Oxygen is essential for human survival. The atmosphere of Earth is made up of roughly 21% oxygen, with the remaining 78% composed primarily of nitrogen. Oxygen levels decrease gradually with increasing altitude, eventually becoming too low to support human life.
How does oxygen decrease with altitude?
As altitude increases, the atmospheric pressure decreases. Lower pressure reduces the number of oxygen molecules that are packed into a given volume of air. At sea level, the oxygen concentration of air is about 21%. At altitudes of 18,000 feet (5,500 meters), the oxygen concentration drops to roughly half this amount, 10.5%.
There are two key factors that cause oxygen levels to fall with increasing elevation:
- Lower air pressure – Less air molecules overall per volume at higher altitudes
- Constant proportion of oxygen – The 21% oxygen fraction stays the same even as overall air pressure drops
So as you climb in elevation, the quantity of oxygen declines rapidly since there are fewer air molecules, even though the 21% oxygen ratio is fixed.
At what altitude is there no oxygen?
There is no specific altitude at which the oxygen level becomes zero. However, oxygen concentrations become dangerously low for humans at high elevations. Most permanent human settlements are located at elevations below 5,000 meters (16,000 feet).
Here are some key elevations to illustrate how oxygen levels decline with height:
| Elevation | Oxygen Level |
|---|---|
| Sea Level | 21% |
| 8,000 ft (2,400 m) | 16% |
| 18,000 ft (5,500 m) | 10.5% |
| 29,000 ft (8,800 m) | 5.5% |
As shown, oxygen concentrations become very low at around 30,000 feet (9,000 m). At the summit of Mount Everest, the highest point on Earth at 29,029 feet (8,848 m), the oxygen concentration is only about 5.5%. This is about one-quarter the oxygen level at sea level.
Impacts of low oxygen at high altitudes
Reduced oxygen has severe physiological impacts on the human body. These effects become extreme at elevations above 8,000 feet (2,400 m):
- Altitude sickness – Headaches, nausea, dizziness due to lack of oxygen
- Pulmonary edema – Fluid accumulation in the lungs
- Cerebral edema – Swelling of brain tissue
Without an oxygen supply, unconsciousness and death will occur within minutes above 16,000 feet (5,000 m). Even with supplemental oxygen, prolonged exposure is dangerous at elevations over 26,000 feet (8,000 m).
Distribution of oxygen with altitude
Although oxygen levels get very low at extreme elevations, oxygen never fully disappears from the atmosphere. Up to an altitude of around 50 miles (80 km), some oxygen molecules remain present. However, the density becomes miniscule at these heights.
The table below shows the oxygen distribution at different elevations:
| Altitude Range | Oxygen Percentage |
|---|---|
| Sea level to 30,000 ft (9,150 m) | 21% to 0% |
| 30,000 to 50 mi (80 km) | Nearly 0% |
| 50 to 100 mi (160 km) | Trace amounts |
| Above 100 mi (160 km) | Extremely low density |
The zone from sea level up to 30,000 feet is known as the homosphere. Here, the composition is fairly uniform. Above this, the heterosphere extends up to about 500 miles (800 km) in altitude. In the heterosphere, composition varies widely depending on lighter elements diffusing higher.
Although oxygen nearly disappears by 50 miles (80 km) up,trace amounts have been detected even at the edge of space at 100 miles (160 km). At these heights, the density is on the order of a few oxygen molecules per cubic centimeter.
Artificial oxygen generation above 30,000 feet
Because oxygen levels get so low at high altitude, extra oxygen is needed for human survival. On airplanes, supplemental oxygen is pumped to avoid danger for passengers and crew. The same applies for mountain climbers and parachutists. Some key methods used to generate oxygen at extreme heights include:
High pressure oxygen tanks
Oxygen is compressed into steel or aluminum tanks at very high pressures of 1800-3000 psi. This compressed gas can then provide elevated oxygen breathing at altitude through masks and regulators. Oxygen tanks are essential above 16,000 feet.
Oxygen concentrators
These devices use zeolite molecular sieves to filter nitrogen from air, generating 90-95% pure oxygen. This eliminates the need for high pressure oxygen tanks. Concentrators are often used to produce oxygen for medical purposes.
Liquid oxygen systems
Oxygen can be cooled below -297°F (-183°C), at which point it becomes a liquid. Liquid oxygen systems store high purity oxygen cryogenically. Heat and pressure convert it to gaseous form for delivery to users. This provides intense oxygen concentration from a compact system.
Chemical oxygen generators
Chemical reactions can produce oxygen as needed for emergency use. One type uses sodium chlorate candles that release oxygen when burned. Another system uses potassium superoxide, which reacts with water to liberate oxygen.
Conclusion
In summary, oxygen concentrations fall gradually with increasing altitude due to lower air pressure. Oxygen enables human survival below 30,000 feet, but becomes dangerously low above this height. Although oxygen never completely disappears from the atmosphere, levels become negligible by 50 miles up. Several technologies exist to produce oxygen artificially for high altitude usage when natural levels are inadequate.
