Air pressure & oxygen at altitude
The air up high still holds about 21% oxygen, but there is less air in every breath. See the pressure, the oxygen your lungs actually get, and the sea-level oxygen percentage it equals.
Oxygen in each breath, compared with sea level
of the sea-level amount
- Air pressure
- 658 hPa
- 65% of sea level
- Oxygen partial pressure
- 103 mmHg
- 13.8 kPa, in dry air
- Inspired oxygen pressure
- 94 mmHg
- 12.5 kPa, in the lungs
- Equivalent oxygen at sea level
- 13.1%
- 13.6% by the simple pressure ratio
- Standard air temperature
- -7.7 °C
- on an average day
- Air density
- 0.863 kg/m³
- 70% of sea level
At 3,500 m, altitude sickness is possible; it usually only affects people above about 2,500 m. This page is not medical advice.
Point at the chart to read off any altitude.
What the numbers mean
Dry air is 20.95% oxygen at every height you can climb to; what changes is the pressure. The oxygen partial pressure is that share of the air pressure. In the lungs the air is warmed to 37 °C and saturated with water vapour (about 47 mmHg, 6.3 kPa), so the inspired oxygen pressure is 0.2095 × (pressure − 47 mmHg). Pressure and inspired oxygen fall to about half the sea-level value near 5,500 m and to about 30% on the summit of Everest.
The equivalent oxygen figure is the oxygen percentage that would give the same inspired pressure at sea level, the way altitude-training tents describe their settings. Many charts use the simpler pressure ratio, which comes out a little higher, so we show both.
Standard atmosphere, real weather
Pressures come from the U.S. Standard Atmosphere 1976, an average. Real pressure changes with weather, season and latitude, and is usually a little higher near the equator: on Everest's summit, researchers in 1983 measured 253 mmHg, well above the standard value of about 236 mmHg. If you have a barometer, tick the box and enter its reading. Airliner cabins are pressurised to no more than an 8,000-foot cabin altitude under normal conditions, a US certification rule.
Not medical advice
Altitude sickness usually starts above about 2,500 m and depends on how fast you climb, not just how high you are. These figures cannot tell you how you will feel. For symptoms, prevention and treatment, read the NHS guide to altitude sickness or the CDC Yellow Book chapter on high-altitude travel, and ask a doctor before travelling if you have a heart or lung condition.
Sources
- NOAA, NASA and US Air Force. U.S. Standard Atmosphere, 1976. NOAA-S/T 76-1562.
- Peacock AJ. ABC of oxygen: oxygen at high altitude. BMJ 1998;317(7165):1063–1066. doi:10.1136/bmj.317.7165.1063
- West JB, Lahiri S, Maret KH, Peters RM, Pizzo CJ. Barometric pressures at extreme altitudes on Mt. Everest: physiological significance. Journal of Applied Physiology 1983;54(5):1188–1194. doi:10.1152/jappl.1983.54.5.1188
- US Code of Federal Regulations, 14 CFR 25.841(a). Pressurized cabins.
- NHS. Altitude sickness.
- CDC Yellow Book. High-altitude travel and altitude illness.
Questions
Is there less oxygen in the air at altitude?
The percentage is the same, about 21%. There is less air, so each breath carries fewer oxygen molecules, which is what the partial pressure measures.
Why are there two equivalent oxygen figures?
The simple one scales 20.9% by the pressure ratio. The other first subtracts the water vapour in your lungs, which matters more as pressure falls, so it is a little lower.
Can this tell me if I will get altitude sickness?
No. It describes the air, not your body. Ascent rate, fitness and individual susceptibility all play a part; follow medical guidance and descend if symptoms get worse.