US Occupational Safety and Health Administration, US Department of Labor.

A substance that gives up oxygen readily. Presence of an oxidizer increases the fire hazard.

Oxygen deficiency
That concentration of oxygen by volume below which atmosphere supplying respiratory protection must be provided. It exists in atmospheres where the percentage of oxygen by volume is less than 19.5 percent oxygen.

Oxygen-enriched atmosphere
An atmosphere containing more than 23.5 percent oxygen by volume.