What percentage of the air we breathe is composed of oxygen?

The composition of the air we breathe is a fundamental aspect of understanding environmental science and HVAC systems. The percentage of oxygen in the Earth's atmosphere is approximately 21% by volume. This value can be expressed in different ways, and it’s essential to recognize that 20.95% is an accurate and scientifically accepted measurement of the oxygen concentration in dry air.

The percentage of oxygen is vital for HVAC professionals because it influences combustion processes in heating systems and affects indoor air quality. Correctly understanding atmospheric oxygen levels is crucial for system design, ensuring efficient operation, and maintaining safety standards within HVAC systems.

While the options presented may reflect various approximations of this percentage, it is the measurement of 20.95% that most accurately represents the average concentration of oxygen in the atmosphere when accounting for different conditions.