Full definition
Work inside an enclosure with restricted entry/exit, not designed for continuous occupancy, with risk of hazardous atmosphere (toxic gases, O₂ deficiency), engulfment or entrapment. Classification: permit-required (major hazard) vs. non-permit (atmospheric hazard only). Controls: continuous gas monitoring, forced mechanical ventilation, retrieval system, external attendant. Standards: NOM-033-STPS, OSHA 29 CFR 1910.146.