At high altitude, barometric pressure may be significantly lower than at sea-level. The result? Oxygen molecules are spread further apart, lowering the oxygen content of each breath one takes. Because of the reduced availability of oxygen in the air, blood oxygen levels decrease, and the body struggles to efficiently deliver oxygen to tissues, muscles and the brain.
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.
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. Understanding how oxygen levels affect the human body at high altitude can save lives — but it involves some math.
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 oxygen levels (percent 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 acclimate 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).
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°C), and 1 atmosphere pressure at sea level.
|Altitude (feet)||Altitude (meters)||Oxygen Levels (%)||Altitude Category||Example|
|0 ft||0 m||20.9 %||Low Altitude||Sea Level|
|1000 ft||305 m||20.1 %||Low Altitude|
|2000 ft||610 m||19.4 %||Low Altitude|
|3000 ft||914 m||18.6 %||Moderate Altitude|
|4000 ft||1219 m||17.9 %||Moderate Altitude|
|5000 ft||1524 m||17.3 %||Moderate Altitude||Boulder, CO (5328 ft)|
|6000 ft||1829 m||16.6 %||Moderate Altitude||Mt. Washington (6288 ft)|
|7000 ft||2134 m||16.0 %||Moderate Altitude|
|8000 ft||2438 m||15.4 %||High Altitude||Aspen, CO (8000 ft)|
|9000 ft||2743 m||14.8 %||High Altitude|
|10,000 ft||3048 m||14.3 %||High Altitude|
|11,000 ft||3353 m||13.7 %||High Altitude||Mt. Phillips (11,711 ft)|
|12,000 ft||3658 m||13.2 %||High Altitude||Mt. Baldy (12,441 ft)|
|13,000 ft||3962 m||12.7 %||Very High Altitude|
|14,000 ft||4267 m||12.3 %||Very High Altitude||Pikes Peak (14,115 ft)|
|15,000 ft||4572 m||11.8 %||Very High Altitude|
|16,000 ft||4877 m||11.4 %||Very High Altitude||Mont Blanc (15,781 ft)|
|17,000 ft||5182 m||11.0 %||Very High Altitude|
|18,000 ft||5486 m||10.5 %||Extreme High Altitude|
|19,000 ft||5791 m||10.1 %||Extreme High Altitude||Kilimanjaro (19,341 ft)|
|20,000 ft||6096 m||9.7 %||Extreme High Altitude||Denali (20,308 ft)|
|21,000 ft||6401 m||9.4 %||Extreme High Altitude|
|22,000 ft||6706 m||9.0 %||Extreme High Altitude||Aconcagua (22,841 ft)|
|23,000 ft||7010 m||8.7 %||Extreme High Altitude|
|24,000 ft||7315 m||8.4 %||Extreme High Altitude|
|25,000 ft||7620 m||8.1 %||Extreme High Altitude|
|26,000 ft||7925 m||7.8 %||Ultra High Altitude|
|27,000 ft||8230 m||7.5 %||Ultra High Altitude|
|28,000 ft||8534 m||7.2 %||Ultra High Altitude||K2 (28,251 ft)|
|29,000 ft||8839 m||6.9 %||Ultra High Altitude||Mt. Everest (29,029 ft)|
Sources for Oxygen Levels:
BMJ. 1998 Oct 17; 317(7165): 1063–1066.
ABC of oxygen
USGS Map Point Elevation Query Service
Source of Effective Oxygen % (oxygen levels):
The answers given by the Barometric Formula equation.