Oxygen-deficient atmosphere
Air with less than 19.5% oxygen by volume.
Definition, in the source's words
29 CFR 1910.134(b)(opens ecfr.gov in a new tab)“Oxygen deficient atmosphere means an atmosphere with an oxygen content below 19.5% by volume.”
What it means
Normal air is about 20.9% oxygen. Inert gases such as nitrogen or argon, rusting steel, and microbial action can all push it lower without any smell or warning. Under 1910.134(d)(2)(iii) oxygen-deficient atmospheres are considered IDLH, with a narrow exception: where the employer shows oxygen stays within the ranges in Table II for the altitude, any atmosphere-supplying respirator may be used.
Example
After a nitrogen purge, a vessel reads 19.0% oxygen. That's below 19.5%, so it's oxygen deficient: a hazardous atmosphere for confined space entry, and IDLH for respirator selection.
Where it appears on the exam
ASP: ASP11 Domain 6, Industrial Hygiene and Occupational Health
Practice: Respiratory Protection, Permit-Required Confined Spaces
CSP: CSP11 Domain 1, Advanced Application of Safety Principles
Practice: Confined Spaces and Lockout/Tagout
CHST: CHST5 Domain 1, Hazard and Risk Identification and Control
Practice: Confined Spaces in Construction
Questions
- Is 19.5% oxygen deficient?
- No. The definition says below 19.5%, so 19.5% itself is not deficient; 19.4% is.
Related
Checked 2026-10-16. Spot a problem? Email hello@passcited.com.