The limits
- Normal air contains about 20.9% oxygen.
- OSHA defines an oxygen-deficient atmosphere as one with less than 19.5% oxygen.
- An oxygen-enriched atmosphere has more than 23.5% oxygen and is a severe fire hazard.
Both are hazardous atmospheres under the confined space standard (1910.146(b)), and acceptable entry conditions on a permit usually require 19.5% to 23.5%.
How oxygen is lost
| Mechanism | Examples |
|---|---|
| Displacement | Nitrogen or argon used for purging or inerting; carbon dioxide from fermentation or dry ice; methane from decomposition; welding shielding gases |
| Consumption | Rusting steel in closed tanks; bacteria in sewage and sludge; decaying organic material; combustion from welding, cutting or engines |
| Absorption | Some materials, such as fresh activated carbon, can absorb oxygen |
Inert gases have no smell or color, and a drop of a few percent isn't noticeable by breathing.
Effects at different oxygen levels
OSHA's Technical Manual and letters of interpretation describe the effects roughly as follows:
| Oxygen | Possible effects |
|---|---|
| Below 19.5% | Oxygen-deficient. Treated as a hazardous atmosphere |
| 16% to 19.5% | Workers doing any exertion can quickly develop symptoms as tissues don't get enough oxygen |
| 12% to 16% | Faster breathing and heart rate; impaired attention, thinking and coordination, even at rest |
| 10% to 14% | Faulty judgment, intermittent breathing and exhaustion with minimal exertion |
| Below 12% | OSHA's Technical Manual notes unconsciousness can come rapidly and without warning; considered IDLH |
| 6% to 10% | Nausea, vomiting, lethargic movements and possible unconsciousness |
Testing and control
- Test oxygen first, then flammables, then toxics, at all levels of the space from outside. See atmospheric testing.
- Watch for small drops: a reading of 20.0% means something is displacing or consuming oxygen, even though it's above 19.5%.
- Ventilate with clean air delivered to the work area; never with pure oxygen.
- Isolate inert gas and process lines feeding the space.
- Monitor continuously during entry, especially near welding gases, purge lines or fermenting materials.
- Inerted spaces and oxygen below 19.5% that can't be corrected require supplied-air respiratory protection and specialized procedures.
Sources and official references
Last reviewed: October 25, 2026 · Free general education based on OSHA 29 CFR 1910.146 and 29 CFR 1926 Subpart AA. It does not replace the site-specific training, written program and hands-on evaluation your employer must provide, and it does not issue a certificate. Nothing you enter on this page is saved or sent.