What is the composition of the air we breathe at the surface?

The composition of the air we breathe at the Earth's surface consists of approximately 21% oxygen and 79% nitrogen. This ratio is considered standard in the troposphere, where most of the Earth's weather occurs and where humans and other terrestrial life exist. The presence of nitrogen is significant because it acts as a diluent for oxygen, balancing the atmosphere and preventing combustion at normal atmospheric temperatures.

Understanding this composition is vital for divers and those in underwater settings since changes in atmospheric pressure and gas composition can significantly affect buoyancy, breathing gas availability, and overall safety during dives. The percentages provided in the other choices do not accurately reflect the atmospheric composition at sea level, which reinforces why 21% oxygen and 79% nitrogen is the accepted figure.