What is the partial pressure of oxygen when breathing air at sea level?

The partial pressure of oxygen when breathing air at sea level is approximately 0.21 ata. This is derived from the composition of air, which is about 21% oxygen and 79% nitrogen. At sea level, the atmospheric pressure is approximately 1 ata. To calculate the partial pressure of oxygen, you multiply the total atmospheric pressure by the fraction of oxygen in the air: 1 ata (total pressure) multiplied by 0.21 (the fraction of oxygen) yields 0.21 ata.

This calculation is essential in understanding how gases behave in different conditions and is particularly significant in diving, where different pressures can affect the availability of oxygen and nitrogen for breathing. The concept of partial pressure is crucial in various applications such as the design of dive tables and understanding the risks of decompression sickness.