What happens to your body at high altitude?
At high altitude, the air is still about 21% oxygen but it is thinner: around 10,000 feet (3,000 m), each breath brings only 69% of the oxygen you get at sea level (CDC, 2025). Your blood carries less of it, so you breathe more, your heart speeds up, and your kidneys release EPO, which tells the body to make red blood cells.
Simplified diagram for a healthy adult coming from sea level. Air pressure calculated with a model (West, 1996). Dashed curves are estimates or relative levels.
What happens to your body at high altitude?
The air is still 21% oxygen, but it is thinner: each breath brings less. Choose an altitude, then tap part of the drawing to see it alone.
Sources: air pressure from the West model (J Appl Physiol, 1996); oxygen per breath: our calculation. Day-1 saturation: studies pooled by Dünnwald (Sensors, 2021) and the CDC Yellow Book (2025). Heart: Naeije (Prog Cardiovasc Dis, 2010); pulse shown, from 66 bpm at sea level to 108 bpm on Everest: our own rough figure, to be checked. EPO: peak around 48 h at 14,200 ft (4,340 m) (Vizcardo-Galindo, High Alt Med Biol, 2020). Red blood cells: about 1.1% more per 100 hours in athletes (Gore, Br J Sports Med, 2013), hence about +5% in 3 weeks (our calculation, estimate). Everest: saturation of about 54% measured at 27,600 ft (8,400 m) in acclimatized climbers (Grocott, N Engl J Med, 2009). Breathing and heart rhythms in the drawing: illustrative.
Keyboard: 1 to 5 the altitude, ← → previous or next altitude, Space pause, L slow motion, Esc the whole scene.
Common questions about high altitude and altitude sickness
At what altitude does altitude sickness start?
Altitude sickness mostly affects people who sleep above about 8,200 ft (2,500 m), and sometimes a little lower. In Colorado, about 1 in 4 visitors who sleep above 8,000 ft (2,450 m) get it (CDC, 2025). It starts 2 to 12 hours after arrival, often after the first night: headache, with nausea, fatigue, or dizziness. The altitude where you sleep matters more than the one you reach during the day (Wilderness Medical Society, 2019).
Is there less oxygen at high altitude?
At high altitude, the air is still about 21% oxygen, but it is thinner, so each breath brings less oxygen. Around 10,000 ft (3,000 m), inspired oxygen has only 69% of its sea-level pressure (CDC, 2025). At the top of Everest, air pressure falls to about a third of what it is at sea level (West model, 1996). That is why you get out of breath faster, even though the air seems the same.
What is a normal oxygen saturation at high altitude?
At high altitude, a lower saturation than at sea level is normal: around 10,000 ft (3,000 m), it often drops to 88 to 91% on the first day in a healthy person (CDC, 2025). At sea level, it is usually 96 to 99%. It climbs back a few points over about a week, as you breathe more (Dünnwald, 2021). The number alone doesn't tell you whether you have altitude sickness: the symptoms are what count.
How can you prevent altitude sickness?
The best way to prevent altitude sickness is to go up gradually: above 9,800 ft (3,000 m), don't sleep more than 1,600 ft (500 m) higher than the night before, and add an extra night every 3,300 ft (1,000 m) (Wilderness Medical Society, 2024, also cited by the CDC). It is better not to sleep at 9,000 ft (2,750 m) or higher on the first day. Arriving by plane or car does not protect you: the speed of the climb is what counts. A preventive medicine exists, on a doctor's advice.
Altitude sickness: when should you go down?
Go down right away if there is confusion, stumbling as if after too much alcohol, or shortness of breath even at rest with a cough: these are signs of swelling of the brain or the lungs (CDC, 2025). Going down 1,000 to 3,300 ft (300 to 1,000 m) is often enough to bring relief (Wilderness Medical Society, 2019). A simple headache means not going any higher while it lasts; if it gets worse despite rest, go down.
True or false?
At altitude, you breathe harder even at rest.
True. The carotid bodies, in the neck, sense the lack of oxygen and make you breathe harder (Kumar and Prabhakar, 2012). This increase keeps going for 3 to 5 days (CDC, 2025).
At altitude, your body makes red blood cells right away.
False. EPO rises more than 13-fold within 48 hours (Vizcardo-Galindo, 2020), but hemoglobin only gains about 1.1% per 100 hours (Gore, 2013). At first, the blood looks richer mainly because plasma goes down (CDC, 2025).
At altitude, your heart beats faster.
True. Heart rate goes up on arrival and stays higher than at sea level, even when the heart's output returns to normal (Naeije, 2010).
This page as text
The same information as the charts, to read without animation: for each chart, the values at the main moments, then the notes with their level of evidence.
What happens as you go up?
The air is still 21% oxygen, but its pressure drops with altitude. Each breath brings less oxygen, and the blood carries less.
| Altitude or days | Air pressure | Oxygen per breath (our calculation) | Day-1 saturation (estimated above 17,100 ft / 5,200 m) | What happens |
|---|---|---|---|---|
| 0 ft | 100 | 100 | 98 | (0 m): air pressure 760 mmHg (100%), oxygen per breath 100%, day-1 saturation about 98% |
| 5,000 ft | 84 | 83 | 95 | (1,524 m): air pressure 639 mmHg (84%), oxygen per breath 83%, day-1 saturation about 95% |
| 10,000 ft | 70 | 68 | 90 | (3,048 m): air pressure 534 mmHg (70%), oxygen per breath 68%, day-1 saturation about 90% |
| 15,000 ft | 58 | 56 | 78 | (4,572 m): air pressure 443 mmHg (58%), oxygen per breath 56%, day-1 saturation about 78% |
| 20,000 ft | 48 | 45 | 70 | (6,096 m): air pressure 365 mmHg (48%), oxygen per breath 45%, day-1 saturation about 71% (estimate, acclimatized climbers) |
| 25,000 ft | 39 | 35 | 60 | (7,620 m): air pressure 299 mmHg (39%), oxygen per breath 35%, day-1 saturation about 60% (estimate, acclimatized climbers) |
Key points
- 21% oxygen everywhere (Throughout, Shown in humans)
The share of oxygen doesn't change. What drops is the air pressure: 605 mmHg at 6,600 ft (2,000 m), 496 mmHg at 12,000 ft (3,650 m), about 253 mmHg at the top of Everest (West model, 1996). Source: West, « Prediction of barometric pressures at high altitude with the use of model atmospheres », J Appl Physiol, 1996 - Less oxygen per breath (From 6,600 to 14,800 ft (4,500 m), Shown in humans)
Around 10,000 ft (3,000 m), inspired oxygen has only 69% of its sea-level pressure (CDC). Our calculation gives 68 to 69%. Source: Hackett and Shlim, « High-altitude travel and altitude illness », CDC Yellow Book 2026, 2025 - Saturation drops (From 4,900 to 17,100 ft (5,200 m), Shown in humans)
About 94% around 6,600 ft (2,000 m), 88 to 91% around 10,000 ft (3,000 m), and around 80% near 14,100 ft (4,300 m) on the first day (studies pooled by Dünnwald, 2021; CDC). Source: Dünnwald et al., « The use of pulse oximetry in the assessment of acclimatization to high altitude », Sensors, 2021 - Above 8,200 ft (2,500 m) (From 8,200 ft (2,500 m) up, Shown in humans)
Altitude sickness becomes common in people who sleep higher without adjusting first (Wilderness Medical Society). Source: Luks et al., « Wilderness Medical Society clinical practice guidelines for the prevention, diagnosis, and treatment of acute altitude illness: 2024 update », Wilderness Environ Med, 2024
How does the body adjust to 11,500 ft (3,500 m)?
Within minutes, you breathe more and your heart speeds up. Within 2 days, EPO peaks. Over a few weeks, the blood gains more red blood cells.
| Altitude or days | Saturation (80 to 100% scale) | Breathing (dotted) | Heart rate (dotted) | EPO (dotted) | Red blood cells (mass) (dotted) | What happens |
|---|---|---|---|---|---|---|
| Arrival | 90 | 0 | 0 | 0 | 0 | saturation about 98%, EPO at baseline, red blood cells +0.0% (estimate based on Gore, 2013) |
| Day 3 | 56 | 87 | 69 | 37 | 14 | 0: saturation about 91%, EPO high, red blood cells +0.8% (estimate based on Gore, 2013) |
| Day 7 | 60 | 98 | 58 | 15 | 33 | 0: saturation about 92%, EPO back down, slightly above normal, red blood cells +1.8% (estimate based on Gore, 2013) |
| Day 14 | 60 | 100 | 55 | 15 | 67 | 0: saturation about 92%, EPO back down, slightly above normal, red blood cells +3.7% (estimate based on Gore, 2013) |
| Day 21 | 60 | 100 | 55 | 15 | 100 | 0: saturation about 92%, EPO back down, slightly above normal, red blood cells +5.5% (estimate based on Gore, 2013) |
Key points
- Saturation rises a little (Until day 10, Shown in humans)
About 89% on day 1 at 11,500 ft (3,500 m), close to 92% by day 9, in children aged 6 to 13 (Scrase, 2009); in adults at 11,200 ft (3,400 m), about 92% from day 2 to day 9 (Gibson, 2015). Two studies included in Dünnwald's review (2021). Source: Dünnwald et al., « The use of pulse oximetry in the assessment of acclimatization to high altitude », Sensors, 2021 - The sensors in your neck make you breathe more (Until day 5, Hypothesis)
The carotid bodies sense the lack of oxygen (Kumar and Prabhakar, 2012); breathing increases for 3 to 5 days (CDC). Shape of the curve: estimate. Source: Kumar and Prabhakar, « Peripheral chemoreceptors: function and plasticity of the carotid body », Compr Physiol, 2012 - The heart stays a bit fast (Throughout, Shown in humans)
At first, the heart pumps more blood thanks to a faster pulse; after a few days, its output is back to normal but the pulse stays higher (Naeije, 2010). Shape: estimate. Source: Naeije, « Physiological adaptation of the cardiovascular system to high altitude », Prog Cardiovasc Dis, 2010 - EPO peaks around 48 hours (Until day 4, Shown in humans)
Measured in 22 men who went up to 14,200 ft (4,340 m): more than 13 times the starting value at 48 h, then falling at 72 h (Vizcardo-Galindo, 2020). Beyond 3 days: estimate. Source: Vizcardo-Galindo et al., « High-altitude hypoxia decreases plasma erythropoietin soluble receptor concentration in lowlanders », High Alt Med Biol, 2020 - Extra red blood cells, slowly (From day 7, Shown in humans)
About 1.1% more hemoglobin mass per 100 hours, in athletes who stayed at 6,900 ft (2,100 m) or higher (Gore, 2013). At first, the blood looks richer mostly because plasma volume drops (CDC). Source: Gore et al., « Altitude training and haemoglobin mass from the optimised carbon monoxide rebreathing method determined by a meta-analysis », Br J Sports Med, 2013
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