Oxygen (O2) deficiency is a hazardous condition in many plants and buildings. The air we breathe is 20.9% oxygen by volume and the human being cannot function properly with even slight deviations of this level. Oxygen deficiency can occur in confined spaces such as manholes, tunnels, pipelines, storage tanks and underground vaults. Often a benign gas such as Nitrogen would not be a hazardous gas, but if that gas pushes the oxygen out of the space, then Oxygen deficiency becomes the hazard. Oxygen deficiency can begin to occur when levels of oxygen in the air drop below 19.5% by volume. Following are the key hazard levels of Oxygen deficiency that industries should be aware of when researching Oxygen deficiency gas detectors and gas detection systems. Concentration of Oxygen (v/v) Effects 20.9% Oxygen concentration of Air 19.5% Minimum