What is Nitrox

Nitrox Enriched Air

You may have seen tanks with markings like this and wondered what exactly is nitrox enriched air? The most common misconception is nitrox is a gas that makes your air last longer, it does not. Some think nitrox allows you to go deeper, this is definitely not true. Lets talk about what nitrox actually is.

The purpose of nitrox is to reduce your nitrogen absorption and give you more bottom time.

Nitrox is any gas containing oxygen (O2) and nitrogen that has an O2 part of more than 21%. We call it gas because an tank air is understood to be 21%. The most common gas mixes for a certified nitrox diver are 32% and 36% O2. You can also get mixes as high as 40%, above that there are special considerations we will talk about later. Many divers customize their gas to their dive. I have had mixes as low as 23% O2.

As you see in the image above, 79% of each breath from an air tank is nitrogen while 32% O2 is 68%nitrogen and 36% O2 64% nitrogen. We know from dive tables, the more nitrogen we adsorb, the closer we get to our NDL (no decompression limit). You get the most advantage from nitrox in the recreational diving range. Many who do multiple dives a day such as on a live aboard use nitrox to keep their nitrogen levels manageable.

Here we have 2 dive tables, one for air (Blue) and one for nitrox 36% (Green and Yellow). On air at 50 feet, you will reach your NDL in 80 minutes. On nitrox 36% at 50 feet you will reach your NDL at 220 minutes, thats a huge difference. On air at 110 feet your NDL is 16 minutes, on 36% nitrox its 29 minutes. You can clearly see the advantages of nitrox over air in regards to nitrogen absorption and how in the recreational range you get the most benefit.

Maximum operating depth (MOD)

You may have noticed the 32% nitrox table stops at 110 feet while air goes to 140 feet. This is because nitrox becomes toxic when exceeding the MOD. We know with air, divers experience nitrogen narcosis at around 100 feet. Its not life threatening physically but it can cause disorientation. With nitrox, nitrogen narcosis isn't generally a concern but O2 toxicity is and it can be life threatening. Nitrox divers not only have to monitor their NDL, they have to monitor their PPo2 (partial pressure of O2) levels. A PPo2 of 1.4 in recreational diving is considered the safe limit. A PPo2 of 1.6 is considered the absolute limit. Like nitrogen narcosis, everyone reacts to PPo2 different. 1.5 may be OK for your buddy, but not for you. The biggest danger to someone suffering from O2 toxicity is they could go into convolutions and lose their regulator. But like narcosis, reducing your depth at the first warning signs will avoid that.

Special considerations when using nitrox

Nitrox has increased levels of O2 which increases the chance of fire with the biggest risk being when filling tanks because they tend to get hot. Some filling stations have pre mixed nitrox gas reducing the risk of fire. Others may do the mixing in the tank itself adding 100% O2 and nitrogen separately, this is the most dangerous. Nitrox tanks must be O2 cleaned and have nitrox ready o-rings. This is why those tanks have special markings. You may notice on the tank valve a nitrox rating, most valves are rated to nitrox 40%, the recreational mix limit. Most regulators are rated for 40% as well. Higher than 40% O2 usage requires special cleaning, lubricants and o-rings. You may notice some regulators say "O2 ready" and may be green.


Nitrox tanks should be analyzed for their mixture by the filler and the diver. A diver MUST analyze the mix themselves, never trust anyone else. I once had a filler mark my tank as 32%, it was actually 36%. Why did it matter? The MOD for what they marked was 130 feet. The MOD for what was in the tank was 110 feet. It can also be bad in reverse. If you think you have 36% (NDL at 90 feet = 40 minutes) and you have 32% (NDL at 90 feet = 35 minutes). You have 5 minutes less than you thought you had which could result in unrealized deco.

Nitrox capable computer

Just about all computers allow the use of nitrox and they are a must for monitoring NDL and PPo2 levels. But they have to be set before the dive. One of the biggest issues in nitrox diving is not setting the computer for the appropriate gas. This isn't a deal breaker if you have a nitrox tank and leave your computer on air, some do this for an extra level of conservatism. It is a big deal if you are diving air and your computer is on nitrox. At 90 feet on air your NDL is 25 minutes, but if you have your computer on 36% you will think your NDL is 40 minutes. That can result in a deco obligation you don't have enough gas to complete, if you realize the error. If not it could result in a DCS (decompressions sickness) situation. I personally analyze my tank and immediately set my computer.

So there you have it, a summary of nitrox. This by no means is a substitute for the actual nitrox course but may help you in your decision if nitrox is right for you. Nitrox has many benefits if used correctly and even though there is no scientific data to support it, many say they have more energy after a nitrox dive. I tend to agree.