Flying into thin Air: Understanding Hypoxia

Undoubtedly cabin pressurisation and oxygen systems have allowed unhindered aviation activities, with a caveat though – never to cross the altitude beyond the capabilities of the system on board. Thus, commercial aircraft fly maintaining a cabin pressure of 6000-8000 ft, and unpressurised small aircraft mostly operate below 10000 ft. Combat aircraft may have a higher service ceiling, but their onboard oxygen systems, invariably, are ‘get-you-down’ devices only. Thus the threat of hypoxia is ever present with accidental loss of cabin pressurisation or when flying beyond the mandated altitude. Although of major interest in military aviation, hypoxia remains a threat in civil aviation as well. Remember the crash of Helios Airways Boeing 737-300, on August 14, 2005 in Greece, where 6 crew and 115 passengers perished due to lack of pressurisation.

So, what is HYPOXIA? 


Hypoxia is defined as lack of Oxygen in the body tissues due to decreased quantity and molecular concentration. In aviation, it occurs due to fall in partial pressure of Oxygen in the inspired air with increasing altitudes, beyond the possible human physiological compatibility.

Conventionally, hypoxia is classified into four different types:-

Hypoxic Hypoxia. This can occur due to reduction of a partial pressure of Oxygen in the inspired air; reduction in alveolar ventilation or hypoventilation; alveolar capillary diffusion block; or,  ventilation perfusion defects due to lung diseases.

Anaemic Hypoxia.  This results when there is a reduction in Oxygen carrying capacity of the blood due to decreased haemoglobin content. This is commonly due to poor nutritional state. Carbon monoxide, nitrates, sulfa drugs etc. could also cause this type of hypoxia by forming stable compounds with haemoglobin and reducing the amount of haemoglobin available to transport Oxygen to the tissues.

Stagnant or Hypokinetic Hypoxia. This form of hypoxia is due to malfunction of the circulatory system where the oxygen carrying capacity of the blood is adequate but there is inadequate circulation of the blood. Conditions such as heart failure, arterial spasm, occlusion of blood vessel, and in aviation pooling of blood in lower limbs during aerial combat manoeuvres (+Gz acceleration) would predispose to stagnant hypoxia.

Histotoxic Hypoxia. This occurs when the utilisation of Oxygen by the body tissues is interfered with. Alcohol, narcotics and certain poisons such as cyanide interfere with the ability of the cells to make use of the oxygen available to them even though the supply is normal in all respects.

Although all the four types of hypoxia may be encountered in flight, the most frequent and important type of hypoxia encountered in aviation is hypoxic hypoxia (a.k.a. Hypobaric hypoxia), caused by breathing air at altitude. The partial pressure of Oxygen in the inspired air progressively reduces as compared to breathing air at sea level. The principal causes of accidental hypoxia in flight are ascent to altitude without supplemental oxygen; failure of personal breathing equipment; or decompression of the pressure cabin.

There are four stages of hypoxia, as per the altitude and the available partial pressure of Oxygen. The stages of hypoxia are as given in table below.

Hypoxia can produce a multitude of effects, with the severity of the effects depending on the degree of hypoxia. It is pertinent to remember that an understanding about hypoxia is vital for the pilots because its onset is insidious and there is no discomfort or pain brought about by hypoxia.

The signs and symptoms of hypoxia become apparent as the degree of hypoxia increases. This includes:-

  • Breathlessness/ air hunger
  • Excessive yawning
  • Tiredness and fatigue
  • Euphoria
  • Impairment of performing recently learnt task
  • Impairment of mental task (learnt tasks)
  • Altered sensorium, including loss of consciousness

Hypoxic changes observed are due to:

  • Compensatory mechanisms of the body
    • Increase in respiratory rate.
    • Increased depth of respiration
  • Impaired cellular functions as a result of decreased oxygen availability
    • Decreased night visual acuity
    • Impaired psychomotor performance

It is important to remember that there are several factors which affect the onset and the severity of the effects of hypoxia. This includes:-

  • Altitude.  Higher the altitude, lower is the partial pressure of alveolar oxygen;  hence shorter is the latent period and greater the severity of effects.
  • Rate of ascent.  The greater the rate of ascent the more rapid the onset of signs and symptoms of hypoxia
  • Duration at Altitude.  The effects of hypoxia are more severe if the duration at altitude is prolonged. This is due to the fact that the effects of hypoxia are cumulative.
  • Ambient Temperature.  High or low environmental temperature favours the development of hypoxia.
  • Physical Activity.  Physical effort at altitude raises the demand for Oxygen and hence the symptoms of hypoxia are more severe. This fact is to be remembered by aircrew other than pilots too.
  • Individuals Susceptibility.  Individuals differ considerably in their ability to withstand hypoxia.
  • Physical Fitness.  A high standard of physical fitness is conducive to a better tolerance of hypoxia. Regular physical training improves the tolerance levels.
  • Smoking.  Smoking makes an individual more liable to suffer from hypoxia due to binding of haemoglobin with Carbon monoxide present in the smoke. A smoker who smokes prior to sortie has already compromised him-/herself to hypoxic insult, where s-/he is at an apparent altitude of 7,000, 14000, 22000 ft as compared to a non-smoker pilot at sea level, 10000 and 20000 ft, respectively. Be informed that if one smokes three cigarettes before a sortie, it is as if s-/he is already at an equivalent altitude of 8000 feet, with its implications due to compromised vision.
  • Organic Diseases.  Effects of hypoxia are more severe in those with disease of the heart, lungs or blood, which interferes with the normal oxygenation and circulation, to restrict adequate physiological compensation.
  • Emotional State.  Apprehension and anxiety make an individual more susceptible to the effects of hypoxia.
  • Acclimatisation.  Acclimatisation while residing at high altitude raises the individuals ability to withstand hypoxia.
  • Equivalent Lung Altitude.  Breathing air at a sea level is associated with a certain partial pressure of oxygen in the lungs (104 mm Hg). By breathing 100% Oxygen, the same partial pressure is brought about at a much greater altitude i.e. at 33,000 ft. The table below shows various equivalent lung altitudes while breathing 100% Oxygen. This concept is found useful in designing the Oxygen system for combat aircraft to ensure adequate oxygenation of the aircrew at different altitude.

Read the Second part: Flying into Thin Air: Neurological Effects of Hypoxia


1. Ernsting’s Aviation Medicine. Rainford DJ, Gradwell DP (Editors). 4th Edition. Hodder Arnold, London 2006.

2. Fundamentals of Aerospace Medicine. DeHart RL, Davis JR (Editors). 3rd Edition. Lippincott, Williams & Wilkins, Philadelphia 2002.

3. Human Performance & Limitations – JAA ATPL Theoretical Knowledge Manual. 2nd Edition. Jeppesen GmbH, Frankfurt 2001.

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