Which two factors determine your total oxygen "dose" during a dive?

The total oxygen "dose" during a dive is fundamentally determined by the partial pressure of oxygen (ppO2) at depth and the duration of the exposure to that pressure. The partial pressure of oxygen increases as a diver descends, which directly affects how much oxygen their body absorbs and processes.

As a diver goes deeper, the increased pressure leads to a higher ppO2, and this, coupled with the time spent at that depth, dictates the overall amount of oxygen taken in. This is essential in managing risks such as oxygen toxicity, which can occur if a diver exceeds safe exposure limits of ppO2. It illustrates the critical interaction between depth and time when planning a dive using enriched air nitrox.

Other options do not directly pertain to determining the total oxygen dose. For instance, the partial pressure of nitrogen or the nitrogen fraction in the tank is relevant to nitrogen exposure and risk but does not affect the oxygen dose. The last dive depth is also not relevant to calculating oxygen dose for the current dive. Thus, focusing on the ppO2 and the dive duration provides the most accurate assessment of oxygen uptake during a dive.