Breathing gas

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As a respiratory gas is referred to in the strict sense, a gas mixture , which for breathing with compressed air breathing equipment is used ( breathing apparatus the fire department , regulators when diving in the anesthesia with the use of anesthetics ).

In the broadest sense, however, the breathing gas mixture commonly referred to as breathing air is also referred to as such in the physiology of breathing ( lung ventilation ). After all, the "breathing air" experiences a corresponding conditioning, such as water vapor saturation, changes in the intraalveolar gas composition (gas partial pressure ), etc.

When using breathing apparatus, the breathing gas is carried in compressed form in a compressed air cylinder .

While breathing equipment only normal air is applied is to be diving both in the commercial sector ( commercial diving ) and increasingly the upscale scuba diving ( Nitrox , Technical Diving used mixtures) which contain, in addition to the components of ordinary air other components or fully are composed differently.


Common breathing gas components and their meaning

The use of mixtures deviating from air has two main reasons, which are related to the increasing pressure with increasing depth :

  • Almost every gas can have a toxic effect on the human organism above a certain gas pressure . This risk can be reduced by either reducing the proportion (in the case of gas mixtures, the partial pressure is usually calculated here) of the hazardous component or by completely replacing it with a gas that is less dangerous at this pressure.
  • The higher the gas pressure, the greater the density of the breathing gas, which causes a “slower” flow of the gas and thus an increase in breathing resistance . This can lead to exhaustion of the respiratory muscles and thus to breathing problems.


Every gas mixture intended for breathing must contain oxygen . While diving, it is assumed that - depending on the ambient and working conditions - oxygen from a partial pressure of about 1.4 bar (cold, heavy work) to 1.6 bar (hot, no effort) increasingly toxic to the central nervous system acting ( Oxygen poisoning ), whereby the risk increases disproportionately at even higher pressures and the poisoning can occur suddenly without warning.

Compared to the inert gases, oxygen also accumulates in the organism when breathing under higher pressure, but this is limited to the nervous system (only becomes important for breathing mixtures with a significantly higher oxygen content compared to air). As a result, desaturation times and residual saturations must also be taken into account for oxygen .

Inert gases

Regardless of the physical / chemical significance of inert gas , respiratory gases are understood to be a gas that is not involved in the metabolic processes and only serves to dilute the oxygen that is essential for life.

Due to Henry's law, the inert gases dissolve and accumulate in the body tissue with increasing pressure during the stay under increased pressure. The speed and the degree of saturation are strongly related to the type of tissue and its blood flow, whereby the following applies: the stronger the blood, the faster and stronger the gases dissolve.

If the pressure is now reduced, the dissolved gases escape from the tissues again. If the pressure drop happens too quickly (for example, surfacing), the inert gases cannot be transported away with the blood and exhaled through the lungs, but instead continue to bubble out in the tissue or blood (soda bottle effect), which leads to life-threatening vascular blockages. In order to avoid decompression sickness , the reduction in pressure must therefore only take place slowly as part of a controlled decompression while observing the required decompression times.


In addition to the previously discussed effects of nitrogen as an inert gas, an increasingly intoxicating effect occurs with increasing pressure, which can have completely different effects from person to person. These can range from anxiety or euphoria to unconsciousness and are generally summarized under the term deep intoxication (inert gas anesthesia). For recreational divers, the first symptoms can generally be expected from a nitrogen partial pressure of 3.2 bar, which corresponds to a diving depth of around 30 meters. The sensitivity to the occurrence of symptoms is very different from one individual to the other and variable in the same person. Various factors can influence sensitivity, for example mood, daily form, environmental conditions or the use of hormonal contraceptives. This effect of nitrogen is the main reason for the generally recommended maximum diving depth of 40 meters for recreational divers with normal compressed air and the recommendation of various diving sports associations that a maximum depth of 30 meters is enough for recreational divers.

The decompression sickness due to gas bubble formation due to excess inert gas in human tissue is mostly back to nitrogen.


After nitrogen, helium is the most frequently used dilution gas in breathing mixtures - mainly in technical and commercial diving - and, in its role as an inert gas, also has the effects discussed above. Due to its small atomic size, however, both dissolution in the tissue and desaturation take place more quickly than with nitrogen. Paradoxically, this greater mobility ( diffusion ) tends to lengthen the decompression time , as the pressure has to be reduced much more carefully than with nitrogen in order to prevent the helium, which quickly escapes from the tissue into the blood, from pearls.

At a greater depth, helium also has an effect on the central nervous system, which becomes noticeable in the so-called High Pressure Nervous Syndrome (HPNS, colloquially "helium tremors"). The process of compression of the nerve tracts combined with the influence of helium is mainly responsible, whereby the speed plays a decisive role: With the diving speeds typical in technical diving, symptoms can be expected from a range of 150 to 200 m, while with the very slow pressure increases in commercial diving depths of up to 600 m can be reached without any effect. Other properties of helium compared to nitrogen are:

  • A lower density, which means that the breathing resistance is significantly lower at the same pressure.
  • A higher thermal conductivity. Therefore, helium mixtures must not be used as taring gas for a dry suit . Common filling gases for dry suits are air or, better, argon.

Neon is a rarely used component. It is considered expensive, and it also has a higher breathing resistance compared to helium. It also acts as an inert gas in the sense described above.


Hydrogen is another exotic component that is rarely used in extreme deep dives. It also acts as an inert gas in the sense described above.

Common breathing gas mixtures


Air ( compressed air ) is the most common breathing gas mixture and, in simple terms, consists of 79% nitrogen and 21% oxygen as well as residues of carbon dioxide and noble gases. The limits for recreational divers for diving with air are the recommended 40 m due to the narcotic effect of nitrogen. At the latest in the area of ​​more than 60 to 70 m, the additional risk of oxygen poisoning makes the risk no longer calculable.

The attempt at diving by a former companion of Cousteau's air at around 130 m ended fatally.


Nitrox is a mixture of nitrogen (engl. Nitrogen ) and oxygen (engl. Ox ygen). The amount of oxygen in the Nitrox mix varies depending on requirements and intended use. The most common Nitrox mixtures used in recreational diving have an oxygen content of 32 to 40%. In technical diving, mixtures with higher oxygen contents are also used. Apart from pure oxygen, a mixture with 50% oxygen (sometimes also referred to as safe air ) for decompression is common here. Due to the reduced nitrogen content, the risk of a decompression sickness is reduced (when using a table / dive computer for air) or used to shorten decompression times. When diving with nitrox tables / computers with nitrox function, longer no- stop times result , but the additional safety aspect is not applicable. The marketing of Nitrox, as a safer variant compared to compressed air, is therefore only correct under the conditions mentioned above in the text. However, there is always an increased risk of oxygen poisoning ( Paul Bert effect ) and this limits the maximum diving depth .


Pure oxygen is used as decompression gas in technical diving in order to shorten decompression times. However, the operating depth is theoretically limited to the last decompression stops at 3 to 6 m.

A special application is the use in oxygen circulation devices ( rebreather ).

Gas mixtures for technical diving

  • Trimix is a mixture of oxygen, nitrogen and helium and is used in technical diving for depths up to 200 m, in commercial diving also deeper.
  • Triox is a normoxic mixture of oxygen, nitrogen and helium and is used when diving up to 60 m. Usual mixtures are 30/30 = 30% oxygen, 30% helium and 40% nitrogen (for diving up to 40 m) and 21/35 = 21% oxygen, 35% helium and 44% nitrogen. Triox is also known as a normoxic trimix.
  • Heliair is a mixture of air and helium and is used in the same way as Trimix .
  • Heliox is a mixture of heli um and oxygen ( Ox ygen), which is used in commercial diving at great to very great (600 m) depths.
  • Neox is a mixture of rare used Ne on and oxygen ( Ox yGen). The mixture is considered expensive.
  • Hydreliox is a mixture of hydrogen ( hydr ogen), H eli to and oxygen ( Ox yGen) and is similar to helium mixtures. With sufficiently low proportions of the two potential combustion partners hydrogen and oxygen in the three-component mixture - below the explosion limit - the explosiveness (and flammability) can be avoided.
  • Hydrox is a mixture of hydrogen ( hydr ogen) and oxygen ( Ox yGen) which is used for extreme dives (1992 Fa. COMEX 701 m in a pressure chamber 534 m in open water). Due to the explosiveness of the oxygen-hydrogen mixture, this mixture can only be used from depths at which the oxygen partial pressure is below the explosion limit .

special cases

See also

Web links

Individual evidence

  1. ↑ Intoxication of depth . Website of the Society for Diving and Hyperbaric Medicine (GTÜM), accessed on June 26, 2018.