O₂ refers to the concentration of O₂ (oxygen) present in air, water, or biological systems. O₂ is essential for life as it plays a critical role in cellular respiration and organism metabolism. In the context of air or water quality, O₂ measurement is used to monitor environmental health, ecosystem balance, and biological activity. Excessively low or high O₂ concentrations can affect organism survival and overall environmental quality.
