From Nicholas McLaren,
Your Guide to Scuba Diving.
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You may have heard about Nitrox (also known as Enriched Air) and be curious what it is and how it works. First it's important to understand what air is and what causes decompression sickness.

Air

The air we breathe consists of approximately 21% oxygen and 79% nitrogen. There are also some other gases present, but only very small amounts.

Decompression Sickness

A simplified description of decompression sickness is that it is the effects of nitrogen bubbles forming in blood, joints, or tissue. When scuba diving, the increased pressure caused by being underwater leads to nitrogen being absorbed into the body's tissues in greater concentrations than on the surface. This in itself is not dangerous, but as you ascend and the pressure decreases the nitrogen comes back out of the body tissue. If a diver ascends too quickly the nitrogen comes out too fast and forms bubbles, causing decompression sickness. That's why you should always ascend slowly and complete a three minute safety stop. Dive tables and computers are designed to ensure that a diver can slowly ascend, perform a safety stop, and surface without the nitrogen coming out too fast.

Nitrox

In simple terms, Nitrox is the name applied to mixtures of oxygen and nitrogen where the oxygen content is greater than 21%. The most common mixtures are 28%, 30%, and 32% oxygen, with the balance consisting of nitrogen.

Why use Nitrox?

But why would we want to have more oxygen in the mixture? Because then we can do longer dives. With increased oxygen in the mixture, there is less nitrogen, which means less nitrogen is being absorbed into the body's tissues. Since less nitrogen is being absorbed into the body's tissues, you can stay underwater longer and still ascend safely. You could also stick to your regular dive time and know that you have an extra level of safety due to breathing less nitrogen. Nitrox is particularly useful for divers who are doing more than two dives a day or repetitive dives over multiple days.

What about the extra oxygen?

Too much oxygen can lead to a problem called Oxygen Toxicity, which is a very dangerous reaction in the body that leads to convulsions. Fortunately, 21% oxygen doesn't cause oxygen toxicity until we go much deeper than recreational diving allows. However, with the increased percentages of oxygen in Nitrox, the depth at which it becomes toxic becomes much less. Different mixtures lead to different maximum depths at which toxicity becomes an issue. There are formulas and tables that can be used to calculate these maximum depths. Most modern dive computers also have Nitrox mode which can calculate maximum depths and revised decompression limits.

How is Nitrox made?

There are two primary methods of mixing Nitrox. One method involves adding pure oxygen to regular air until the ratio is correct. The other method involves using a filter to remove nitrogen from the air until the mixture is correct. It is necessary to complete a formal course to be qualified to mix Nitrox and it is always necessary for the diver to personally test and verify the mixture in their own tank before a dive.

Nitrox Training

In order to obtain tanks filled with Nitrox or to dive with Nitrox you must be certified as a Nitrox Diver. You can learn to safely use Nitrox by completing a one day course. The course only requires a theory component, but may also include dives. During the course you'll learn how to calculate partial pressures of oxygen and nitrogen, how to use Nitrox dive tables, how to calculate maximum depths, theory about Decompression Sickness, and what equipment can be used with Nitrox.