Hypoxia and time of useful consciousness
Hypoxia is the condition of too little oxygen reaching the body’s tissues, and the part that makes it dangerous is that your judgement goes first. The exam asks for the four types, the time you have at each altitude and a handful of numbers behind them. This page puts them in one place.
Time of useful consciousness
Time of useful consciousness (TUC) is the time from the loss of oxygen supply, or from exposure to an oxygen-poor environment, until you can no longer take corrective action. It shrinks fast with altitude:
| Flight level | TUC (seated, at rest) |
|---|---|
| FL180 | 20–30 minutes |
| FL250 | 3–5 minutes |
| FL300 | 1–2 minutes |
| FL350 | 30–60 seconds |
| FL400 | 15–20 seconds |
These are the ranges used in EASA ATPL Human Performance (040 01 02). Textbooks differ slightly, so check your course notes for the exact figures you will be examined on. Exertion, fatigue, smoking and a rapid decompression all shorten the time.
The four types of hypoxia
| Type | What is wrong | Example |
|---|---|---|
| Hypoxic | The pressure of oxygen in the air is too low | Altitude, rapid decompression |
| Anaemic (hypaemic) | The blood carries too little oxygen | Carbon monoxide, anaemia, blood loss |
| Stagnant | The blood does not circulate well enough | High G, cold, a tight cuff |
| Histotoxic | The cells cannot use the oxygen they get | Alcohol, cyanide, some drugs |
A way to remember them: hypoxic is height, anaemic is the carrier, stagnant is the flow, histotoxic is the cell.
The numbers behind it
- Oxygen is about 21 % of the air, and the share is the same at every altitude you fly at. It is the pressure that falls, not the percentage.
- Pressure is about half of sea level at roughly 18 000 ft. That is why FL180 is the usual threshold for “oxygen needed”, and why the oxygen pressure there is half of what you are used to.
- Night vision starts to suffer from about 5 000 ft. The rods in the eye are the first to complain, long before you feel anything else.
- Haemoglobin binds carbon monoxide about 200–250 times more strongly than oxygen. Smokers carry a permanent handicap, and a faulty heater can cause anaemic hypoxia at any altitude. Classic texts describe a cherry-red skin colour, which is rarely seen in practice.
Where students lose marks
- Mixing up the types. Alcohol is histotoxic, not hypoxic. High G is stagnant, not hypoxic.
- Mixing up the 21 %. The percentage does not fall with altitude. The partial pressure does.
- Quoting TUC for the wrong level. Learn the order of magnitude: minutes at FL250, a minute or two at FL300, seconds at FL400.
- Forgetting that hypoxia hides itself. Euphoria, poor judgement and a feeling of being fine are classic symptoms, which is why checklists and oxygen discipline matter more than how you feel.
If hypoxia is suspected: oxygen first, then descend. This page is a study aid; follow your aircraft’s procedures and your syllabus.
Quick answers
What is the time of useful consciousness at FL350?
About 30 to 60 seconds for a person seated at rest. At FL400 it is about 15 to 20 seconds.
What are the four types of hypoxia?
Hypoxic (too little oxygen pressure, for example at altitude), anaemic or hypaemic (the blood carries too little oxygen, for example with carbon monoxide), stagnant (poor circulation, for example under high G) and histotoxic (the cells cannot use the oxygen, for example with alcohol or cyanide).
At what altitude is the air pressure half of sea level?
About 18 000 ft, which is why oxygen pressure at FL180 is roughly half of what it is at sea level, although the oxygen percentage is still about 21 %.
Why is carbon monoxide so dangerous?
Haemoglobin binds carbon monoxide about 200 to 250 times more strongly than oxygen, so even a small amount blocks the blood from carrying oxygen.
Make it stick
Fly the Hypoxia & Altitude drill: every item is a streak you have to land.