This is the cliff. Above ~2,500 m the curve turns down.
The air does not change.
Air at the summit of Everest is still 20.9% oxygen, exactly the same mix as the air on a beach.
The pressure does.
What thins as you climb is the pressure pushing that oxygen across your lungs and into your blood. Watch the pressure bar empty as you rise.
For a while, your blood holds.
Blood oxygen barely moves through the foothills. The curve runs flat. This is why the first 2,000 m feel like nothing.
Then it falls off a cliff.
Past about 2,500 m the curve turns down hard. At Leh, 3,524 m, an unacclimatised arrival reads around 90%. A few nights claw some of it back, which is why a good Leh plan holds two nights before going higher.
At the top, the model stops being a trek prediction.
On the summit of Everest, this simple unacclimatised model falls toward 20%. Treat that as a stress-test number, not a real itinerary prediction.2 Climbers reach that zone only after staged acclimatisation, and often with bottled oxygen.
Your blood at Leh, 3,524 m, unacclimatised on day one: about 90%.
A reading like this is expected while you acclimatise. A low number on its own is not altitude sickness.
If you are feeling symptoms rather than just reading a number, Part 3 has the symptom self-check.
| Altitude | Air pressure | O₂ to lungs | Blood O₂ |
|---|---|---|---|
| Sea level 0 m | 760 mmHg | 149 mmHg | 97% |
| Lohajung 2,300 m | 574 mmHg | 110 mmHg | 94% |
| Leh 3,524 m | 492 mmHg | 93 mmHg | 90% |
| Roopkund 5,029 m | 404 mmHg | 75 mmHg | 80% |
| Everest Base Camp 5,364 m | 386 mmHg | 71 mmHg | 77% |
| Yunam 6,111 m | 349 mmHg | 63 mmHg | 68% |
| Everest summit 8,848 m | 236 mmHg | 40 mmHg | 21% |
Your body does not take this lying down.
A few nights at altitude claw some oxygen back. The next part is about that fight: breathing, kidneys, blood chemistry, and the red-cell clock.
Sources
- Burtscher J, Swenson ER, Hackett PH, Millet GP, Burtscher M. Flying to high-altitude destinations: Is the risk of acute mountain sickness greater? Journal of Travel Medicine, 2023. PMC10289512.
- Severinghaus JW. Simple, accurate equations for human blood O₂ dissociation computations. J Appl Physiol, 1979. International Standard Atmosphere used for pressure–altitude.
This guide is educational and is not a diagnosis or a substitute for a doctor. If symptoms are worsening at altitude, the safe default is to stop ascending and descend.