Hazardous atmosphere
Air that could kill, injure, incapacitate or keep a worker from escaping, from one of five listed causes.
Definition, in the source's words
29 CFR 1910.146(b)(opens ecfr.gov in a new tab)“Hazardous atmosphere means an atmosphere that may expose employees to the risk of death, incapacitation, impairment of ability to self-rescue (that is, escape unaided from a permit space), injury, or acute illness from one or more of the following causes:”
What it means
The five causes in 1910.146(b) are: flammable gas, vapor or mist above 10% of its lower flammable limit (LFL); airborne combustible dust at or above its LFL; oxygen below 19.5% or above 23.5%; a substance above its permissible exposure limit where a dose or exposure limit applies; and any other atmosphere that is immediately dangerous to life or health.
Example
A meter at a tank opening reads 12% LFL for a solvent vapor and 20.8% oxygen. The oxygen is fine, but 12% is above 10% of the LFL, so the atmosphere is hazardous and entry conditions aren't acceptable until ventilation brings it down.
Where it appears on the exam
ASP: ASP11 Domain 2, Safety Programs and Concepts
Practice: 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
- What oxygen levels make an atmosphere hazardous under 1910.146?
- Below 19.5% (oxygen deficient) or above 23.5% (oxygen enriched).
Related
- Oxygen-deficient atmosphere
- IDLH (immediately dangerous to life or health)
- Permit-required confined space
- Regulation: 29 CFR 1910.146
Checked 2026-10-16. Spot a problem? Email hello@passcited.com.