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Passenger well-being in airplanes
Autonomic Neuroscience: Basic and Clinical 129 (2006) 80 – 85 www.elsevier.com/locate/autneu
Review
Passenger well-being in airplanes H. Hinninghofen ⁎, P. Enck University Hospitals Tübingen, Internal Medicine VI, Psychosomatic Medicine and Psychotherapy, Fronsbergstraße 23, D-72070 Tübingen, Germany
Abstract Passenger well-being is influenced by cabin environmental conditions which interact with individual passenger characteristics like age and health conditions. Cabin environment is composed of different aspects, some of which have a direct influence on gastrointestinal functions and may directly generate nausea, such as cabin pressure, oxygen saturation, and motion or vibration. For example, it has been shown that available cabin pressure during normal flight altitude can significantly inhibit gastric emptying and induce dyspepsia-like symptoms when associated with a fibre-rich meal. Other aspects of the cabin environment such as space and variability of seating, air quality, and noise, also have been shown to modulate (reduce or increase) discomfort and nausea during flights. Individual passenger characteristics and health status also have been demonstrated to increase vulnerability to adverse health outcomes and discomfort. © 2006 Elsevier B.V. All rights reserved. Keywords: Aircraft cabins; Atmospheric pressure; Air quality; Humidity; Deep vein thrombosis (DVT); Hypoxia
Contents 1. 2. 3.
Introduction . . . . . . . . . . . . . Cabin pressure and oxygen saturation Cabin pressure and gas expansion . . 3.1. Other effects of pressure . . . 4. Motion and vibration of the aircraft . 5. Seating and immobility. . . . . . . . 6. Deep vein thrombosis . . . . . . . . 7. Cabin air quality . . . . . . . . . . . 7.1. CO2 concentration. . . . . . . 7.2. Humidity . . . . . . . . . . . 7.3. Noise . . . . . . . . . . . . . 8. Conclusions . . . . . . . . . . . . . 9. Future perspectives . . . . . . . . . . References . . . . . . . . . . . . . . . . .
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1. Introduction
⁎ Corresponding author. Tel.: +49 7071 9387372; fax: +49 7071 9387379. E-mail address: [email protected] (H. Hinninghofen). 1566-0702/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.autneu.2006.07.018
Modern commercial airplanes are quiet and comfortable compared with previous generations of aircrafts. Nevertheless, there are still some aspects of the cabin environment which may cause stress and affect health in vulnerable individuals. Stressors in flight include cabin pressure and
H. Hinninghofen, P. Enck / Autonomic Neuroscience: Basic and Clinical 129 (2006) 80–85
oxygen saturation, motion and vibration of the aircraft, space and variability of seating, air quality and humidity, and noise. While pressure or oxygen and motion or vibration may exhibit direct effects on gastrointestinal function that may result in nausea and vomiting, other factors may modulate well-being and amplify or reduce these direct effects. 2. Cabin pressure and oxygen saturation Relevant to passenger health are flight altitude and cabin altitude (Fig. 1). Modern airplanes cruise normally at an altitude between 30,000 and 40,000 ft (9150 and 12,200 m) where the atmospheric pressure is less than 30% of that at sea level. In pressurized airplanes, cabin pressure will vary from sea level (760 mmHg) to 8000 ft (564 mmHg), depending upon aircraft altitude and pressurization schedules. These pressure guidelines were established in the early days of pressurized cabins (McFarland, 1953), based on medical knowledge, aeroengineering requirements, and economics (Ernsting, 1978). The ambient pressure of oxygen is approximately 20% of barometric pressure. Because the ambient oxygen concentration remains fixed, any decrease in atmospheric pressure will lead to decreases in inspired oxygen tensions (PAO2) and arterial oxygen tension. The physiologic response to lowered PAO2 is hyperventilation as a first step to meet the body's needs. Continuing hypoxia is compensated for with tachycardia to increase cardiac output. Hypoxia is also a stimulus for atrial arrhythmias and is associated with premature ventricular contractions (Gong, 1992). The blood haemoglobin oxygen saturation level is normally 95% for healthy people at sea level. At higher altitudes (and lower pressure), saturation levels fall. Oxygen saturation levels less than 85% may lead to impairment of mental function, and appreciable handicaps will occur at 70% saturation (Peacock, 1998). Oxygen saturation in resting people in an airplane cabin with
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a cabin altitude of 8000 ft will be around 90% with a range of 85%–93% after 30 min. In dozing subjects saturation levels of around 80% were found (Simons and Krol, 1996). In general, mild hypoxia with an oxygen saturation level of 90% is of no significance to healthy individuals, but due to wide individual differences, prediction of hypoxemia in an individual air traveller is difficult. Of concern nowadays is that in-flight cabin measurements show that cabin altitudes substantially higher than 8000 ft may be allowed (Cotrell, 1988) with consequent lowering of oxygen saturation that may be dangerous for those with ischemic heart disease, chronic respiratory disease and anaemia, or sickle cell anaemia (Gendreau and DeJohn, 2002). In passengers with such conditions, the consequences of lack of oxygen may include angina, acute myocardial infarction and congestive cardiac failure. Cardiac disorders are the most common cause of in-flight death (Cummins et al., 1988). Older passengers are at increased risk because arterial oxygen tension tends to fall after 40 years of age (James, 1996). 3. Cabin pressure and gas expansion Low cabin pressure also leads to expansion of gases: in accordance with Boyle's law, gases expand up to 35% when ascending from sea level to 8000 ft of altitude (Table 1). The body has a number of air-filled cavities: the middle ear and sinuses, the gut, pleural cavities, tooth fillings and the skull. Because pressure changes during ascent and descent occur very slowly, the majority of travellers experience no adverse effects apart from slight bloating or a sensation in the middle ear. Nevertheless, gas in body cavities can cause discomfort if equilibration with the ambient barometric pressure is not possible. Expansion of gas in the stomach or duodenum can lead to discomfort, nausea or vomiting. Gas expansion within the gut
Fig. 1. Cruising altitude and corresponding cabin altitude. Cruising altitude of modern airplanes is shown on the left Y-axis, while cabin altitude is shown on the right Y-axis.
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Table 1 Gas expansion in body cavities in relation to environmental pressure (Hagelsten and Nolte, 1963) Cabin altitude Meters
Feet
Sea level 1600 3300 5000 6600
Sea level 5000 10 000 15 000 20 000
Relative volume of gas in body cavities 1.0 1.2 1.5 1.9 2.4
contributed to significant increase in dyspeptic symptoms among flying staff compared with ground personnel of a commercial airliner (Enck et al., 1995). On long-distance flights, cabin crew reported significantly more bloating as compared to a ground-based period (Vejvoda et al., 2000). Fiber intake is often increased to compensate flight-related intestinal problems such as constipation (Enck et al., 1995). High fibre diets may in turn contribute to intestinal gas volume and amplify the pressure-induced problems. In an attempt to identify the mechanisms by which dyspeptic symptoms (bloating, nausea) are generated during flights, we simulated an 8-h flight in a hypobaric chamber in 16 healthy males (25.5 ± 5.9 years), subjected to two meal conditions on separate days in randomized sequence (with and without a fibre-supplemented standard meal, 2 g or 20 g, respectively), but randomly assigned to either a flight altitude of 2500 m (8200 ft = 565 mmHg cabin pressure) or 1000 m (3280 ft = 637 mmHg). The subjects were uninformed about altitude and fibre content of the meal. Gastrointestinal symptom ratings were taken every hour, and gastric emptying was assessed by conventional 13C-octanoic acid breath-test. The test based on the use of 13C-octanoic acid, a medium chain fatty acid which after ingestion is rapidly absorbed in the duodenum and metabolised in the liver. Following oxidation, the resulting 13CO2 is excreted into breath at a level which can be detected and measured by infrared spectrometry. In a separate experiment, we examined the effect of the two test meals (2 g vs. 20 g of fibre) on the same measures in 30 healthy males (26.7± 6.2 years) under conventional laboratory conditions. We found (Hinninghofen et al., 2006), gastric emptying (half emptying time =T1/2) was significantly delayed at 2500 m altitude when a high fibre meal was given (146.3± 58.4 min vs. 193.9 ± 54.3 min) (Fig. 2). Symptom reports of gastric distension and bloating were increased at 2500 m for the high fibre meal compared with the low fibre meal. There were no differences between groups on any measure for the ground control condition. This supports the claim that cabin pressure and oxygen saturation affect gastrointestinal functions such as gastric emptying and this may mediate dyspeptic symptoms such as bloating, distension or nausea during flights. 3.1. Other effects of pressure Failure to equalize pressure within the middle ear and the environment can cause otitic barotrauma. In up to 9% of air
travellers, otitic barotrauma occurs with attendant ear pain, tinnitus, vertigo or hearing loss (Csortan et al., 1994). Therefore, individuals with conditions causing a blockade of the Eustachian tube such as upper respiratory infection, allergy, or sinusitis should not fly until the blockade is cleared. People with recent surgery should contact their physician before they fly. To facilitate the gas equalization during ascent and descent, passengers should swallow frequently or increase pressure in the nasopharynx by performing a Valsalva manoeuvre (Hold the nose closed by pinching, close the mouth and blow gently. This raises the pressure in the pharynx, forcing air up the Eustachian tubes into the middle ear). 4. Motion and vibration of the aircraft Humans normally are in an environment where the force of gravity is constant in direction and magnitude. During air travel, passengers may perceive unusual vibration, motion, and centrifugal forces. Additionally, air turbulence can cause a linear vertical motion on the vestibular organ. In combinations, these may induce discomfort and motion sickness. A motion sickness study during air travel found that 0.5% of the passengers had vomited and 8.4% reported nausea during flight (Turner et al., 2000). Motion sickness tends to be severe during exposure to turbulent flight conditions. In vulnerable individuals, protracted vomiting may cause dehydration and electrolyte imbalance. Predisposed passengers should be advised to take preventive medication and to choose a seat over the wings, in the centre of gravity of the aircraft. Flying at night might be helpful by reducing visual stimulation (AMA, 1982). In general, aircraft vibration induced by the engine is usually well tolerated, but together with aircraft motion, noise and low humidity, it may cause some degree of discomfort and contribute to travel fatigue. A more detailed description of the general contribution of motion and vibration to the development of motion sickness, nausea and vomiting can be found in the contributions of E. Muth and J. Golding in this volume.
Fig. 2. Displayed gastric emptying time (T1/2) as a function of the fibre content of the test meals for both simulated altitudes. Gastric emptying was significantly delayed at 2500 m altitude, if a high fibre diet was given (p = 0.039) (from: Hinninghofen et al., 2006).
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5. Seating and immobility Planes today are able to fly non-stop more than 15 h, which makes seat comfort important for passenger wellbeing. Seat comfort is associated with seat pitch, seat width, leg room, quality of upholstery, and possible angle of recline. Seat pitch is the distance between the back of one seat to the same point on the back of the seat in front. The range for seat pitch is from 71 cm (28 in., charter planes) to 152 cm (60 in., first class). Seat pitch in economy travel is 76–86 cm. Close seat pitch is associated with poor seat comfort and restricts the degree of tilt of the seat, a feature which is important for night travel. Seat width is also important for comfortable seating, with 30% of seats narrower than the recommended 42 cm (17 in.) (Roggla et al., 1999). Cramped seating is not only uncomfortable. It makes it difficult to leave the seat for regular exercise, disturbs respiration, restricts the gastrointestinal transit and normal blood circulation, and can cause oedema and ischemia of the lower limbs. Surveys on charter and economy class flights have consistently rated seat comfort and leg room as being the two least satisfactory characteristics of air travel (Anonymous, 2000). 6. Deep vein thrombosis The term “economy class syndrome “ refers to the development of deep vein thrombosis (DVT) and pulmonary embolism (PE) in air travellers flying for long distances in cramped economy class seats. But, as shown recently, there is no clear evidence of a specific risk associated with air travel; DVT and pulmonary embolism are potentially a general problem of prolonged travel in any confined spaces (Bagshaw, 2001). The incidence of pulmonary embolism among 135 million passengers arriving at Charles de Gaule airport was much greater among those who had travelled more than 10,000 km (4.8 per million) (Ansell, 2001). Flight duration and individual predisposing seems to be a critical trigger. Both complications are likely to occur in those with risk factors such as venous disease, heart failure, nephrotic syndrome, thromboembolic illness, hypercoagulable disorders, diabetes or age. Women are at higher risk than men (Giangrande, 2000). Leg oedema (jet flight leg), which may mimic DVT, is usually benign and resolves after landing. Possible measures to prevent DVT are: in-flight muscle-contracting exercises, regular walks, use of compression stockings, adequate (non-alcoholic) fluid intake, and the use of drugs such as aspirin and subcutaneous heparin (Scurr et al., 2001).
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recycled air in the aircraft cabin environment, the relative humidity, and the number of people in the cabin. The ventilation rate on board an aircraft is determined to provide fresh air to dilute odour to an acceptable level for 80% of passengers. Odours and volatile organic compounds can be added from furnishings and humans, lubricating oils, hydraulic fluids and de-icing fluids entering through open doors whilst the plane is on the ground. Even if not toxic, odorous air can itself cause nausea, headache and loss of appetite. 7.1. CO2 concentration Air travellers remain for prolonged time with the smallest available air space per person of any other current social setting (Hocking, 1998). Without any air change, it would take only 2.3 min for the concentration of CO2 in that space to exceed 1000 ppm. CO2 concentration is normally taken as an important measure of air quality. Recommended concentration is less than 1000 ppm. Measurements in business class non-smoking flights not uncommonly shows concentrations greater than 1000 ppm with peaks of 2900 ppm (Lee et al., 1999). Extremely high CO2 concentrations were found during take-off and landing, when power requirements reduce the amount of ventilation. Since the 1980s, half of the air exchange consists of recycled air from the passenger cabin. As the proportion of recirculated air increases energy savings can be made, but at the same time health risks are increased. Early experiments on aircraft found 9 L/s/p (Litre/second/person) of fresh air were adequate for the comfort for most passengers (McFarland, 1953). Today most commercial aircraft are only capable to provide 2.8 L/s/p of outside air to their passenger cabins. The airline industry saves more than $30 million annually by reducing energy costs due to recycling air with inadequate ventilation (Consumer Reports, 1994). Beside the reported symptoms, of particular concern is the increased risk of disease transmission in this setting. Organisms transmitted by droplets less than 10 μm in size, such as measles, influenza viruses, and tuberculosis may spread between
7. Cabin air quality Acceptable air quality in commercial aircraft is important for the comfort and well being of passengers and flight crews. Inadequate air quality may cause symptoms of fatigue, headache, and dizziness, as well as respiratory and ocular discomfort (Pierce et al., 1999). Cabin air quality depends on the amount of air flow, cabin air distribution, the proportion of
Fig. 3. Humidity and temperature recorded on a flight London–Mailand; from Barnes (1973).
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passengers (Kenyon et al., 1996; Olsen et al., 2003). Aircraft operators should provide a minimum of 7 L/s/p of outside air per passenger at all times, because this is the minimum necessary to maintain CO2 concentrations below 1000 ppm. Use of recirculated air should be limited to not more than 50% (Dechow et al., 1997). Unfortunately, increasing the amount of fresh air also decreases the relative humidity of cabin air, because air at high altitude has very low water content.
sickness such as nausea, dizziness, and headache, or to gastrointestinal distress. Most of the descriptive factors related to cabin environment are not of significance for a healthy person. But an interaction of factors and maintained exposure over more than 12 h may result in adverse effects even in healthy people. In order to avoid the stresses of air travel and improve passenger well being, airlines sometimes provide information to their passengers as to what to do (e.g., drink fluids and inseat exercise) and what to avoid (e.g., alcohol).
7.2. Humidity
9. Future perspectives
Comfortable humidity conditions are considered to be 40–70% relative humidity at sea level (Fig. 3). The guidelines for humidity in aircraft cabins are between 12% and 22% relative humidity. Cabin air humidity depends upon aircraft type, cruising altitude, ventilation rate and the number of people on board. In-flight measurements have shown relative cabin humidity dropping from at least 47% to 11% within 30 min of ascent (Eng et al., 1982). At the end of a long distance flight, the humidity can be as low as 2–3%. Low humidity leads to dryness of the lips and skin and other exposed membranes in the nose, eyes, mouth, and throat. It may lead to dehydration, especially compounded by diuretic effects of drugs, alcohol or caffeine. Passengers wearing contact lenses may experience corneal discomfort as a result of the low humidity, and they should be advised to consider wearing spectacles during long flights. Questionnaire surveys on long-haul cabin crews found that 60% felt that the cabin humidity was too low. The main symptoms reported were dry, itchy or irritated eyes, dry or stuffy nose, and dryness of the skin or irritation (Lee et al., 1999). Increased fluid intake, rather than humidification is recommended (Thibeault and Krol, 1997).
The aircraft industry is building a new generation of airplanes with the Dreamliner from Boeing and the A380 from Airbus. Both models are bigger and faster than any airplane before. The aircraft manufacturers are interested in improving the comfort and safety of air travel for passengers and aircraft crews. In this new generation of airplanes there are some design improvements that can affect health and comfort. The A380, the biggest passenger jet ever built, is environmentally friendly. While carrying 35% more passengers than its competitor, it produces half the noise on take off and burns 12% less fuel per passenger. Airbus has built the industry's largest cabin simulator to investigate the consequences of cabin vibrations and flight motion for passenger comfort. The Boeing 787 Dreamliner will improve the flying comfort for passengers and flight crews by offering higher air humidity, better air quality, lower cabin altitude and also a smoother, quieter flight. Cabin altitude will be reduced to 2000 ft (600 m). Economy seats will be more spacious (4 cm wider) and both aisles will be 6 cm wider (up to 55 cm). Further research on the interactions of different factors of cabin environment and individual health conditions reduce nausea and distress and could contribute further to passenger comfort and well being.
7.3. Noise Travelling by aircraft includes exposure to a high level of engine noise, as well as noise from cabin ventilation systems, airflow over the external surfaces, service on board, and other passengers. Although noise within the cabin may sometimes seem excessive on some commercial aircrafts, in-flight measurements have not revealed prolonged levels above 85 dB, a level at which ear protection is advisable because of the risk of acoustic trauma. For sensitive persons or frequent travellers, an active noise reduction headset may be useful. 8. Conclusions Some aspects of air travel are associated with adverse health effects, but major medical incidents are rare during flying. Reports of minor medical incidents during flights show large differences between airlines: The range is from 1 : 202,000 passengers to 1 : 1300 passengers (Brundrett, 2001). Not every symptom will be reported to the flight attendant. Most unreported symptoms are likely related to mild forms of motion
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H. Hinninghofen, P. Enck / Autonomic Neuroscience: Basic and Clinical 129 (2006) 80–85 Cummins, R.O., Chapman, P.J.C., Chamberlain, D.A., 1988. In-flight deaths during commercial air travel. J. Am. Med. Assoc. 258, 1983–1988. Dechow, M., Sohn, H., Steinhanses, J., 1997. Concentrations of selected contaminants in cabin air of airbus aircrafts. Chemosphere 35 (1–2), 21–31. Enck, P., Mueller-Sacks, E., Holtmann, G., Wegmann, H., 1995. Gastrointestinal problems in airline crew members. Z. Gastroenterol. 33, 513–516. Eng, W., Harada, L., Jagerman, L., 1982. The wearing of hydrophilic contact lenses aboard a commercial jet aircraft. Aviat. Space Environ. Med. 53 (3), 235–238. Ernsting, J., 1978. Prevention of hypoxia-acceptable compromises. Aviat. Space Environ. Med. 3, 495–502. Gendreau, MA., DeJohn, C., 2002. Responding to medical events during commercial airline flights. N. Engl. J. Med. 346, 1067–1073. Giangrande, P.L.F., 2000. Thrombosis and air travel. J. Travel Med. 7, 149–154. Gong, H., 1992. Air travel and oxygen therapy in cardiopulmonary disorders. Chest 101, 1104–1113. Hagelsten, J., Nolte, H., 1963. Medizinische Probleme beim Lufttransport Kranker und Verletzter (Medical problems during air transportation of patients). Anaesthesist 12, 271–277. Hinninghofen, H., Musial, F., Kowalski, A., Enck, P., 2006. Gastric emptying effects of dietary fiber during 8 hours at two simulated cabin altitudes. Aviat. Space Environ. Med. 77 (2), 121–123. Hocking, M.B., 1998. Indoor air quality: recommendations relevant to aircraft passenger cabins. Am. Ind. Hyg. Assoc. J. 59, 446–454. James, P.B., 1996. Jet “leg”, pulmonary embolism and hypoxia. Lancet 347, 1697. Kenyon, T.A., Valway, S.E., Ihle, W.W., Onorato, I.M., Castro, K.G., 1996. Transmission of multidrug-resistant Mycobacterium tuberculosis during a long airplane flight. N. Engl. J. Med. 334 (15), 933–938.
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Lee, S.C., Poon, C.S., Li, X.D., Luk, F., 1999. Indoor air quality investigation on commercial aircraft. Indoor Air 9 (3), 180–187. McFarland, R.A., 1953. Human Factors in Air Transport: Occupational Health and Safety. McGraw Hill, New York. Olsen, S.J., Chang, H.L., Cheung, T.Y., Tang, A.F., Fisk, T.L., Ooi, S.P., 2003. Transmission of the severe acute respiratory syndrome on aircraft. N. Engl. J. Med. 349 (25), 2416–2422. Peacock, A.J., 1998. ABC of oxygen: oxygen at high altitude. BMJ 317, 1063–1066. Pierce, W.M., Janczewski, J.N., Roethlisberger, B., Janczewski, M.G., 1999. Air quality on commercial aircraft. ASHRAE J. 41 (9), 26–34. Roggla, G., Moser, B., Roggla, M., 1999. Seat space on airlines. Lancet 353 (9163), 1532. Scurr, J.H., Machin, S.J., Bailey-King, S., Macki, I.J., McDonald, S., Smith, P.D., 2001. Frequency and prevention of symptomless deep-vein thrombosis in long-haul flights: a randomised trial. Lancet 357, 1485–1489. Simons, R., Krol, J., 1996. Jet lag, pulmonary embolishment and hypoxia. Lancet 348, 146. Thibeault, R., Krol, J., 1997. Cabin air quality. Aviat. Space Environ. Med. 68 (1), 80–82. Turner, M., Griffin, M.J., Holland, I., 2000. Airsickness and aircraft movement during short haul flights. Aviat. Space Environ. Med. 71, 1181–1189. Vejvoda, M., Samel, A., Maas, H., Luks, N., Linke-Hommes, A., Schulze, M., Mawet, L., Hinninghofen, H., 2000. Study on strain, workload, and circadian rhythm in cabin crews during transmeridian flights. Research Report 2000-32. Deutsches Zentrum für Luft-und Raumfahrt (DLR), Cologne.
Review
Passenger well-being in airplanes H. Hinninghofen ⁎, P. Enck University Hospitals Tübingen, Internal Medicine VI, Psychosomatic Medicine and Psychotherapy, Fronsbergstraße 23, D-72070 Tübingen, Germany
Abstract Passenger well-being is influenced by cabin environmental conditions which interact with individual passenger characteristics like age and health conditions. Cabin environment is composed of different aspects, some of which have a direct influence on gastrointestinal functions and may directly generate nausea, such as cabin pressure, oxygen saturation, and motion or vibration. For example, it has been shown that available cabin pressure during normal flight altitude can significantly inhibit gastric emptying and induce dyspepsia-like symptoms when associated with a fibre-rich meal. Other aspects of the cabin environment such as space and variability of seating, air quality, and noise, also have been shown to modulate (reduce or increase) discomfort and nausea during flights. Individual passenger characteristics and health status also have been demonstrated to increase vulnerability to adverse health outcomes and discomfort. © 2006 Elsevier B.V. All rights reserved. Keywords: Aircraft cabins; Atmospheric pressure; Air quality; Humidity; Deep vein thrombosis (DVT); Hypoxia
Contents 1. 2. 3.
Introduction . . . . . . . . . . . . . Cabin pressure and oxygen saturation Cabin pressure and gas expansion . . 3.1. Other effects of pressure . . . 4. Motion and vibration of the aircraft . 5. Seating and immobility. . . . . . . . 6. Deep vein thrombosis . . . . . . . . 7. Cabin air quality . . . . . . . . . . . 7.1. CO2 concentration. . . . . . . 7.2. Humidity . . . . . . . . . . . 7.3. Noise . . . . . . . . . . . . . 8. Conclusions . . . . . . . . . . . . . 9. Future perspectives . . . . . . . . . . References . . . . . . . . . . . . . . . . .
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1. Introduction
⁎ Corresponding author. Tel.: +49 7071 9387372; fax: +49 7071 9387379. E-mail address: [email protected] (H. Hinninghofen). 1566-0702/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.autneu.2006.07.018
Modern commercial airplanes are quiet and comfortable compared with previous generations of aircrafts. Nevertheless, there are still some aspects of the cabin environment which may cause stress and affect health in vulnerable individuals. Stressors in flight include cabin pressure and
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oxygen saturation, motion and vibration of the aircraft, space and variability of seating, air quality and humidity, and noise. While pressure or oxygen and motion or vibration may exhibit direct effects on gastrointestinal function that may result in nausea and vomiting, other factors may modulate well-being and amplify or reduce these direct effects. 2. Cabin pressure and oxygen saturation Relevant to passenger health are flight altitude and cabin altitude (Fig. 1). Modern airplanes cruise normally at an altitude between 30,000 and 40,000 ft (9150 and 12,200 m) where the atmospheric pressure is less than 30% of that at sea level. In pressurized airplanes, cabin pressure will vary from sea level (760 mmHg) to 8000 ft (564 mmHg), depending upon aircraft altitude and pressurization schedules. These pressure guidelines were established in the early days of pressurized cabins (McFarland, 1953), based on medical knowledge, aeroengineering requirements, and economics (Ernsting, 1978). The ambient pressure of oxygen is approximately 20% of barometric pressure. Because the ambient oxygen concentration remains fixed, any decrease in atmospheric pressure will lead to decreases in inspired oxygen tensions (PAO2) and arterial oxygen tension. The physiologic response to lowered PAO2 is hyperventilation as a first step to meet the body's needs. Continuing hypoxia is compensated for with tachycardia to increase cardiac output. Hypoxia is also a stimulus for atrial arrhythmias and is associated with premature ventricular contractions (Gong, 1992). The blood haemoglobin oxygen saturation level is normally 95% for healthy people at sea level. At higher altitudes (and lower pressure), saturation levels fall. Oxygen saturation levels less than 85% may lead to impairment of mental function, and appreciable handicaps will occur at 70% saturation (Peacock, 1998). Oxygen saturation in resting people in an airplane cabin with
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a cabin altitude of 8000 ft will be around 90% with a range of 85%–93% after 30 min. In dozing subjects saturation levels of around 80% were found (Simons and Krol, 1996). In general, mild hypoxia with an oxygen saturation level of 90% is of no significance to healthy individuals, but due to wide individual differences, prediction of hypoxemia in an individual air traveller is difficult. Of concern nowadays is that in-flight cabin measurements show that cabin altitudes substantially higher than 8000 ft may be allowed (Cotrell, 1988) with consequent lowering of oxygen saturation that may be dangerous for those with ischemic heart disease, chronic respiratory disease and anaemia, or sickle cell anaemia (Gendreau and DeJohn, 2002). In passengers with such conditions, the consequences of lack of oxygen may include angina, acute myocardial infarction and congestive cardiac failure. Cardiac disorders are the most common cause of in-flight death (Cummins et al., 1988). Older passengers are at increased risk because arterial oxygen tension tends to fall after 40 years of age (James, 1996). 3. Cabin pressure and gas expansion Low cabin pressure also leads to expansion of gases: in accordance with Boyle's law, gases expand up to 35% when ascending from sea level to 8000 ft of altitude (Table 1). The body has a number of air-filled cavities: the middle ear and sinuses, the gut, pleural cavities, tooth fillings and the skull. Because pressure changes during ascent and descent occur very slowly, the majority of travellers experience no adverse effects apart from slight bloating or a sensation in the middle ear. Nevertheless, gas in body cavities can cause discomfort if equilibration with the ambient barometric pressure is not possible. Expansion of gas in the stomach or duodenum can lead to discomfort, nausea or vomiting. Gas expansion within the gut
Fig. 1. Cruising altitude and corresponding cabin altitude. Cruising altitude of modern airplanes is shown on the left Y-axis, while cabin altitude is shown on the right Y-axis.
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Table 1 Gas expansion in body cavities in relation to environmental pressure (Hagelsten and Nolte, 1963) Cabin altitude Meters
Feet
Sea level 1600 3300 5000 6600
Sea level 5000 10 000 15 000 20 000
Relative volume of gas in body cavities 1.0 1.2 1.5 1.9 2.4
contributed to significant increase in dyspeptic symptoms among flying staff compared with ground personnel of a commercial airliner (Enck et al., 1995). On long-distance flights, cabin crew reported significantly more bloating as compared to a ground-based period (Vejvoda et al., 2000). Fiber intake is often increased to compensate flight-related intestinal problems such as constipation (Enck et al., 1995). High fibre diets may in turn contribute to intestinal gas volume and amplify the pressure-induced problems. In an attempt to identify the mechanisms by which dyspeptic symptoms (bloating, nausea) are generated during flights, we simulated an 8-h flight in a hypobaric chamber in 16 healthy males (25.5 ± 5.9 years), subjected to two meal conditions on separate days in randomized sequence (with and without a fibre-supplemented standard meal, 2 g or 20 g, respectively), but randomly assigned to either a flight altitude of 2500 m (8200 ft = 565 mmHg cabin pressure) or 1000 m (3280 ft = 637 mmHg). The subjects were uninformed about altitude and fibre content of the meal. Gastrointestinal symptom ratings were taken every hour, and gastric emptying was assessed by conventional 13C-octanoic acid breath-test. The test based on the use of 13C-octanoic acid, a medium chain fatty acid which after ingestion is rapidly absorbed in the duodenum and metabolised in the liver. Following oxidation, the resulting 13CO2 is excreted into breath at a level which can be detected and measured by infrared spectrometry. In a separate experiment, we examined the effect of the two test meals (2 g vs. 20 g of fibre) on the same measures in 30 healthy males (26.7± 6.2 years) under conventional laboratory conditions. We found (Hinninghofen et al., 2006), gastric emptying (half emptying time =T1/2) was significantly delayed at 2500 m altitude when a high fibre meal was given (146.3± 58.4 min vs. 193.9 ± 54.3 min) (Fig. 2). Symptom reports of gastric distension and bloating were increased at 2500 m for the high fibre meal compared with the low fibre meal. There were no differences between groups on any measure for the ground control condition. This supports the claim that cabin pressure and oxygen saturation affect gastrointestinal functions such as gastric emptying and this may mediate dyspeptic symptoms such as bloating, distension or nausea during flights. 3.1. Other effects of pressure Failure to equalize pressure within the middle ear and the environment can cause otitic barotrauma. In up to 9% of air
travellers, otitic barotrauma occurs with attendant ear pain, tinnitus, vertigo or hearing loss (Csortan et al., 1994). Therefore, individuals with conditions causing a blockade of the Eustachian tube such as upper respiratory infection, allergy, or sinusitis should not fly until the blockade is cleared. People with recent surgery should contact their physician before they fly. To facilitate the gas equalization during ascent and descent, passengers should swallow frequently or increase pressure in the nasopharynx by performing a Valsalva manoeuvre (Hold the nose closed by pinching, close the mouth and blow gently. This raises the pressure in the pharynx, forcing air up the Eustachian tubes into the middle ear). 4. Motion and vibration of the aircraft Humans normally are in an environment where the force of gravity is constant in direction and magnitude. During air travel, passengers may perceive unusual vibration, motion, and centrifugal forces. Additionally, air turbulence can cause a linear vertical motion on the vestibular organ. In combinations, these may induce discomfort and motion sickness. A motion sickness study during air travel found that 0.5% of the passengers had vomited and 8.4% reported nausea during flight (Turner et al., 2000). Motion sickness tends to be severe during exposure to turbulent flight conditions. In vulnerable individuals, protracted vomiting may cause dehydration and electrolyte imbalance. Predisposed passengers should be advised to take preventive medication and to choose a seat over the wings, in the centre of gravity of the aircraft. Flying at night might be helpful by reducing visual stimulation (AMA, 1982). In general, aircraft vibration induced by the engine is usually well tolerated, but together with aircraft motion, noise and low humidity, it may cause some degree of discomfort and contribute to travel fatigue. A more detailed description of the general contribution of motion and vibration to the development of motion sickness, nausea and vomiting can be found in the contributions of E. Muth and J. Golding in this volume.
Fig. 2. Displayed gastric emptying time (T1/2) as a function of the fibre content of the test meals for both simulated altitudes. Gastric emptying was significantly delayed at 2500 m altitude, if a high fibre diet was given (p = 0.039) (from: Hinninghofen et al., 2006).
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5. Seating and immobility Planes today are able to fly non-stop more than 15 h, which makes seat comfort important for passenger wellbeing. Seat comfort is associated with seat pitch, seat width, leg room, quality of upholstery, and possible angle of recline. Seat pitch is the distance between the back of one seat to the same point on the back of the seat in front. The range for seat pitch is from 71 cm (28 in., charter planes) to 152 cm (60 in., first class). Seat pitch in economy travel is 76–86 cm. Close seat pitch is associated with poor seat comfort and restricts the degree of tilt of the seat, a feature which is important for night travel. Seat width is also important for comfortable seating, with 30% of seats narrower than the recommended 42 cm (17 in.) (Roggla et al., 1999). Cramped seating is not only uncomfortable. It makes it difficult to leave the seat for regular exercise, disturbs respiration, restricts the gastrointestinal transit and normal blood circulation, and can cause oedema and ischemia of the lower limbs. Surveys on charter and economy class flights have consistently rated seat comfort and leg room as being the two least satisfactory characteristics of air travel (Anonymous, 2000). 6. Deep vein thrombosis The term “economy class syndrome “ refers to the development of deep vein thrombosis (DVT) and pulmonary embolism (PE) in air travellers flying for long distances in cramped economy class seats. But, as shown recently, there is no clear evidence of a specific risk associated with air travel; DVT and pulmonary embolism are potentially a general problem of prolonged travel in any confined spaces (Bagshaw, 2001). The incidence of pulmonary embolism among 135 million passengers arriving at Charles de Gaule airport was much greater among those who had travelled more than 10,000 km (4.8 per million) (Ansell, 2001). Flight duration and individual predisposing seems to be a critical trigger. Both complications are likely to occur in those with risk factors such as venous disease, heart failure, nephrotic syndrome, thromboembolic illness, hypercoagulable disorders, diabetes or age. Women are at higher risk than men (Giangrande, 2000). Leg oedema (jet flight leg), which may mimic DVT, is usually benign and resolves after landing. Possible measures to prevent DVT are: in-flight muscle-contracting exercises, regular walks, use of compression stockings, adequate (non-alcoholic) fluid intake, and the use of drugs such as aspirin and subcutaneous heparin (Scurr et al., 2001).
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recycled air in the aircraft cabin environment, the relative humidity, and the number of people in the cabin. The ventilation rate on board an aircraft is determined to provide fresh air to dilute odour to an acceptable level for 80% of passengers. Odours and volatile organic compounds can be added from furnishings and humans, lubricating oils, hydraulic fluids and de-icing fluids entering through open doors whilst the plane is on the ground. Even if not toxic, odorous air can itself cause nausea, headache and loss of appetite. 7.1. CO2 concentration Air travellers remain for prolonged time with the smallest available air space per person of any other current social setting (Hocking, 1998). Without any air change, it would take only 2.3 min for the concentration of CO2 in that space to exceed 1000 ppm. CO2 concentration is normally taken as an important measure of air quality. Recommended concentration is less than 1000 ppm. Measurements in business class non-smoking flights not uncommonly shows concentrations greater than 1000 ppm with peaks of 2900 ppm (Lee et al., 1999). Extremely high CO2 concentrations were found during take-off and landing, when power requirements reduce the amount of ventilation. Since the 1980s, half of the air exchange consists of recycled air from the passenger cabin. As the proportion of recirculated air increases energy savings can be made, but at the same time health risks are increased. Early experiments on aircraft found 9 L/s/p (Litre/second/person) of fresh air were adequate for the comfort for most passengers (McFarland, 1953). Today most commercial aircraft are only capable to provide 2.8 L/s/p of outside air to their passenger cabins. The airline industry saves more than $30 million annually by reducing energy costs due to recycling air with inadequate ventilation (Consumer Reports, 1994). Beside the reported symptoms, of particular concern is the increased risk of disease transmission in this setting. Organisms transmitted by droplets less than 10 μm in size, such as measles, influenza viruses, and tuberculosis may spread between
7. Cabin air quality Acceptable air quality in commercial aircraft is important for the comfort and well being of passengers and flight crews. Inadequate air quality may cause symptoms of fatigue, headache, and dizziness, as well as respiratory and ocular discomfort (Pierce et al., 1999). Cabin air quality depends on the amount of air flow, cabin air distribution, the proportion of
Fig. 3. Humidity and temperature recorded on a flight London–Mailand; from Barnes (1973).
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passengers (Kenyon et al., 1996; Olsen et al., 2003). Aircraft operators should provide a minimum of 7 L/s/p of outside air per passenger at all times, because this is the minimum necessary to maintain CO2 concentrations below 1000 ppm. Use of recirculated air should be limited to not more than 50% (Dechow et al., 1997). Unfortunately, increasing the amount of fresh air also decreases the relative humidity of cabin air, because air at high altitude has very low water content.
sickness such as nausea, dizziness, and headache, or to gastrointestinal distress. Most of the descriptive factors related to cabin environment are not of significance for a healthy person. But an interaction of factors and maintained exposure over more than 12 h may result in adverse effects even in healthy people. In order to avoid the stresses of air travel and improve passenger well being, airlines sometimes provide information to their passengers as to what to do (e.g., drink fluids and inseat exercise) and what to avoid (e.g., alcohol).
7.2. Humidity
9. Future perspectives
Comfortable humidity conditions are considered to be 40–70% relative humidity at sea level (Fig. 3). The guidelines for humidity in aircraft cabins are between 12% and 22% relative humidity. Cabin air humidity depends upon aircraft type, cruising altitude, ventilation rate and the number of people on board. In-flight measurements have shown relative cabin humidity dropping from at least 47% to 11% within 30 min of ascent (Eng et al., 1982). At the end of a long distance flight, the humidity can be as low as 2–3%. Low humidity leads to dryness of the lips and skin and other exposed membranes in the nose, eyes, mouth, and throat. It may lead to dehydration, especially compounded by diuretic effects of drugs, alcohol or caffeine. Passengers wearing contact lenses may experience corneal discomfort as a result of the low humidity, and they should be advised to consider wearing spectacles during long flights. Questionnaire surveys on long-haul cabin crews found that 60% felt that the cabin humidity was too low. The main symptoms reported were dry, itchy or irritated eyes, dry or stuffy nose, and dryness of the skin or irritation (Lee et al., 1999). Increased fluid intake, rather than humidification is recommended (Thibeault and Krol, 1997).
The aircraft industry is building a new generation of airplanes with the Dreamliner from Boeing and the A380 from Airbus. Both models are bigger and faster than any airplane before. The aircraft manufacturers are interested in improving the comfort and safety of air travel for passengers and aircraft crews. In this new generation of airplanes there are some design improvements that can affect health and comfort. The A380, the biggest passenger jet ever built, is environmentally friendly. While carrying 35% more passengers than its competitor, it produces half the noise on take off and burns 12% less fuel per passenger. Airbus has built the industry's largest cabin simulator to investigate the consequences of cabin vibrations and flight motion for passenger comfort. The Boeing 787 Dreamliner will improve the flying comfort for passengers and flight crews by offering higher air humidity, better air quality, lower cabin altitude and also a smoother, quieter flight. Cabin altitude will be reduced to 2000 ft (600 m). Economy seats will be more spacious (4 cm wider) and both aisles will be 6 cm wider (up to 55 cm). Further research on the interactions of different factors of cabin environment and individual health conditions reduce nausea and distress and could contribute further to passenger comfort and well being.
7.3. Noise Travelling by aircraft includes exposure to a high level of engine noise, as well as noise from cabin ventilation systems, airflow over the external surfaces, service on board, and other passengers. Although noise within the cabin may sometimes seem excessive on some commercial aircrafts, in-flight measurements have not revealed prolonged levels above 85 dB, a level at which ear protection is advisable because of the risk of acoustic trauma. For sensitive persons or frequent travellers, an active noise reduction headset may be useful. 8. Conclusions Some aspects of air travel are associated with adverse health effects, but major medical incidents are rare during flying. Reports of minor medical incidents during flights show large differences between airlines: The range is from 1 : 202,000 passengers to 1 : 1300 passengers (Brundrett, 2001). Not every symptom will be reported to the flight attendant. Most unreported symptoms are likely related to mild forms of motion
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