# 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 9-10â€‰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 37Â°C). As oxygen is 21% of dry air, the inspired oxygen pressure is 0.21Ã—(100âˆ’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 5500â€‰m and only 30% of the sea level value at 8900â€‰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 CalculatorÂ

# 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 donâ€™t acclimatize properly, you greatly increase your chance of developing AMS (Acute Mountain Sickness), or even worse,Â HAPE (High Altitude Pulmonary Edema) orÂ 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 20.9Â % Low Sea Level 1,000 ft 305 m 20.1Â % Low 2,000 ft 610 m 19.4Â % Low 3,000 ft 914 m 18.6Â % Medium 4,000 ft 1,219 m 17.9Â % Medium 5,000 ft 1,524 m 17.3Â % Medium Boulder, CO Â (5328') 6,000 ft 1,829 m 16.6Â % Medium Mt. Washington (6288') 7,000 ft 2,134 m 16.0Â % Medium 8,000 ft 2,438 m 15.4Â % High Aspen, CO Â (8,000') 9,000 ft 2,743 m 14.8Â % High 10,000 ft 3,048 m 14.3Â % High 11,000 ft 3,353 m 13.7Â % High Mt. Phillips Â (11,711') 12,000 ft 3,658 m 13.2Â % High Mt. Baldy Â (12,441') 13,000 ft 3,962 m 12.7Â % Very High 14,000 ft 4,267 m 12.3Â % Very High Pikes Peak Â (14,115') 15,000 ft 4,572 m 11.8Â % Very High 16,000 ft 4,877 m 11.4Â % Very High Mont Blanc Â (15,781') 17,000 ft 5,182 m 11.0Â % Very High 18,000 ft 5,486 m 10.5Â % Extreme 19,000 ft 5,791 m 10.1Â % Extreme Kilimanjaro Â (19,341') 20,000 ft 6,096 m 9.7Â % Extreme Denali Â (20,308') 21,000 ft 6,401 m 9.4Â % Extreme 22,000 ft 6,706 m 9.0Â % Extreme 23,000 ft 7,010 m 8.7Â % Extreme Aconcagua Â (22,841') 24,000 ft 7,315 m 8.4Â % Extreme 25,000 ft 7,620 m 8.1Â % Extreme 26,000 ft 7,925 m 7.8Â % Ultra 27,000 ft 8,230 m 7.5Â % Ultra 28,000 ft 8,534 m 7.2Â % Ultra K2 Â (28, 251') 29,000 ft 8,839 m 6.9Â % Ultra Mt. Everest Â (29,029')

Sources:

BMJ. 1998 Oct 17; 317(7165): 1063â€“1066.
doi: 10.1136/bmj.317.7165.1063
PMCID: PMC1114067
PMID: 9774298
ABC of oxygen

USGS Map Point Elevation Query Service
https://nationalmap.gov/epqs/

Source of Effective Oxygen %:
The answers given by the Barometric Formula equation.

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