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What is oxygen deficiency?

By Bob Henderson | Last updated: June 4, 2023
Presented by GfG Instrumentation

Fresh air contains approximately 20.9% oxygen (O2). According to OSHA, atmosphere that contains less than 19.5% by volume is hazardous due to oxygen deficiency. As the O2 concentration drops below 19.5%, people begin to experience symptoms. The lower the concentration, the worse the symptoms become. Initial symptoms may include tiredness, clumsiness, and behavioral changes. When the concentration drops below 16% more severe symptoms associated with anoxia begin to appear. Below 10% the symptoms include loss of consciousness leading to death. Below 6% the loss of consciousness is immediate.

Oxygen deficiency is the leading cause of death in confined space accidents. Before a permit space that may have a hazardous atmosphere can be entered, the atmosphere must be tested for oxygen, as well as combustible gases and any toxic gases and vapors that may be potentially present.

Any oxygen concentration other than 20.9% indicates an abnormal condition. A less than normal concentration of oxygen by definition indicates a greater than normal concentration of some other component present in the atmosphere. Even when the oxygen concentration does not constitute a statutory hazardous condition, the user should determine the cause of the abnormal reading prior to entering a confined space.


Oxygen sensors by themselves cannot indicate what other gases are present. The only gas that is measured by an oxygen sensor is oxygen. Toxic gases such as carbon monoxide and hydrogen sulfide can be present in immediately lethal concentrations without displacing enough oxygen to result in an oxygen deficiency. Just because the O2 reading is higher than 19.5% does not mean the atmosphere is safe for entry. The atmosphere must be directly evaluated for all relevant hazards before entry.

Common causes of oxygen deficiency include:

  1. Combustion
    - Internal combustion engines
    - Welding and “hot work” that includes use cutting torches
  2. Microbial decomposition
    - Rotting foods, sewage, plant life and fermentation
  3. Oxidation of metals
    - Rusting
  4. Inerting
    - Deliberate replacement of the atmosphere in a confined space by an inert gas such as nitrogen or carbon dioxide
  5. Consumption
    - Ripening fruit
    - Presence of workers in a poorly ventilated space
  6. Displacement
    - Occurs when oxygen in the air is replaced by other gases or vapors accidentally introduced into the space. Typical examples include heavier than air combustible gases such as propane, cryogenic fluids such as liquid nitrogen, heavier than air solvent vapors, heavier than air toxic gases such as and CO2. Lighter than air gases such as methane can also displace oxygen in confined spaces where the gas can become trapped and unable to escape.
  7. Absorption
    - Can occur when materials in the space absorb oxygen directly from the air. Examples include paints and sealants that are catalyzed by oxygen as they dry or cure. Other examples include limestone and activated charcoal.

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Written by Bob Henderson | President

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Bob Henderson is President of GfG Instrumentation, Inc. in Ann Arbor, Michigan.Robert has been a member of the American Industrial Hygiene Association since 1992. He is an active member of the AIHA Real Time Detection Systems Technical Committee, and the AIHA Confined Spaces Committee. He is also a past chair of the Instrument Products Group of the International Safety Equipment Association. Robert has over 37 years of experience in the design, sale and marketing of atmospheric monitoring instruments used in confined space, industrial safety, and industrial hygiene monitoring applications.

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