Ask any well-trained employee entering confined spaces what their main concern is, and they will most likely say, “oxygen deficiency.” Notice the use of the words “well-trained.” Oxygen deficiency in confined spaces isn’t something we can see with our eyes. However, if we can educate employees on the causes, symptoms, and health effects oxygen deficiency as well as the importance of atmospheric monitoring, we can send them home safely.

oxygen deficiency

The air we breathe contains approximately 20.9% oxygen, 78% nitrogen, and smaller quantities of gases such as argon and carbon dioxide. The immediate effects of low oxygen environments can be traced to the body’s oxygen transport system. Blood absorbs oxygen from air in the lungs to fuel cells in the body. The body organ most sensitive to lack of oxygen is the brain. A person will immediately begin to feel the health effects associated with lack of oxygen with decreases of 1 to 2 percent.

Healthy individuals are unable to work strenuously or lose coordination when oxygen levels are between 15 and 19 percent. When oxygen levels are between 10 to 12 percent respiration increases, lips turn blue, and judgement is impaired. Fainting and unconsciousness occur when oxygen levels are between 8 and 10 percent. Death can occur within eight minutes if the oxygen level is between 6 and 8 percent. These values may vary based on several factors including the person’s health, the level of physical activity, and the environment.

OSHA Respiratory Protection Standard 1910.134 (link)

The Occupational Safety and Health Administration’s (OSHA) Respiratory Protection Standard 1910.134 defines oxygen deficiency as an atmosphere with an oxygen content below 19.5% by volume. OSHA’s Permit Required Confined Space Entry Standard 1910.146 and Grain Handling Standard 1910.272 also recognize that an atmosphere with less than 19.5% oxygen is hazardous. Many safety professionals agree that entry into any hazardous space or other area where oxygen deficiency may exist requires the oxygen levels to be as close to normal as possible. For instance, an atmospheric level of 20.5%, while still acceptable to OSHA, signals that there is an issue that needs to be investigated and addressed.


A number of factors can cause oxygen deficiency when in confined spaces. In general, oxygen deficiency occurs when another gas displaces oxygen in a poorly ventilated space. For example, decomposition of organic material or smoldering grain can consume oxygen within a space such as a grain bin, silo, or tank. Grain that is high in moisture or poor in quality can result in elevated levels of carbon dioxide. Carbon dioxide can displace oxygen within spaces such as grain bins, silos, tanks, and even flat storage buildings. Carbon Dioxide is denser than air. Therefore, it can also accumulate in low-lying areas adjacent to confined spaces.


A worker cannot see, taste, or even smell an oxygen deficient atmosphere in a confined space. The only way to identify oxygen deficiency is to test the air with a calibrated atmospheric monitor. According to OSHA, calibration refers to an instrument’s measuring accuracy relative to a known traceable concentration of test gas. Atmospheric monitors must be calibrated according to manufacturer guidelines. Keep in mind that calibration gases have an expiration date. Calibration gases that have expired should be replaced immediately.

In addition to calibration, a “bump test” should be performed prior to each use. According to OSHA, a bump test is a qualitative function check in which a challenge gas is passed over the sensor(s) at a concentration and exposure time sufficient to activate all alarm settings. The purpose of this check is to confirm that gas can get to the sensor(s) and that all the instrument’s alarms are functional. The bump test does not provide a measure of the instrument’s accuracy.


Both OSHA’s Permit Required Confined Space and Grain Handling Standards require atmospheric testing prior to entry into hazardous spaces. Many safety professionals agree that performing a pre-entry test alone is not adequate. The atmosphere in an area where oxygen deficiency may exist should be continually monitored throughout the entry. Atmospheres within confined spaces can change. Monitoring the atmosphere continually during entry is the best way to protect employees working in confined spaces.

Remember, safety doesn’t have to be difficult, simple steps, such as training employees on the causes, symptoms, and health effects of oxygen deficiency, and implementing pre-entry and continual atmospheric monitoring can send employees home safely.

Related Links:


  • Pearce, Nancy. “Oxygen Deficient Environments in Confined Spaces”. Web blog post. nfpa.org. NFPA today, 13, Nov. 2012.
  • Naranjo, Edward. “Oxygen Deficiency: The Silent Killer.” EHS Today, 12 Dec. 2007.
  • Nolte, Jim, Wisconsin Agribusiness Association. High Moisture New Crop Corn Has Potential to Generate Low Oxygen and High Carbon Dioxide Levels in Boot Pits, Bins and Basements.


Joe Mlynek is a partner and subject matter expert at Safety Made Simple, LLC. He has over 25 years of experience in safety at the corporate level and as a consultant. He is a Certified Safety Professional (CSP) and Occupational Safety and Health Technician (OHST). Joe can be reached at [email protected]