Diving is a sport that requires not only extensive preparation but also strict control over the equipment to be used. Let’s not forget—underwater, we still need to breathe!
 

Reasons for Using Gas Mixes in Diving

Why would we choose to breathe gas mixes other than air if our bodies are perfectly adapted to breathing air from birth? Well, this isn’t entirely accurate. Our bodies are adapted to breathe air at atmospheric pressure, but when breathing it at high pressures—such as in diving—air ceases to be the optimal gas.

Know the reasons for mixing gases in diving


If we treat the gas mix we breathe like any other piece of diving equipment, we’ll aim for a mix optimised for the type of dive we’re undertaking.

Why isn’t air an optimal mix?

  • It contains too much nitrogen, forcing us to limit our dive time or perform decompression stops to eliminate excess gas accumulated in our tissues.
  • Below -30 metres, air becomes narcotic, impairing cognitive function and complicating even mildly complex tasks.
  • Below -40 metres, the increased ambient pressure causes the density of breathed air to rise, significantly increasing respiratory effort and potentially inducing a sensation of suffocation.
  • Below -66 metres, air becomes outright toxic due to the high partial pressure of oxygen.


Practice with nitrox


So, what alternative gases can we use for breathing?

Main Types of Gas Mixes
 

  • Oxygen-enriched mixes, known as Nitrox or enriched air. These reduce nitrogen levels to extend dive times without increasing decompression requirements or serve as decompression gases to accelerate the process.
  • Helium-based mixes. These mitigate nitrogen narcosis, oxygen toxicity, and reduce gas density. Recently, experimental helium mixes are also being used for decompression. Helium mixes include Heliox (helium + oxygen) and Trimix (helium + oxygen + nitrogen).

Some Trimix blends are also labelled differently, such as Heliair—a specific Trimix variant made by blending helium with air—or Helitrox, a newer term for mixes of helium + Nitrox.

Two other gases are used: pure oxygen for decompression stops at -6 m and -3 m, and occasionally argon—not as a breathing gas but as an insulating gas for drysuit inflation.

Point out the air cylinders to have them identified

Differences Between Mixed-Gas Diving and Air Diving

In recreational diving, using Nitrox mixes (21–40% oxygen):

  • Equipment: Standard diving gear can be used, but decompression tables must be adjusted or Nitrox-specific ones employed. A Nitrox-capable dive computer is also an option. Nitrox cylinders must be clearly labelled as NITROX.
  • Pre-dive: Verify the oxygen content of the mix and its maximum operating depth, clearly marking this depth on the cylinder.
  • During the dive: Monitor depth to avoid exceeding the mix’s maximum limit.
  • Decompression: No-stop dive times increase significantly, or decompression times reduce. For example, with Nitrox 36 (36% oxygen), you can spend 35 minutes at 28 metres without decompression, versus 25 minutes on air—a 40% no-decompression time gain.
  • Post-dive: Remember your cylinder contains Nitrox, not air. If reused by others, they must be informed.

In technical diving below -40 metres, differences from air diving are substantial (covered in future articles), but key distinctions include:

  • Multiple gas mixes per dive (descent, bottom, decompression), requiring multiple cylinders.
  • Specialised equipment to manage additional gear efficiently.
  • Critical dive planning—summarised by the adage, Practice the necessary diving techniques