How does an increased depth affect the ppO2 experienced by a diver?

At increased depths, the partial pressure of oxygen (ppO2) that a diver experiences rises due to the relationship between pressure and gas volume described by Boyle's Law. As a diver descends, the surrounding water pressure increases, which consequently increases the total pressure acting on the diver's body.

Since the concentration of gases in a mixture remains the same while the total pressure increases, the partial pressure of each gas in that mixture, including oxygen, also increases. For example, at sea level (1 atmosphere of pressure), oxygen makes up about 21% of the air we breathe, giving a ppO2 of approximately 0.21 atmospheres. However, as a diver goes deeper, for every 10 meters of seawater, the pressure increases by approximately 1 atmosphere. Therefore, at 10 meters, the total pressure is 2 atmospheres, and the ppO2 becomes 0.42 atmospheres (21% of 2 atm).

This concept is critical when planning dives using enriched air nitrox, as higher ppO2 levels can cause oxygen toxicity if they exceed safe limits. Understanding the relationship between depth and ppO2 is essential for safe diving practices.