How is the partial pressure of a breathing gas calculated?

The partial pressure of a breathing gas is calculated by multiplying the ambient pressure by the fraction of that gas in the mixture. This is based on Dalton's Law of Partial Pressures, which states that in a mixture of gases, each gas exerts a pressure proportional to its concentration in the mixture.

When you take the ambient pressure—which is the total pressure at a certain depth or atmospheric level—and multiply it by the fraction of the gas in the mixture, you arrive at the partial pressure of that specific gas. For example, if you are breathing air that is composed of 21% oxygen, you can calculate the partial pressure of oxygen at a given ambient pressure, such as at sea level (approximately 1 atmosphere or 1013 hPa), by taking 0.21 (the fraction of oxygen) and multiplying it by the ambient pressure.

This approach forms the basis for understanding how gases behave under pressure and is crucial for managing risks associated with breathing gases at various depths during scuba diving.