Breathing gas mixtures for scuba divers

Breathing underwater is a unique experience for every scuba diver. At first, breathing through the regulator in your mouth may feel a bit strange and unusual; however, as you gain more experience, breathing compressed gases from your cylinder will become second nature. So which gases do scuba divers actually breathe underwater?

Compressed Gases

Scuba divers breathe compressed gases. Scuba cylinders contain breathing gas mixtures that are pressurized to a particular level, commonly to 200 bars (as a reference, atmospheric pressure on the surface, at sea level, is equal to 1 ATM or 1 bar). This means that a scuba cylinder filled with a compressed gas mixture can last a long time. For example, a 10 litre cylinder filled with air pressurized to 200 bars will contain 10 x 200 or 2000 litres of air.

Breathing Gas Mixtures

There are, in fact, different types of breathing gas mixtures that scuba divers can use. The most common breathing gas mixture in recreational scuba diving is air, the same air we breathe on the surface. However, there are also other gas mixtures that can be used in scuba diving, such as Enriched Air Nitrox and Trimix.

AirTrimix Scuba Diver

The air we breathe on the surface consists of approximately 78.08% nitogen, 20.95% oxygen, 0.93% argon, 0.03% carbon dioxide and other gases that make up the remaining 0.003%. The percentage of gases other than nitrogen and oxygen are so small that we normally refer to air in our daily lives as 79% nitrogen and 21% oxygen. Nitrogen is an inert gas, while oxygen is essential in supporting life.

Air is the most commonly used breathing gas mixture in recreational scuba diving.

Enriched Air Nitrox

Nitrox is any gas mixture consisting of oxygen and nitrogen. Enriched Air Nitrox (EAN) is a gas mix that contains a higher percentage of oxygen that air i.e. more than 21% oxygen and, therefore, a lower percentage of nitrogen. For example, you may see scuba divers performing dives on EAN32 (32% oxygen) or EAN40 (40% oxygen). EAN mixtures allow scuba divers to stay longer underwater at shallower depths as compared to air.

EAN is becoming a popular breathing gas mixture among recreational scuba divers. However, using EAN requires special training.


Trimix stands for a gas mixture that contains three different gases: nitrogen, oxygen and helium. Helium is commonly used in technical and commercial diving. Below certain depths nitrogen can have a narcotic effect on the human body, resulting in nitrogen narcosis, while oxygen can become altogether toxic. Helium is introduced in the gas mix to help counter these effects.

Using Trimix requires special training that is beyond the scope of recreational scuba diving.

A variety of specialty programmes and courses are available to learn about the effects of pressure and different gases on the scuba diver’s body, such as Deep Diving, EAN Nitrox, Altitude Diving and Science of Diving.

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