What is the maximum oxygen content allowed in a breathing gas mixture in terms of percentage?

The maximum oxygen content allowed in a breathing gas mixture for recreational diving purposes is 40%. This limitation is set to manage the risks associated with oxygen toxicity, particularly at higher pressures underwater. When divers breathe gas mixtures with elevated levels of oxygen, such as those found in enriched air (Nitrox), there is an increased risk of harmful effects on the body, especially at depths beyond 30 meters (100 feet).

In the context of recreational diving, a 40% oxygen concentration is often used because it provides a beneficial balance, allowing for extended bottom times and reduced nitrogen absorption while still keeping the risk of toxicity within acceptable limits for most diving conditions. This particular formulation aligns with industry standards and safety protocols, ensuring that divers can utilize Nitrox effectively without jeopardizing their safety.

Other options, such as 50% or higher concentrations, would significantly heighten the risk of oxygen toxicity, particularly for prolonged exposure or deeper dives. On the other hand, a 20% oxygen content is typical for air, which is not enriched in oxygen and is generally considered safe but does not provide the benefits associated with using Nitrox for dive scenarios. This is why 40% strikes the optimal balance for safe and effective diving practices using enriched air.