Oxygen Levels at Altitude
Although the percentage of oxygen in inspired air is constant at different altitudes, the fall in atmospheric pressure at higher altitude decreases the partial pressure of inspired oxygen and hence the driving pressure for gas exchange in the lungs. An ocean of air is present up to 9-10 000 m, where the troposphere ends and the stratosphere begins. The weight of air above us is responsible for the atmospheric pressure, which is normally about 100 kPa at sea level. This atmospheric pressure is the sum of the partial pressures of the constituent gases, oxygen and nitrogen, and also the partial pressure of water vapor (6.3 kPa at 37°C). As oxygen is 21% of dry air, the inspired oxygen pressure is 0.21×(100−6.3)=19.6 kPa at sea level.
Atmospheric pressure and inspired oxygen pressure fall roughly linearly with altitude to be 50% of the sea level value at 5500 m and only 30% of the sea level value at 8900 m (the height of the summit of Everest). A fall in inspired oxygen pressure reduces the driving pressure for gas exchange in the lungs and in turn produces a cascade of effects right down to the level of the mitochondria, the final destination of the oxygen.
RESOURCE: Altitude / Air Pressure Calculator
Why is there Less Oxygen at High Altitude?
We all live underneath a huge ocean of air that is several miles deep: the atmosphere. The pressure on our bodies is about the same as ten metres of sea water pressing down on us all the time. At sea level, because air is compressible, the weight of all that air above us compresses the air around us, making it denser. As you go up in elevation (while mountaineering, for example), the air becomes less compressed and is therefore thinner.
The important effect of this decrease in pressure is this: in a given volume of air, there are fewer molecules present. This is really just another way of saying that the pressure is lower (this is called Boyle's law). The percentage of those molecules that are oxygen is exactly the same: 21% (20.9% actually). The problem is that there are fewer molecules of everything present, including oxygen.
Although the percentage of oxygen in the atmosphere is the same, the "thinner air" means there is less oxygen to breathe. Try using the Barometric Pressure Calculator to see how air pressure changes at high altitudes. Or use the altitude oxygen graph (below) to see how much less oxygen is available at any altitude.
The body makes a wide range of physiological changes in order to cope better with the lack of oxygen at high altitude. This process is called acclimatization. If you don’t acclimatize properly, you greatly increase your chance of developing AMS (Acute Mountain Sickness), or even worse, HAPE (High Altitude Pulmonary Edema) or HACE (High Altitude Cerebral Edema).
RESOURCE: Barometric Pressure Calculator
Use the table below to see how the effective amount of oxygen in the air varies at different altitudes. Although air contains 20.9% oxygen at all altitudes, lower air pressure at high altitude makes it feel like there is a lower percentage of oxygen. The chart is based on the ideal gas law equation for pressure versus altitude (Barometric Formula), assuming a constant atmospheric temperature of 32 degrees Fahrenheit (0 Celsius), and 1 atmosphere pressure at sea level.
|Altitude (feet)||Altitude (meters)||Effective Oxygen %||Altitude Category||Example|
|0 ft||0 m||20.9 %||Low||Sea Level|
|1,000 ft||305 m||20.1 %||Low|
|2,000 ft||610 m||19.4 %||Low|
|3,000 ft||914 m||18.6 %||Medium|
|4,000 ft||1,219 m||17.9 %||Medium|
|5,000 ft||1,524 m||17.3 %||Medium||Boulder, CO (5328')|
|6,000 ft||1,829 m||16.6 %||Medium||Mt. Washington (6288')|
|7,000 ft||2,134 m||16.0 %||Medium|
|8,000 ft||2,438 m||15.4 %||High||Aspen, CO (8,000')|
|9,000 ft||2,743 m||14.8 %||High|
|10,000 ft||3,048 m||14.3 %||High|
|11,000 ft||3,353 m||13.7 %||High||Mt. Phillips (11,711')|
|12,000 ft||3,658 m||13.2 %||High||Mt. Baldy (12,441')|
|13,000 ft||3,962 m||12.7 %||Very High|
|14,000 ft||4,267 m||12.3 %||Very High||Pikes Peak (14,115')|
|15,000 ft||4,572 m||11.8 %||Very High|
|16,000 ft||4,877 m||11.4 %||Very High||Mont Blanc (15,781')|
|17,000 ft||5,182 m||11.0 %||Very High|
|18,000 ft||5,486 m||10.5 %||Extreme|
|19,000 ft||5,791 m||10.1 %||Extreme||Kilimanjaro (19,341')|
|20,000 ft||6,096 m||9.7 %||Extreme||Denali (20,308')|
|21,000 ft||6,401 m||9.4 %||Extreme|
|22,000 ft||6,706 m||9.0 %||Extreme|
|23,000 ft||7,010 m||8.7 %||Extreme||Aconcagua (22,841')|
|24,000 ft||7,315 m||8.4 %||Extreme|
|25,000 ft||7,620 m||8.1 %||Extreme|
|26,000 ft||7,925 m||7.8 %||Ultra|
|27,000 ft||8,230 m||7.5 %||Ultra|
|28,000 ft||8,534 m||7.2 %||Ultra||K2 (28, 251')|
|29,000 ft||8,839 m||6.9 %||Ultra||Mt. Everest (29,029')|