There are several advantages to using nitrox as a scuba diving gas. Only the first benefit is commonly taught at novice level diving.
Reduced Inert-Gas Absorption
This is the primary, sometimes only, advantage taught on many nitrox diver courses. It is simply that by raising the fraction (%) of oxygen in a breathing mixture we consequently lower the fraction (%) of inert gas in the mix.
Reducing the fraction of inert gas (i.e. nitrogen) in what we breath equals a proportional reduction in the inert gas our body absorbs.
Inert gas is what can cause decompression sickness in divers. Oxygen is metabolised and, therefore, does not create damaging bubbles.
This results in the diver appreciating longer bottom-time (dive length) using nitrox compared to when breathing regular air and/or surfacing with less inert gas dissolved in their body tissues for a dive of equal duration.
Accelerated Inert Gas Removal
In addition to absorbing less inert gas, a higher O2% in the breathing gas also promotes faster elimination of inert gas when the diver ascends to the surface.
This occurs because lowering the fraction (%) of inert gas being breathed causes greater gas partial-pressure differential between the gas dissolved in the tissues and the gas in the lungs.
The exact same principle of increasing gas partial-pressure differential is what technical divers utilise when “accelerating” decompression by switching to a rich oxygen mix on their shallower ascent stops.
In short, nitrox accelerates the removal of nitrogen, not just slows the absorption of it.
Nitrox promotes more effective micro-emboli (micro-bubble) resolution on the ascent.
Microscopic bubbles form in ALL divers when they ascend. These bubbles aren’t big enough to cause any immediate harm and don’t produce symptoms known as decompression sickness.
However, those bubbles do stimulate the body’s immune system (which recognises the bubbles and attacks them). If there are many of these bubbles, a large immune response may cause noticeable effects on the diver once they’ve surfaced. This is because the immune system response (complement system) involves the release of histamine and then serotonin into the blood. Serotonin also happens to be the hormone that regulates sleep.
A diver with a significant amount of micro-emboli in their venous system may feel sleepy or lethargic post-dive as a result of that prior serotonin release.
This is known as “decompression stress”.
Nitrox raises the inert gas partial-pressure differential across bubble walls – and promotes more bubbles to crush in size and or dissipate entirely during ascent to the surface.
In theory, and as shown in scientific studies, nitrox is believed to reduce that decompression stress.
Hence, divers who typically experience some decompression stress may feel that using nitrox makes them “less tired” after diving.
For more details, and references, see:
Subclinical DCS, Decompression Stress and Post-Dive Fatigue