[1][3] The techniques used to fill diving cylinders with gases other than air or pure oxygen are called gas blending.

It is common to provide the additional oxygen as a pure gas added to the breathing air at inhalation, or though a life-support system.

The boundaries set by authorities may differ slightly, as the effects vary gradually with concentration and between people, and are not accurately predictable.

The tissues and organs within the body (notably the heart and brain) are damaged if deprived of oxygen for much longer than four minutes.

As oxygen supports combustion and causes rust in diving cylinders, it should be handled with caution when gas blending.

Below this partial pressure the diver may be at risk of unconsciousness and death due to hypoxia, depending on factors including individual physiology and level of exertion.

For this reason normoxic or hyperoxic "travel gases" are used at medium depth between the "bottom" and "decompression" phases of the dive.

[1][2][3][7][11] For therapeutic recompression and hyperbaric oxygen therapy partial pressures of 2.8 bar are commonly used in the chamber, but there is no risk of drowning if the occupant loses consciousness.

Many divers find that the level of narcosis caused by a 30 m (100 ft) dive, whilst breathing air, is a comfortable maximum.

[27] Helium's low molecular weight (monatomic MW=4, compared with diatomic nitrogen MW=28) increases the timbre of the breather's voice, which may impede communication.

[1][3][28] This is because the speed of sound is faster in a lower molecular weight gas, which increases the resonance frequency of the vocal cords.

Helium is found in significant amounts only in natural gas, from which it is extracted at low temperatures by fractional distillation.

[1][3][32][33] It is sometimes used for dry suit inflation by divers whose primary breathing gas is helium-based, because of argon's good thermal insulation properties.

They can enter diving cylinders as a result of contamination, leaks,[clarification needed] or due to incomplete combustion near the air intake.

[5][30] This is good for corrosion prevention in the cylinder but means that the diver inhales very dry gas.

The dry gas extracts moisture from the diver's lungs while underwater contributing to dehydration, which is also thought to be a predisposing risk factor of decompression sickness.

This problem is reduced in rebreathers because the soda lime reaction, which removes carbon dioxide, also puts moisture back into the breathing gas,[9] and the relative humidity and temperature of exhaled gas is relatively high and there is a cumulative effect due to rebreathing.

This is particularly important for breathing gas mixtures where errors can affect the health and safety of the end user.

It is difficult to detect most gases that are likely to be present in diving cylinders because they are colourless, odourless and tasteless.

[4] Standards for breathing gas quality are published by national and international organisations, and may be enforced in terms of legislation.

In the UK, the Health and Safety Executive indicate that the requirements for breathing gases for divers are based on the BS EN 12021:2014.

[40] Water content is limited by risks of icing of control valves, and corrosion of containment surfaces – higher humidity is not a physiological problem – and is generally a factor of dew point.

Methods used include batch mixing by partial pressure or by mass fraction, and continuous blending processes.

Closed circuit systems may be used to conserve the breathing gas, which may be in limited supply – in the case of mountaineering the user must carry the supplemental oxygen, and in space flight the cost of lifting mass into orbit is very high.

Medical use of breathing gases other than air include oxygen therapy and anesthesia applications.

[47][45] Oxygen can be given in a number of ways including nasal cannula, face mask, and inside a hyperbaric chamber.

This relationship exists because the drugs bind directly to cavities in proteins of the central nervous system,[clarification needed] although several theories of general anaesthetic action have been described.

Inhalational anesthetics are thought to exact their effects on different parts of the central nervous system.

Modern machines incorporate a ventilator, suction unit, and patient monitoring devices.

Exhaled gas is passed through a scrubber to remove carbon dioxide, and the anaesthetic vapour and oxygen are replenished as required before the mixture is returned to the patient.