Oxygen is essential for human life. The oxygen concentration in normal air should be 20.9% by volume. Under normal conditions, fluctuations in this important oxygen concentration are extremely small. The human body is very sensitive to changes in the level of oxygen in breathing air. Oxygen deficiency leads to tiredness as one of the first symptoms. This means that the operator’s efficiency and safety while operating plant and machinery is quickly impaired. A decrease of oxygen concentration of only 5 % can cause breathing difficultly and damage to health.