Oxygen deficiency is still the leading cause of death in confined space incidents.
The concentration of oxygen in normal air is approximately 20.9%. The balance (over 78%) consists primarily of nitrogen. The remaining fraction includes small amounts of water vapor, carbon dioxide and argon, as well as traces of other gases.
OSHA 29 CFR 1910.146, "Permit Required Confined Spaces," contains the requirements for practices and procedures to protect employees in general industry from the hazards of entry into permit-required confined spaces.
The OSHA standard defines an oxygen deficient atmosphere as any atmosphere containing less than 19.5% oxygen by volume.
Any atmosphere that contains less than 19.5% oxygen is hazardous, and may not be entered by unprotected workers.