The map shows the mean annual value of BOD observed in river monitoring stations since 1993, reported by EEA member countries via WISE. BOD (Biological Oxygen Demand or Biochemical Oxygen Demand) is the amount of dissolved oxygen, expressed in milligrams of oxygen per litre of water, consumed by micro-organisms to decompose the organic matter present in water. BOD is commonly reported as BOD5, the amount of oxygen consumed over a 5-day period of incubation. Some countries report BOD7, the amount of oxygen consumed over a 7-day period. For comparability, the values of BOD7 are converted to BOD5 (BOD7 = 1.16 BOD5). Large quantities of organic matter (microbes and decaying organic waste) in water are a potential risk to aquatic ecosystems and human health. The reduction in the amount of dissolved oxygen as a result of the decomposition of organic matter can endanger aquatic life through asphyxiation and disrupt the ecological balance of the water. Organic matter can also pollute drinking water and bathing water. High levels of BOD can indicate such organic pollution. The purpose of the map is to provide an overview of the mean annual value of BOD in rivers across Europe and to enable the user to compare values per country, River Basin District (RBD) or individual monitoring station, depending on the scale of visualisation. Historical data since 1992 can be displayed using the time slider.