Oxygen Deficient Environments in Confined Spaces

It is generally recognized that lack of oxygen is the leading cause of death in confined spaces. You cannot see or smell a oxygen deficiency therefore the hazard is not readily apparent to someone entering an oxygen deficient environment. The only way to determine if a confined space has sufficient oxygen is to test the atmosphere with a calibrated gas monitor.

The air we breathe contains approximately 20.9 % oxygen. Most of the remaining 79% is made up of nitrogen with smaller quantities other gases such as argon and carbon dioxide. Interestingly, contrary to what most people think, the percentage of oxygen in the air remains the same even at higher elevations. However because the air at higher elevations is less “dense”, there are fewer molecules of everything present, including oxygen. Less oxygen molecules means it is it potentially harder to breathe despite the fact that a gas monitor will still read 20.9%.

Low levels of oxygen can lead to impaired judgment, lack of coordination, behavior changes, dizziness, fatigue and ultimately collapse and death. Sometimes workers think they can “hold their breath” for a second to enter a space quickly without testing or ventilation.  But even one breath of oxygen deficient air could prevent your muscles from responding so that you cannot have the strength to escape the space even if conscious.  Those with coronary, pulmonary, or circulatory disease may feel symptoms before others. I once investigated a confined space incident in which only one of three workers was dizzy and passed out.  The atmosphere was later tested and found to
have a slightly lower oxygen level of approximately 18-19.5 %.  The only worker affected was the one who had a pre-existing cardiac condition.  

Low oxygen levels occur from chemical or biological processes or reactions that either consume or displace oxygen from the confined space. Common causes of oxygen
deficiency include:

  • Rusting-(rusting is an oxidation process that consumes oxygen).
  • Combustion-(all sources of combustion such as propane heaters, welding, consume oxygen).
  • Displacement by other gases- (such as Nitrogen purging, inerting, welding gases)
  • Decomposition of Organic Matter (Micro-organisms consume oxygen and produce flammable methane gas that can also displace oxygen

While most gas monitors will not alarm until 19.5% (OSHA allowable lower limit for entry), it is recommended that you establish a policy to require 20.9 % oxygen prior to entry. If you test the atmosphere in a confined space and it is anything OTHER THAN 20.9% you should investigate the source of this oxygen deficiency and ventilate the space prior to entry, retesting until the oxygen level is maintained at 20.9%.  With

so many variables and potential hazards in confined spaces, you should strive to maintain the atmosphere as close to “normal” as possible.  

NFPA is in the process of developing a Best Practices Document for Confined Space Entry.  One item that we will likely include as a best practice is to prohibit entry into confined spaces where oxygen levels are less than 20.9% and to ventilate the space until the levels reach 20.9%.   You may wish to sign up for the alerts for the document that is being developed by going to and clicking on the SIGN UP FOR EMAIL ALERTS link above the tabs. An email will be sent notifying you of any meetings or additions to the document information page related to the confined space document. If you have ideas for what should be included in this document or would like to be involved in document development please let us know! Task groups to develop draft chapters of the document are now being formed.  If you have an interest or special expertise in a particular area let us know how to contact you! 
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