Hypoxia: how does it work? It’s a term we all get hammered with during Human Performance and Limitations training during the ATPL’s.

For most though, while the facts don’t lie, it sometimes feels like a term that is very distant and one of those things that ‘you know about’ but probably won’t ever encounter, especially for helicopter pilots who rarely fly above 6000’ AMSL.

And while that might be true, helicopters are not even pressurised, and pilots are often misinformed of the dangers and levels at which hypoxia can become an issue.

The pilots in the audience who fly in mountainous areas or have performed rescue operations at high altitude are probably very aware of their oxygen related SOP’s and pitfalls.

So whether you’re an airline pilot, helicopter pilot, or anyone else interested in the dangers, let’s go over what exactly it is, and what the dangers are.

To kick things off, let’s go over this short summary of a hypoxia related fatal airline crash that happened in 2005, which perfectly illustrates how relentless it can be.

Helios Airways Flight 522

On 14 August 2005, a Boeing 737-300 aircraft, registration number 5B-DBY, operated by Helios Airways, departed Larnaca, Cyprus at 06:07 h for Prague, Czech Republic, via Athens, Hellas.

After passing 16000’, there was no response to radio calls to the aircraft. During the climb, at an aircraft altitude of 18 200 ft, the passenger oxygen masks deployed in the cabin. The aircraft leveled off at FL340 and continued on its programmed route.

At 07:21 h, the aircraft flew over the KEA VOR, then over the Athens International Airport, and subsequently entered the KEA VOR holding pattern at 07:38 h. At 08:24 h, during the sixth holding pattern, the Boeing 737 was intercepted by two F-16 aircraft of the Hellenic Air Force.

One of the F-16 pilots observed the aircraft at close range and reported at 08:32 h that the Captain’s seat was vacant, the First Officer’s seat was occupied by someone who was slumped over the controls, the passenger oxygen masks were seen dangling and three motionless passengers were seen seated wearing oxygen masks in the cabin.

No external damage or fire was noted and the aircraft was not responding to radio calls. At 08:49 h, he reported a person not wearing an oxygen mask entering the cockpit and occupying the Captain’s seat. The F-16 pilot tried to attract his attention without success. At 08:50 h, the left engine flamed out due to fuel depletion and the aircraft started descending. At 08:54 h, two MAYDAY messages were recorded on the CVR.

At 09:00 h, the right engine also flamed out at an altitude of approximately 7 100 ft. The aircraft continued descending rapidly and impacted hilly terrain at 09:03 h in the vicinity of Grammatiko village, Hellas, approximately 33 km northwest of the Athens International Airport. The 115 passengers and 6 crew members on board were fatally injured. The aircraft was destroyed.


This was quite an eye opener for this particular airline, but also the entire industry. The direct causes summarised to be three-fold:

  • Lack of recognition that the cabin pressurisation mode was in the MAN (manual) position during departure.
  • Lack of identification of the warnings and their reasons
  • Crew incapacitation due to hypoxia

While some of these might sound basic, the latent causes that were identified by the AAIASB were more focussed on organisational culture, education of human factors, and a general oversight in safety SOP’s.

Recognising the signs and symptoms of hypoxia could have helped this crew. However, one of the reasons this is so tricky to do is the fact that hypoxia lowers one’s spatial awareness and cognitive reasoning, which we’ll go into deeper later on. First, let’s go over the basics.

What Gasses are in the Atmosphere?

The atmosphere’s amount of oxygen actually stays relatively constant all the way up to very high altitudes (around 70.000’). The air’s consistency looks like this:

  • 21.0% Oxygen
  • 78.0% Nitrogen
  • 0.93% Argon
  • 0.03% Carbon Dioxide
  • 0.04% Rare gases

The reason we have access to less oxygen at higher altitudes is because of the reduction in pressure, not the reduction of the amount of oxygen within that air. This is because of Dalton’s Law:

“In a mixture of non-reacting gases, the total pressure exerted is equal to the sum of the partial pressures of the individual gases”

So if we have a quick look at what pressures we can expect for certain altitudes (compared to sea level atmospheric pressure as a percentage), it would make a graph that looks like the inverse of an exponential function, with 40.000’ giving about 20% of atmospheric pressure at sea level:

Hypoxia: How does it work?

The pressure of our air at sea level is 1013 HPa, 21% of that pressure is made of by the pressure created by the oxygen part, 78% of the nitrogen part, etc. So therefore, if we go to 40.000 feet, where the average pressure is roughly 200 Hpa, oxygen’s partial pressure is 21% of 200 HPa (which is around 42 Hpa – not a lot!).

Healthy people are able to function relatively normally up to 10.000 ft, provided they are not exerting themselves. If you go above this level, you will need to increase the amount of oxygen in the air, to give your alveoli (the part of the lung that processes oxygen) an oxygen partial pressure that can sustain human cells.

The altitudes that require extra oxygen, or even pressurised oxygen to increase this partial pressure even further are as follows:

Oxygen at Altitude

Cabin pressure during the cruise does not represent sea level, most airlines are calibrated for a pressure representing around 6000’ to 8000’ in altitude. This is why even with normal pressurised cabins, you can still feel bloated or different than you might on the ground.

What is Hypoxia?

So, if any of the conditions in the above table do not get achieved, hypoxia becomes an issue. Hypoxia is defined as:

“A state in which oxygen is not available in sufficient amounts at the tissue level to maintain adequate homeostasis”

In other words, the oxygen saturation in your body is too low for normal brain and overall cell function. While there are many causes for this, in the aviation world, we are mostly having to deal with altitude as the biggest risk factor (but it isn’t the only one), this type of hypoxia is called hypoxic hypoxia, however there are a total of 4:

  • Hypoxic Hypoxia: Hypoxia due to lack of air pressure
  • Anaemic Hypoxia: Hypoxia due to a reduction in your blood’s ability to carry oxygen
  • Hypokinetic Hypoxia: Circulatory failures such as a heart attack or bloodvessel blockage
  • Histotoxic Hypoxia: Hypoxia due to a reduction in your body’s ability to absorb oxygen (usually due to substances in your system such as alcohol).

We’ll be focussing on hypoxic hypoxia. Your haemoglobin oxygen saturation is around 97.5% at sea level. At 10.000’ this is reduced to about 87%, and at 20.000’ down to 65%. The symptoms will get worse very quickly at high altitude, and less quickly at lower altitudes.

How quickly? Well, one of the more objective symptoms that are more tangible to understand is a concept known as Time of Useful Consciousness (TUC).

It is essentially your battle against time to recognise something is wrong, if you lose your “useful consciousness” then it’s too late to recognise and trigger corrective actions. It is measured not to the point of unconsciousness, but rather a point where you are no longer able to initiate steps to help yourself in the situation presented.

This time can be put in a table for various altitudes:

Time of Useful Consiousness

As you can see, at higher cruising altitudes for airlines (between 30.000’ to 40.000’, TUC varies from 2 minutes to 20 seconds. If you take away the time it takes to even realise something is wrong, you’ll realise just how deadly a lack of oxygen can become at such altitudes.

To demonstrate how ruthless it is to see how someone can be ‘not-unconscious’ but completely helpless at the same time to correct for hypoxia, Destin Sandlin from Smarter Everyday has put together an amazing video where he and NASA astronaut Don Pettit are put to the test in a Barometric chamber.

Skip to the 4 minute mark for the start of the test, and the 6:30 minute mark for the point where Destin gets told to correct for hypoxia and that if he doesn’t, he will die (I won’t spoil the result), video below or via this link.

So (Spoiler Alert), even after he was literally told he was going to die if he did not do anything about it (attach his mask), he still only managed to say ‘I don’t want to die’ with a smile on his face.

This can and will happen to most people, including you and me. TUC has passed, and even telling someone in that state ‘if you don’t do X you will die’ is not enough anymore. At this point it’s too late, and whatever the trend of the situation currently is, it will continue (with usually a bad ending).

So how do we arm ourselves for not getting into this position? We start by learning to help ourselves with:

How to Recognise Hypoxia?

Usually, symptoms gradually develop rather than go from 0 to 100. There are 4 stages of hypoxia, which gradually evolve until you reach the end of your TUC:

  • The Indifferent Stage (MSL – 10.000’): Your night vision is affected, new tasks will be harder to execute, increase in heart rate and breathing rate.
  • The Compensatory Stage (10.000 – 15.000’): Your body starts to compensate harder for the lack of oxygen in various ways such as an increase in cardiac output and even higher respiratory volume, judgment and memory are impaired at this stage and mental alertness drops significantly.
  • The Disturbance stage (15.000 – 20.000’): Euphoria starts to set in, dizziness, fatigue, slower cognitive processing and an impairment of basic motor function.
  • The Critical Stage (20.000’ – 23.000’): Mental performance now rapidly deteriorates, total incapacitation or the end of TUC can occur without warning.

While these zones and altitudes apply to most people, everyone will experience them at slightly different rates. There are quite a few variables that determine how likely you are to become hypoxic:

  • Time: The longer you are exposed to hypoxic conditions, the worse the results
  • Altitude: The higher the altitude the smaller the TUC is
  • Alcohol / Drugs: Metabolism is affected and causes histotoxic hypoxia as discussed above, increasing symptoms
  • Fatigue or Illness: Increased energy demands which accelerates the hypoxia stage
  • Temperature Extremes: The body will spend more energy on temperature regulation, which requires more oxygen and therefore accelerates the hypoxia stage


While hypoxia can be deadly, it is up to us as pilots to learn how to recognise the symptoms and manage risks accordingly. A call for action will have to be initiated within your TUC for most situations to still end well.

Categories: Human Factors

Jop Dingemans

AW169 HEMS Commander | Founder of Pilots Who Ask Why | Aerospace Engineer | Former Flight Instructor


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