What percentage of oxygen is considered normal air?

Normal air is composed of approximately 20.9% oxygen by volume under standard atmospheric conditions. This percentage is significant for various processes, particularly in water treatment and aquatic environments, as it impacts the respiration of aquatic organisms and the overall health of water systems. Accurate knowledge of oxygen levels is vital for maintaining balanced ecosystems and ensuring the efficiency of processes like aeration in water treatment.

The values listed in the other choices represent percentages that deviate from the accepted standard. Recognizing that the correct answer reflects commonly understood atmospheric composition helps in understanding the impact of oxygen levels in both environmental and industrial contexts. Thus, knowledge of oxygen concentration is essential for those working in fields related to water treatment and environmental health.