As organic matter enters waterways, the micro-organisms in the water begin to break it down with the help of oxygen. Biological oxygen demand means the volume of oxygen consumed by this activity; the more organic matter the waste water contains, the higher the biological oxygen demand is. Oxygen demand is important because, especially during winter, all oxygen in the water may be used up, which may lead to the mass death of the living organisms in the water. Biological oxygen demand is usually abbreviated as BOD. An index after the abbreviation signifies the number of days over which the oxygen demand is calculated. Thus, the abbreviation BOD7 means the oxygen consumed by a given sample of waste water in seven days. Chemical oxygen demand means the same as biological oxygen demand, except that here the oxygen is not consumed by micro-organisms, but by chemical reactions. The COD, as it is usually abbreviated, primarily describes the oxygen consumption by slowly-decaying organic compounds.