BOD₅ measures the amount of dissolved oxygen (mg O₂/L) consumed by microorganisms when oxidising organic matter in a water sample at 20°C over 5 days in the dark. It is a standard indicator of organic pollution in wastewater. High BOD indicates high organic load, which depletes oxygen in receiving waters, causing fish kills. BOD differs from COD (chemical oxygen demand), which measures total oxidisable material by chemical oxidation.