Air is the most common and only natural breathing gas. Other artificial gases, either purifications or blends of gases, are used in closed breathing environments such as SCUBA equipment, recompression chambers, submarines and space suits.

A safe breathing gas has three essential features:

• it must contain sufficient oxygen to support the life, consciousness and work rate of the breather.
• it must not contain harmful gases. Carbon monoxide and carbon dioxide are common poisons in breathing gases. There are many others.
• it must not become toxic when being breathed at high pressure such as when underwater. Oxygen and nitrogen are examples of gases that become toxic under pressure.

Most breathing gases are a mixture of oxygen and one or more inert gases. The techniques used to fill diving cylinders with gases other than air are called gas blending.

Common diving breathing gases

Common diving breathing gases are:

• Air is a mixture of 21% oxygen and 79% nitrogen. It is the commonest diving gas being cheap and simple to use. As its nitrogen component causes nitrogen narcosis it has a safe depth limit of 40 metres/ 130 feet for most divers.
• Pure Oxygen is mainly used during the shallow decompression stops at the end of a technical dive for accelerated decompression stops.
• Nitrox is a mixture of oxygen and air. It is mainly used instead of air, to accelerate decompression or to decrease the risk of Decompression sickness.
• Trimix is a mixture of oxygen, nitrogen and helium and is often used during the deep phase of a technical dive.
• Heliox is a mixture of oxygen and helium and is often used during the deep phase of a commercial deep dive.
• Heliair is a mixture of oxygen, nitrogen and helium. It suitable for during the deep phase of a technical dive. It is easily blended from helium and air and so always has a 21:79 ratio of oxygen and nitrogen and the rest is helium.
• Neox is a mixture of oxygen and neon that is rarely used due to its cost.

Individual component gases

Oxygen

Oxygen (O₂) must be present in every breathing gas. This is because it is essential to the human body's metabolic process, which sustains life. The human body cannot store oxygen for later use as it does with food. If the body is deprived of oxygen for more than a few minutes, unconsciousness results. The tissues and organs within the body (notably the heart and brain) are damaged if deprived of oxygen for much longer than four minutes.

The proportion of oxygen in a breathing gas determines the depth at which the mixture gas can safely be used:

• hypoxic mixes have lower proportion of oxygen than air, 21%, or more strictly less than 16% oxygen and are designed only to be breathed at depth as a "bottom gas". Trimix, Heliox and Heliair are used to create typical hypoxic mixes and are used in technical diving as deep breathing gases.
• normoxic mixes have the same proportion of oxygen as air, 21%
• hyperoxic mixes have a higher proportion of oxygen than air 21% and are designed only to be used in shallow water and generally to accelerate decompression. Nitrox is a typical hyperoxic breathing gas.

The minimum safe partial pressure of oxygen in a breathing gas is 0.16 bar. Below this partial pressure the diver risks unconsciousness and death due to hypoxia.

The maximum safe partial pressure of oxygen in a breathing gas depends on exposure time, but for dives of less than 3 hours is commonly considered to be 1.4 bar, although the U.S. Navy has been known to authorize dives with a partial oxygen pressure of as much as 1.8 bar. At high partial pressures or longer exposures, the diver risks oxygen toxicity including a seizure similar to an epileptic fit. Each breathing gas has a maximum operating depth which is determined by its oxygen content.

Oxygen analysers are used to measure the concentration of oxygen in the gas mix.

Filling a diving cylinder with pure oxygen costs around five times more than filling it with compressed air. As oxygen supports combustion and causes rust in diving cylinders, it should be handled with respect when gas blending.

Nitrogen

Nitrogen (N₂) is an inert gas that causes nitrogen narcosis in the diver, so its use is limited to shallower dives. Nitrogen can cause decompression sickness. Air is the cheapest diving, breathing gas.

Equivalent air depth is used often used to help design a breathing gas mix by determining the maximum nitrogen content for a particular depth of dive. Many divers find that the level of narcosis caused by a 30 metre / 100 feet dive, whilst breathing air, is a comfortable maximum. The partial pressure of nitrogen at this depth on air is 3.16 bar (Fraction of nitrogen x Absolute pressure = 0.79 x 4 bar). So, what fraction of nitrogen would cause the same narcosis at 60 metres? The answer is 45% nitrogen. (3.16 / 7 bar)

Helium

Helium (He) is an inert gas that is less narcotic than nitrogen at diving pressures, so it is more suitable for deeper dives than nitrogen. But helium can still cause decompression sickness. It is not very suitable for dry suit inflation due to its poor thermal insulation properties — helium is a very good conductor of heat, as opposed to air which is a rather poor conductor of heat. Helium fills typically cost ten times more than an equivalent air fill. Helium also distorts the diver's voice, which may impede communication.

Neon

Neon (Ne) is an inert gas sometimes used in deep, commercial diving but is very expensive. Like helium, it is less narcotic than nitrogen, but unlike helium, it does not distort the diver's voice.

Hydrogen

Hydrogen (H₂) has been used in deep diving gas mixes but is very explosive when mixed with more then about 4-5% oxygen (such as the oxygen found in breathing gas). This limits use of hydrogen to deep dives and complicated protocols to insure that oxygen is cleared from the lungs, the blood stream and the breathing equipment before breathing hydrogen starts. Like helium, it distorts the diver's voice. See "Exotic diving gases (http://www.techdiver.ws/exotic_gases.shtml#3.1)." Tech Diver Web. Accessed on Jan 9, 2005.

Unwelcome components of breathing gases

Many gases are not suitable for use in diving breathing gases. Here is an incomplete list.

Argon

Argon (Ar) is an inert gas that is more narcotic than nitrogen, so is not suitable as a diving, breathing gas. It is used for dry suit inflation because of its good thermal insulation properties. Argon is much more expensive than air.

Carbon dioxide

Carbon dioxide (CO₂) is produced by the metabolism of the human body and causes carbon dioxide poisoning.

Carbon monoxide

Carbon monoxide (CO) is produced by incomplete combustion. Two common sources are:-

Internal combustion engine exhaust gas in the air being drawn into a diving air compressor.

Lubricants of the compressor firing under compression like happens in a diesel engine.

It causes carbon monoxide poisoning.

Hydrocarbons

Hydrocarbons (C_xH_y) can occur due to compressor lubricants leaking, or due to incomplete combustion near the air intake, as for carbon monoxide.

They cause explosions, especially in high-oxygen mixtures.

Oil mist in breathed air can slowly damage the lungs and finally cause the lungs to degenerate into severe emphysema.

Moisture content

The process of compressing gas into a diving cylinder removes moisture from the gas. This is good for corrosion prevention in the cylinder but means that the diver provided with very dry gas. The dry inhaled gas extracts moisture from the divers lungs while underwater contributing to dehydration, which is also thought to be a predisposing risk factor of decompression sickness. It is also uncomfortable making the diver thirty. This problem is reduced with rebreathers because the soda lime reaction to remove carbon dioxide puts moisture back into the breathing gas. In a hot tropical climate, open circuit diving can accelerate heat exhaustion because of dehydration.

Gas detection and measurement

Divers find it difficult to detect most gases that are likely to be present in diving cylinders because they are colourless, odourless and tasteless. Electronic sensors exist for some gases, such as oxygen analysers, helium analyser, carbon monoxide detectors and carbon dioxide detectors. Oxygen analysers are commonly found underwater in rebreathers. Oxygen and helium analysers are often used on the surface during gas blending. Chemical and other types of gas detection methods are not often used in diving.

Gas cylinder color coding

In the European Union gas cylinders are colour coded. The "shoulder" is the top of the cylinder close to the pillar valve.

• Air has a white and black quartered shoulder.
• Heliox has a white and brown quartered shoulder.
• Nitrox has a white and black quartered shoulder.
• Pure Oxygen has a white shoulder.
• Pure Helium has a brown shoulder.
• Trimix has a white, black and brown segmented shoulder.

Worldwide, in many recreational diving settings where air and nitrox are the widely used gases, nitrox cylinders are colour-coded with a green stripe on yellow bottom. Air scuba cylinders are often all yellow, so they can be seen easily underwater.