The International Society for Respiratory Protection

pdf Minimum oxygen concentration in breathing gas: Effects of altitude (Vol. 41, No. 1, 2024) Popular


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Minimum oxygen concentration in breathing gas:

Effects of altitude


Dan Warkander1,2,*, Richard Arnold1, Alberto Spasciani3

  1. Naval Medical Research Unit Dayton, Dayton, OH, USA,
  2. Leidos, Reston, VA, USA,
  3. Spasciani Spa, Origgio, Italy.


* Corresponding author email: ,



Background. Oxygen is essential for life and a minimum safe level shall be determined before using air purifying respiratory protective devices. However, no consensus among regulatory standards exists on what is a safe low limit of oxygen concentration (O2%) – the values range from 17% to 19.5%. A person’s fundamental need is the partial pressure of oxygen, not the O2%. The partial pressure of O2 is proportional to the number of molecules of O2 in the air. Since the barometric pressure decreases with altitude, the number of O2 molecules in a breath of air also decreases. Therefore, it makes little sense to express a safe low limit as a concentration, unless the altitude of use is specified. For instance, if an altitude of 2,400 m (8,000 feet) is considered safe when breathing air, the equivalent partial pressure of O2% would be provided at sea level from a gas containing 15.3% O2.

Objective. To highlight the different levels of O2% or altitude that are considered safe in various situations and provide means to determine how the necessary O2% level changes with altitude.

Methods. The alveolar gas equation was used to determine the equivalence between O2% and altitude for a given partial pressure of O2.

Results. The equivalences between O2% and altitude are shown in graphs for easy interpretation. For instance, if it is deemed acceptable to breathe air at 2,400 m, then the equivalent O2% is 19.3% at 1,800 m or 17.3% at 1,000 m. Breathing gas containing 23.5% O2 at an altitude of 3,300 m is equivalent to breathing air at 2,400 m.

Conclusion. The step-by-step approach described will allow a Safety Officer or user of respiratory protective devices to determine equivalent O2% based upon a generally accepted safe condition from a known altitude.


Keywords: partial pressure of oxygen, partial pressure of carbon dioxide, O2, CO2, hypoxia, hyperoxia, alveolar gases.