What factor is most critical in determining the appropriate mix of Nitrox for a dive?

The depth of the dive is the most critical factor in determining the appropriate mix of Nitrox because it directly influences the partial pressures of oxygen and nitrogen that divers are exposed to during the dive. When diving at greater depths, the pressure increases, which increases the partial pressure of gases in the breathing mix.

To safely use Nitrox and manage the risk of oxygen toxicity, divers must consider the maximum operating depth (MOD) for the specific Nitrox mix they plan to use. Each mixture has a maximum depth at which it can be used safely, owing to the rise in partial pressure of oxygen beyond certain limits. For example, higher concentrations of oxygen can become toxic when the partial pressure exceeds 1.4 ATA, which occurs at specific depths depending on the Nitrox blend.

While dive time, water temperature, and equipment compatibility are also important factors in diving, they do not directly dictate the choice of Nitrox mix as significantly as depth does. Depth sets the stage for what levels of oxygen and nitrogen divers will experience and, consequently, what mix will keep them within safe limits for both oxygen toxicity and nitrogen narcosis. Therefore, it is depth that primarily guides the formulation of a safe Nitrox mix.