The amount of oxygen that is required for the complete oxidation of microbiologically degradable material in water. It is measured by the direct oxygen consumption of materials such as sewage and contaminated water, and represents a measure of the amount of organic pollutants in water. As a form of index used for the degree of organic pollution in water, it is expressed as the amount of oxygen used for biochemical oxidation by a unit volume of water at a given temperature and for a given time. It involves incubating a sample at 25°C for a fixed time (often five days), and the amount of oxygen removed determined by chemical analysis. The result is expressed in milligrams of oxygen per dm3 of water. The higher the value, the greater the level of pollution. It is also known as the biological oxygen demand.