What nitrox mixture can be used at any depth shallower than 99 feet?

The correct answer is EAN32 because it has a maximum partial pressure of oxygen that complies with safety limits at depths shallower than 99 feet. In particular, EAN32, which contains 32% oxygen, achieves a partial pressure of approximately 1.4 atm at around 110 feet. Since the generally accepted maximum partial pressure of oxygen is 1.4 atm for recreational diving, EAN32 ensures that divers can safely use it below the recommended limit.

As for other options, EAN36, which has a higher oxygen content of 36%, approaches the upper limits of oxygen exposure at shallower depths and would exceed the safe partial pressure threshold at 99 feet. EAN28, which contains 28% oxygen, has a safer profile for depth but doesn’t utilize the nitrogen advantage as effectively as EAN32 for longer dives. Finally, EAN40, with 40% oxygen, comes close to the limit very quickly and is only safe at even shallower depths, potentially exceeding the maximum partial pressure of oxygen if used at 99 feet or deeper. Therefore, EAN32 represents the most versatile and safe option for depths shallower than 99 feet.