Publisher Summary This chapter discusses about occupational hygiene. Occupational hygiene deals not only with overt threats to health but also in a positive sense with the achievement of optimal “comfort conditions” for workers, that is, the reduction of discomfort factors that may cause irritation, loss of concentration, impaired work efficiency, and general decreased quality of life. When the potential for harm of a particular stress in the workplace has been recognized, it is necessary to evaluate it, and some common environmental measurement techniques together with their interpretation as they relate to accepted standards. Deficiency of oxygen in the atmosphere of confined spaces is often experienced in industry, for example, inside large fuel storage tanks when empty. Before such places may be entered to carry out inspections or maintenance work, a check must be made on the oxygen content of the atmosphere throughout the vessel. Hygrometers are instruments used in the measurement of the water vapor content of air, that is, the humidity.