)

4.49 The smell of fumes from other aircraft while manoeuvring on the ground is a matter of annoyance for some rather than a health hazard. Problems could be reduced by the kind of odour-eliminating filters noted in paragraph 4.34 and, to increase passenger comfort, airlines may wish to consider installing these.

4.50 Airbus Industrie (Q 427) and Boeing (p 204) noted in supplementary material that traces of other chemical contaminants had been identified from time to time in cabin atmospheres including sulphur dioxide, oxides of nitrogen, and specific volatile organic compounds such as acetone and formaldehyde. However, we have received no evidence that these are of either concern or significance to the flying public or to aircrew, and we consider them no further.

39 At normal or lower (as in the aircraft cabin) pressures. Nitrogen can have dangerous effects at significantly higher pressures as experienced by divers and caisson or tunnelling workers who, as noted in paragraph 5.47, may then be at risk from the reduced pressure in the aircraft cabin. Back

40 See paragraph 3.29. Back

41 5,000 feet for Concorde. Back

42 Or, in other words, the maximum effective cabin altitude. Back

43 Indeed, in retrospect, Professor Denison considered the 5,000 feet effective cabin altitude in Concorde was unnecessarily cautious (Q 236). Back

44 See footnote 29, paragraph 3.35. Back

45 As noted in paragraph 5.3, the typical air supply is some 20 cfm per person, including up to 50% re-circulated air from which substantial oxygen is also available. Back

46 As a natural consequence of respiration, the lungs normally contain about 5% carbon dioxide. Back

47 Including microbiological organisms such as bacteria, viruses and spores. Back

48 These standards are generally not maximum limits but time-weighted averages for an 8-hour working period. Excursions above the levels are permitted provided that they are balanced by time below the limits. Back

49 A micron is one millionth of a metre or one thousandth of a millimetre. Back

50 A microgram is one millionth of a gram. Back