What is the normal oxygen content in the atmosphere?

The correct answer is based on established scientific measurements of atmospheric composition. The normal oxygen content in the atmosphere is approximately 20.9%. This value represents the concentration of oxygen that is essential for aerobic life and is significantly consistent across the globe, providing a stable environment for living organisms.

This percentage is crucial because it affects processes such as respiration in animals and photosynthesis in plants. Understanding this standard is vital for water treatment and environmental monitoring since oxygen levels can influence aquatic ecosystems' health and the efficiency of water treatment processes.

Other values presented, such as 21.5% or 22.0%, are higher than the actual atmospheric concentration, which would not align with actual measurements. Values like 19.5% are lower than the standard and could indicate low oxygen conditions, which may not support human and other aerobic life sufficiency in environments such as confined spaces or areas lacking good ventilation.