Hypoxia is a state of oxygen deficiency in the body sufficient to impair functions of the brain and other organs. Because of the nature of flight, flight crews are much more likely to suffer from hypoxia than “normal” people. Knowing what to look for and how to react to resolve the situation is essential to maintain flight safety. This Briefing Note defines hypoxia and describes the symptoms and performance decrements that can result from it. It is important for flight crews to understand the warning signs of hypoxia and how the human body responds to reduced levels of oxygen. Also included are some techniques that can help a flight crew member defend against the effects of hypoxia.
Hypoxia from exposure to altitude is due entirely to the reduced barometric pressures encountered at higher altitudes. The concentration of oxygen in the atmosphere does not change as altitude increases; rather it stays constant at about 21%. Because of the decrease in barometric pressure, however, there is less atmosphere (air) at higher altitudes, which results in less available oxygen.
The first hypoxia-related casualties were reported in 1878 by Paul Bert as balloonists traveled high into the atmosphere. Despite technological advances in aviation, hypoxia still occurs today.
Hypoxia can occur quickly, and the body’s ability to adapt to a low-oxygen condition is poor when the onset is fast. Flight crews must be well-informed of the causes and consequences of hypoxia in flight and know how to protect themselves from such conditions.
Stages of Hypoxia
In aeronautics, hypoxia typically results from a decompression or lack of pressurisation of the aircraft cabin. Hypoxia occurs within a few minutes if the cabin pressure altitude rises to between 5,000-6,000 m (about 16,000 - 20,000 ft). Acute hypoxia is characterised by impaired cognitive performance and sometimes a loss of consciousness.
If there is a cabin rupture or other cabin depressurisation that occurs extremely quickly, hypoxia can occur within a few seconds, especially if cabin pressure altitude is higher than 7,500 m (about 25,000 ft). This sudden onset hypoxia is termed fulminant hypoxia. At high altitudes, loss of consciousness occurs within a few seconds without any warning symptom. A “normal” person generally feels nothing prior to loss of consciousness and will be unable to recall the incident.
If the loss of cabin pressure can be resolved quickly, the crew can regain consciousness within 20 seconds. A person will, however, experience painful earaches due to rapid descent or emergency repressurisation.
Acute hypoxia is the term classically used to describe the different temporary clinical effects of hypoxia on cognitive performance, behaviour, mood and the senses. Hypoxia is particularly dangerous because its signs and symptoms do not usually cause discomfort or pain. The onset of symptoms is insidious, and tolerance can vary both among individuals and on a daily basis for any particular person.
The effects of hypoxia begin immediately upon exposure to any altitude above sea level. Below 3,500 m (about 11,000 ft), the performance decrements are minimal and normally go unnoticed. Decreased night vision and drowsiness are usually the only detectable issues at low altitudes.
Hypoxia can be recognised from both objective (i.e., capable of being perceived by an observer) and subjective (i.e., perceived by the pilot only) symptoms. Objective signs include increased rate and depth of breathing, tachycardia, cyanosis (blue coloured lips and nails), mental confusion, anger, euphoria, poor judgment, loss of muscle coordination, slouching and loss of consciousness. Behavioural changes may be noted by the hypoxic individual, as well as by the observer. The subjective symptoms include breathlessness, apprehension, headache, dizziness, fatigue, nausea, hot and cold flashes, blurred vision, tunnel vision, tingling, and numbness.
The Four Stages of Hypoxia
Hypoxia can be classified into four stages based on altitude and the associated performance decrements and physiological symptoms.
Indifferent Stage, 0 - 1,500 m (0 - 5,000 ft)
No physiological responses or performance decrements related to hypoxia are typically observed between these altitudes for a person in good health.
Complete Compensatory Stage, 1,500 - 3,500 m (5,000 - 11,400 ft)
Visual sensitivity at night is decreased by 10 percent at 1,500 m (5,000 ft) and by 30 percent at 3,000 m (10,000 ft). Performance of new tasks may be impaired due to memory issues. The nervous system, however, is able to maintain its primary functions and performance, for the most part, is unaffected.
Partial Compensatory Stage, 3,500 - 6,000 m (11,400 - 20,000 ft)
Between these altitudes, a drastic increase in breathing is needed to maintain proper cardiovascular function. Nervous system functioning begins to degrade, but there can also be great individual variability in the symptoms for a given altitude.
Cognitive disturbances are typical at these altitudes. They are characterised by two main components:
- Loss of self-monitoring and cognitive feedback
- Difficulty in thinking
The absence of self-monitoring makes it impossible for an individual to recognise whether his or her actions are hazardous. This, combined with slow thinking, can be extremely dangerous. Many times fixation occurs or there is a tendency to repeat an action without realising that the action was just completed moments before. Judgment becomes extremely poor and physical movement becomes uncoordinated.
A pilot often will have trouble concentrating or may have difficulty reading instruments. Delayed and/or imprecise communications may result. Frequently, alterations in a pilot’s voice are the first signs that something is wrong. An example is when a pilot attempts to deliver altitude information to the controller, but there is a noticeable delay and the pilot has a lazy, dull tone to his or her voice. Many potential accidents have been prevented when a controller recognises these symptoms and notifies the pilot of the need to take corrective action.
Such situations demonstrate the importance of knowing the symptoms of hypoxia and the correct actions that must be taken to resolve the situation. In a case such as the one described above, it is important that the controller use strong instructions to the pilot to take corrective action. Because the crew may already be impaired by hypoxia, they may have to be convinced that there actually is a problem.
Effects of hypoxia on behaviour and mood
Sometimes a pilot receives instructions but is unable to properly perform the mental or physical tasks needed to perform the required actions. Figure 1 (below) shows how a simple mental task becomes very difficult at high altitudes. A person was asked to do a simple arithmetic task in an altitude chamber. The altitude level was set at 6,000 m (20,000 ft). The test consisted of counting backwards from 1,000 by increments of two. Almost immediately, the subject made a large calculation error (went from 990 to 888), and writing was impaired to the point where it was almost illegible. The subject was able to recover a few seconds after receiving oxygen.
Figure 1: Simple arithmetic task in altitude chamber (6,000 m,; 20,000 ft); 
It is difficult to predict at what altitude behavioural disturbances will occur and how long a person must be exposed to a particular altitude before the onset of a disturbance. Experiences may differ for the same pilot on different days. Thus, it is nearly impossible for a pilot to know exactly how his or her body will respond under certain conditions.
Mood disturbances are generally extreme and can include deep sorrow, uncontrollable laughing, nervous exhaustion, attacks of aggressiveness and antisocial actions. Sometimes a crew may appear to be drunk, and fighting between crew members may occur. In most instances, only breathing more oxygen will resolve the situation.
Sometimes a crew will enter a deep depressive state and will experience a complete lack of will to conduct a task. The crew may still be able to analyse the situation, but they are unable to mount any practical response to it. “Nothing can be done” is a frequent comment by a pilot who is unable to act appropriately. Other times, a crew may react with a behaviour that is the exact opposite of the behaviour that should be implemented. For example, there have been cases of crews intentionally depressurising the cabin when there was a failure of the oxygen system.
The end of a hypoxia-involved crisis is usually very evident. It often ends with a euphoric phase. This is often reported in debriefings of military pilots who will sometimes submit themselves to hypoxia intentionally. Some cases of addiction to hypoxia and the euphoric state it can induce have been reported among military pilots.
- Vision is the first of the senses to be affected by a lack of oxygen. This is especially true of night vision, which may be affected as low as 1,500 m (5,000 ft) of altitude. Color vision starts to deteriorate between 1,500-3,000 m (5,000 - 10,000 ft). At higher altitudes, the ocular muscles become weakened and uncoordinated. The range of accommodation is decreased which causes blurring of near vision and subsequently difficulty in carrying out near-vision-related tasks. Above 5,000 m (10,000 ft) of altitude, the visual disturbances are more severe as reaction time and responses to visual stimuli becoming sluggish. Accommodation and convergence are weakened and cause double vision. All problems, however, can usually be reversed by the use of oxygen or a return to sea level.
- Hearing is particularly resistant to hypoxia which is one reason many crew members have been saved from near tragedy. Radio is the best way to communicate with a pilot suffering from acute hypoxia. Simple orders from pilots in other planes, controllers, or flight engineers have been used to guide pilots to safety.
Critical Stage, above 5,500 m (18,000 ft)
Above this altitude, complete incapacitation can occur with little or no warning. All senses fail, and a pilot will become unconscious within a very short period of time. No stimuli such as the radio will be able to help a pilot suffering from hypoxia, especially fulminant hypoxia, above 5,500 meters (18,000 feet).
Time of useful consciousness (effective performance time)
Time of Useful Consciousness is defined as the amount of time an individual is able to perform proper corrective or protective actions under hypoxia in flight. This definition explains why it is more useful to talk about effective performance time (EPT) rather than time of useful consciousness.
It is difficult to estimate universal values for EPT due to individual variability influenced by endurance, experience, physical exercise and the situation under which exposure to high altitude has occurred. Two factors are crucial: the proportion of O2 in the inspired gas prior to the decompression and the level of metabolic activity at the time of decompression. Approximated values of time of useful consciousness under air or O2 breathing, and for these two conditions at rest or under moderate physical exercise are shown in Table 1.
Table 1: Time of useful consciousness
Figure 2 emphasises the deleterious effect of rapid decompression at high altitude on EPT.
At the optimal level of cabin altitude of 2,500 m (8,000 ft), the pressure of O2 in the lungs and in the pulmonary vessels (PAO2 and PvO2, respectively) are equal to 96 and 40 hPa, such that O2 will flow from the lungs to the blood. Upon rapid decompression at 12,000 m (39,000 ft), PAO2 plummets so drastically and so quickly that it becomes lower than PvO2. As a result, there is an immediate reversal of oxygen flow from the blood to the lung within four to five seconds following the decompression. This depletes the blood’s oxygen reserve and reduces the EPT at rest by up to 50 percent. Loss of consciousness usually occurs within 10 seconds. However, loss of consciousness does not mean that breathing will stop. If a pilot puts the O2 mask on his face within the 5 seconds following the decompression, the lung pressure in O2 increases to an effective value (80 hPa) and as result, recovery from hypoxia occurs within seconds.
Two operational consequences result from this phenomenon:
- When a transport aircraft flies above 7,600 m (25,000 ft), the rapidness of loss of consciousness makes it mandatory to provide the crew an O2 device that can be fitted on the face within five seconds. In case of cabin decompression, putting the mask on the face becomes an immediate and time critical emergency. It is mandatory that the crew be effectively trained with the procedures for donning the mask effectively. Crews must be conditioned to apply this procedure by reflex prior to taking any other action.
- The threat of large variations in PAO2 following a decompression supports the delivery of a gas enriched in O2 into the cockpit in case of decompression when flying above 2,500 m (8,000 ft). If the inhaled air contains at least 40 percent additional oxygen, PAO2 will generally remain within proper limits when between 2,500 - 12,000 m (8,000 - 39,000 ft). FAR 121.33e in the U.S. requires that above 25,000 feet (7,600 m) one of the two pilots always be fitted with an O2 mask. If the O2 mask is of the quick-fitting type, it is required to be worn only above 41,000 ft (12,500 m).
Figure 2: Evolution of the lung pressure in O2
) following a rapid decompression. The O2
mask is put on the face within 5 seconds
There are two methods for preventing hypoxia. One method involves increasing barometric pressure to a minimal value such that the concentration of oxygen is sufficient to prevent hypoxia from occurring. This is generally done by pressurising the aircraft cabin. The other method is to increase the breathable oxygen in a system through the use of an O2 device. This is usually accomplished by having a pilot wear an oxygen mask. It is also possible to combine the methods to provide greater assurance that hypoxia is prevented.
Pressurisation keeps the cabin barometric pressure (PBc) at a value higher than the barometric pressure (PBz) corresponding to the flight level. The inner-outer pressure difference is generated by the difference between the incoming and outgoing air flows.
What is controlled is either the cabin pressure or the inner-outer pressure difference. Figure 3 demonstrates the relationship between barometric pressure and altitude and how pressurisation affects this relationship. In the initial part of the ascent, the cabin pressure equals the outer pressure, meaning that pressurisation is not triggered. In the second part of the ascent, the pressurisation device works to keep a constant cabin pressure. In the last part of the ascent, the pressurisation device works to keep a constant pressure difference, allowing pressure to decrease so long as a constant difference between the cabin pressure and outside pressure is maintained.
Figure 3: Cabin pressurisation during the ascent of the aircraft
Because aircraft pressurisation is effectively controlled by on-board systems, crew and passengers can travel at high altitudes safely and in comfort. Supplemental oxygen devices are usually not required, and everyone is free to move about the cabin unhampered by oxygen masks or other equipment.
As required in FAR and JAR 25, §841 concerning civilian transport aircraft, hypoxia is prevented by maintaining a cabin altitude below 8,000 feet (2,500 meters) in normal flight conditions and below 15,000 feet (4,500 meters) in case of “reasonably” likely conditions of failure; the main components of the pressurisation device must be at least redundant. Supplemental oxygen is required only when cabin pressurisation fails.
O2 devices are used in emergencies involving pressurisation failure or the presence of smoke or fumes. In normal night flying conditions, it can also be advantageous for a pilot to use supplemental oxygen, especially in final approach in order to overcome the impairment of nocturnal vision normally experienced at 2,500 meters (8,000 feet) of cabin pressure altitude.
FAR 121 prescribes the use of O2 devices as a function of altitude (§327 to 337). Its key points are as follows:
- Supplemental O2 is mandatory for all pilots in the range of flight levels 100-120, except if the flight at this altitude lasts less than 30 minutes; above FL 120, the use of supplemental O2 is mandated without condition.
- O2 must be available for at least 10 percent of the passengers in the range of FL80-FL120, except if the flight at this altitude lasts less than 30 minutes; same demand for at least 30 percent of the passengers between FL120 and FL140. O2 delivery for all passengers above FL150.
So far, there is no corresponding JAR requirement.
Above 12,200 m (40,000 ft), 100 percent oxygen must be breathed with additional pressure to achieve adequate oxygenation to prevent hypoxia. This is termed Positive Pressure Breathing (PPB). The level of positive pressure is generated by the O2 device as a function of the altitude level required to maintain PAO2 at the minimal value that will allow a pilot to perform flight-saving procedures within a few minutes. In other words, positive pressure is equal to the difference between the environmental barometric pressure and the needed lung pressure (196 hPa) allowing PAO2 to be kept at 80 hPa.
Technique for pressure breathing
During PPB, breathing patterns are inverted as inspiration becomes passive and easy and expiration becomes active and difficult. As a result, breathing must be consciously controlled to avoid hyperventilation. Practice is required to become accustomed to this reversed breathing pattern.
The best technique for PPB is as follows:
- Establish mental discipline to control breathing
- When inhaling, maintain a conscious tension of the respiratory muscles (diaphragm and abdominal muscles). Control the expansion of the thorax through muscle tension. As inhalation progresses, steadily decrease muscle tension to allow progressive lung inflation
- Pause when the desired lung inflation has occurred
- When ready to exhale, positively increase muscle tension for a steady, smooth exhalation
- Pause and breathe at a rate slower than normal.
Summary of key points
- Hypoxia remains a concern for all individuals involved in flight safety.
- Hypoxia is dangerous because it impairs cognitive and physical performance, sometimes without the flight crew realising that anything is wrong.
- There is no way for physiological adaptation to hypoxia in the aeronautical environment when there is a rapid decrease in barometric and lung pressure.
- Protection against hypoxia is mandatory. The main protection tool is the pressurisation of the cabin. In case of cabin pressurisation failure, supplemental oxygen is required, according to some international mandates.
- Pilots must be aware of issues related to hypoxia and must be trained to wear and breathe through an O2 mask.
- Above FL 400 and in case of pressurisation failure, O2 is delivered to the pilots with a positive pressure. A pilot must be trained as to the proper technique for positive pressure breathing.
The following Briefing Notes and Checklists complement the above issues:
- ^ Marotte (H.).- L’hypoxie. In: Physiologie aéronautique. Editions S.E.E.S., Lognes, France, 2004, pp. 27-50. (from Dr Henri Marotte, personal documentation)
- ^ Sheffield (P.) ; Heimbach (R.D.).- Respiratory physiology.- In: Fundamentals of Aerospace Medicine, DeHart (R.) eds., Lea and Febiger, Philadelphia, 1985, pp. 91-102
- ^ a b Marotte (H.), Toureé (C.), Clère (J-M.), Vieillefond (H.).- Rapid decompression of a transport aircraft cabin: protection against hypoxia. Aviat. Space Environ Med, 1990, 28(111), pp. 201-203.
- Aviation medicine.- Ernsting (J.) ; Nicholson (A.) ; Rainford (DJ) eds., 3rd ed., Butterworth-Heinemann, Oxford, 1999.
- Hackworth (C.A.) ; Peterson (L.M.) ; Jack (D.J.), Williams (C.A.), Hodges (B.E.).- Examining hypoxia: a survey of pilots’ experiences and perspectives on altitude training. Report DOT/FAA/AM-03/10, 2003.