Nitrox - A breath beyond

Most of you have heard of Nitrox as a diving gas, and Crawley Divers will soon be able to supply Nitrox to suitably qualified divers, so I thought I would write a little article about the subject.

Let's start with Air ….

Air has been the diving gas that we all learned to dive with. As such, in our training, we have come to know some basic facts regarding this mixture. These include:

a) Air is made up of 21% Oxygen & 79% Nitrogen (and some trace gases).
b) We metabolise the oxygen and our bodies do nothing with the Nitrogen part.
c) Nitrogen under pressure causes divers problems including D.C.I. and Nitrogen Narcosis.

Nitrox is basically any mixture of oxygen and nitrogen, so in fact air (21% O2 + 79% N2) can be said to be a Nitrox mix. In fact, Air is classed as a Normoxic Mix.

True Nitrox mixes contain a higher percentage of oxygen than air. This means that the percentage of Nitrogen is lower than the 79% found in air.

This new gas mix has several advantages over air when diving. It also has some major disadvantages too!


Because D.C.I. and Nitrogen Narcosis effect us due to our exposure to Nitrogen, any lowering of the amount we are exposed to will reduce the possible effects of these diving problems.

So a Nitrox mix that contains 28% Oxygen will be better than air (at 21% Oxygen). A Nitrox mix with 36% Oxygen will be better than both these other mixes.

We, as divers, are not actually using any greater amount of O2 in our bodies that when we dive with air, it’s just that if we want to lower the N2 level, we need to replace it with some other gas. Oxygen is a metabolic gas i.e. it’s used by the body therefore it won’t give us D.C.I. as if nothing else our bodies would resolve the D.C.I. by using the oxygen.

You can also see, we’re defining our Nitrox mix by the percentage of oxygen it contains. You will often hear people refer to a “Nitrox 32” or a “Nitrox 36”, the number refers to the percentage of oxygen.

So if a higher percentage of Oxygen gives a lower percentage of Nitrogen – why aren’t we diving using pure O2 (100%)?

Well here’s one of the downsides.. Oxygen is a poison. Yes you heard right - a poison. We need it but only because our bodies use it as an oxidising agent by combining it with other chemicals to make them safer and to keep us living. Hence why you see those skin care adverts going on about free radicals, ozone etc.

The higher the percentage of O2 in a Nitrox mix, the shallower the depth at which it becomes toxic to us.

So to keep things simple, if we dived using 100%, it would become dangerous below 6 metres – not good!

The basic formula we use (in the BSAC) is, if the partial pressure is greater than 1.44 bar, it’s considered dangerous.

“Partial Pressure?” I hear you say ….

Well, at the surface, Air contains 21% O2 / 79% N2. This equates to 0.21 ppO2 / 0.79ppN2.

If we went to 10 metres on air, the pressure is 2 bar. Hence the partial pressures would be twice what they are on the surface, so percentage wise, our air at 10 metres still contains 21% O2 / 79% N2 but the partial pressures are now 0.42ppO2 / 1.58ppN2.

So to follow this through, on air, when the ambient pressure is 6.6 bar (56 metres), the partial pressure of O2 would be 1.4 which is the danger limit – hence why the maximum diving depth allowed by BSAC is 50 metres (shallower than the depth where O2 becomes dangerous).

So, a Nitrox mix where the Percentage O2 is higher than air means that your max depth is actually shallower than air. Nitrox is not a deep diving gas!

The 2 standard Nitrox mixes people often use are Nitrox 32 and Nitrox 36.

  Max Depth ppO2  at Max depth
Air  50m  1.26
Nitrox 32  35m  1.44
Nitrox 36  30m  1.44

These fit nicely with the depth limits for Sports Divers.

So what happens if you go deep on high O2 mixes? Well, there are a couple of things that can happen – the main one is Oxygen Toxicity. Underwater, you can suffer symptoms that include convulsing and unconsciousness. These by themselves won’t kill you, but you do tend to spit out your regulator and not being able to breathe underwater normally doesn’t do you any good!

So what are the other positive sides to using Nitrox?

Well, if you use a Nitrox Dive Table or Nitrox Dive Computer it will either give you less decompression or a longer dive time without deco.

Note : Nitrox does not stop D.C.I.

Other reported effects include reduced tiredness after a dive and possible reduced sub-clinical D.C.I. bubbles. If you are old or overweight, again Nitrox is an advantage as Nitrogen is absorbed by fat tissue.

So any other downsides?

Well, not so much downsides but special considerations.

Firstly, when Nitrox is blended, it needs to be done by properly qualified gas blenders. As you are dealing with pure O2 and high pressures – it is quite dangerous if mishandled.

As you are filling cylinders with pure O2, they need to be Oxygen Clean, to stop explosions from contact with carbon based materials. This needs to be done once a year by a dive shop and costs £15 to £20.

From then on, the cylinder must only be filled with O2 Clean Air, to stop cross contamination with O2 clean equipment.

For diving using Nitrox mixes up to 40%, that’s all you need to do. You can use your existing regulators – beyond 40% these too need to be O2 cleaned once a year.

So, if you’re interested in changing to Nitrox, you need to do a Nitrox skill development course. BSAC offer two different courses:

Basic Nitrox – is a one-day course that allows you to use Nitrox 32 and Nitrox 36.

Advanced or Combined Nitrox – is a two-day course that allows you to use Nitrox up to 50% for diving and decompression stop use.

Any further questions chat to me in the pub!

Peter Smith

This page was last updated on : 06 Sep 2011