Approximately what percentage of oxygen does the air we breathe contain?

The air we breathe consists of approximately 21% oxygen, which is crucial for the survival of most life forms on Earth. This percentage is a well-established fact based on atmospheric composition studies, indicating that oxygen is not present in excess but at a balanced level that supports biological processes.

The human respiratory system is adapted to efficiently utilize this oxygen for cellular respiration, where it plays a vital role in converting food into usable energy. Maintaining this oxygen concentration is essential for sustaining life, as both lower and higher levels can lead to detrimental effects on health and the environment.

Understanding the composition of air is fundamental in disciplines ranging from environmental science to various engineering fields, where the impact of oxygen levels on combustion and respiratory processes is often considered.