Heliox is a breathing gas
Breathing gas
Breathing gas is a mixture of gaseous chemical elements and compounds used for respiration.Air is the most common and only natural breathing gas...

 composed of a mixture of helium
Helium is the chemical element with atomic number 2 and an atomic weight of 4.002602, which is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas that heads the noble gas group in the periodic table...

 (He) and oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...


Heliox has been used medically since the 1930s, and although the medical community adopted it initially to alleviate symptoms of upper airway obstruction, its range of medical uses has since expanded greatly, mostly because of the low density of the gas. Heliox is also used in saturation diving
Saturation diving
Saturation diving is a diving technique that allows divers to reduce the risk of decompression sickness when they work at great depth for long periods of time....

 and sometimes during the deep phase of technical dives
Technical diving
Technical diving is a form of scuba diving that exceeds the scope of recreational diving...


Medical uses

In medicine
Medicine is the science and art of healing. It encompasses a variety of health care practices evolved to maintain and restore health by the prevention and treatment of illness....

 heliox generally refers to a mixture of 21% O2 (the same as air) and 79% He, although other combinations are available.

Heliox generates less airway resistance than air and thereby requires less mechanical energy to ventilate the lungs. "Work of Breathing" (WOB) is reduced. It does this by two mechanisms:
  1. increased tendency to laminar flow
    Laminar flow
    Laminar flow, sometimes known as streamline flow, occurs when a fluid flows in parallel layers, with no disruption between the layers. At low velocities the fluid tends to flow without lateral mixing, and adjacent layers slide past one another like playing cards. There are no cross currents...

  2. reduced resistance in turbulent flow.

Heliox has a similar viscosity to air but a significantly lower density (0.5 g/l versus 1.2 5g/l at STP
Standard conditions for temperature and pressure
Standard condition for temperature and pressure are standard sets of conditions for experimental measurements established to allow comparisons to be made between different sets of data...

). Flow of gas through the airways comprises laminar flow, transitional flow and turbulent flow. The tendency for each type of flow is described by the Reynolds number. Heliox's low density produces a lower Reynolds number and hence higher probability of laminar flow for any given airway. Laminar flow tends to generate less resistance than turbulent flow.

In the small airways where flow is laminar, resistance is proportional to gas viscosity and is not related to density and so heliox has little effect. The Hagen–Poiseuille equation describes laminar resistance. In the large airways where flow is turbulent, resistance is proportional to density, so heliox has a significant effect.

Heliox has been used medically since the early 1930s. It was the mainstay of treatment in acute asthma
Asthma is the common chronic inflammatory disease of the airways characterized by variable and recurring symptoms, reversible airflow obstruction, and bronchospasm. Symptoms include wheezing, coughing, chest tightness, and shortness of breath...

 before the advent of bronchodilators. Currently, heliox is mainly used in conditions of large airway narrowing (upper airway obstruction from tumors or foreign bodies and vocal cord dysfunction
Vocal cord dysfunction
Vocal cord dysfunction is a condition that affects the vocal folds, commonly referred to as the vocal cords, which is characterized by full or partial vocal fold closure that usually occurs during inhalation for short periods of time; however, can occur during both inhalation and exhalation....

). There is also some use of heliox in conditions of the medium airways (croup
Croup is a respiratory condition that is usually triggered by an acute viral infection of the upper airway. The infection leads to swelling inside the throat, which interferes with normal breathing and produces the classical symptoms of a "barking" cough, stridor, and hoarseness...

, asthma and chronic obstructive pulmonary disease
Chronic obstructive pulmonary disease
Chronic obstructive pulmonary disease , also known as chronic obstructive lung disease , chronic obstructive airway disease , chronic airflow limitation and chronic obstructive respiratory disease , is the co-occurrence of chronic bronchitis and emphysema, a pair of commonly co-existing diseases...


Patients with these conditions may suffer a range of symptoms including dyspnea
Dyspnea , shortness of breath , or air hunger, is the subjective symptom of breathlessness.It is a normal symptom of heavy exertion but becomes pathological if it occurs in unexpected situations...

 (breathlessness), hypoxemia (below-normal oxygen content in the arterial blood) and eventually a weakening of the respiratory muscles due to exhaustion, which can lead to respiratory failure and require intubation and mechanical ventilation. Heliox may reduce all these effects, making it easier for the patient to breathe. Heliox has also found utility in the weaning of patients off mechanical ventilation, and in the nebulization of inhalable drugs, particularly for the elderly. Research has also indicated advantages in using helium–oxygen mixtures in delivery of anaesthesia.

Diving uses

Owing to the expense of helium, heliox is most likely to be used in deep commercial diving
Commercial Diving
Professional diving is a type of diving where the divers are paid for their work. There are several branches of professional diving, the most well known of which is probably commercial diving...

. It is also sometimes used by diving enthusiasts, particularly those using rebreather
A rebreather is a type of breathing set that provides a breathing gas containing oxygen and recycled exhaled gas. This recycling reduces the volume of breathing gas used, making a rebreather lighter and more compact than an open-circuit breathing set for the same duration in environments where...

s, which conserve the breathing gas at depth much better than open circuit scuba.

The proportion of oxygen in a diving mix depends on the maximum depth of the dive plan, but it is often hypoxic and typically 10%. Each mix is bespoke
Bespoke is a term employed in a variety of applications to mean an item custom-made to the buyer's specification...

 and is created using gas blending
Gas blending
Gas blending or gas mixing is the filling of diving cylinders with non-air breathing gases such as nitrox, trimix and heliox.Filling cylinders with a mixture of gases has dangers for both the filler and the diver. During filling there is a risk of fire due to use of oxygen and a risk of explosion...

 techniques, which often involve the use of booster pumps to achieve typical diving cylinder
Diving cylinder
A diving cylinder, scuba tank or diving tank is a gas cylinder used to store and transport high pressure breathing gas as a component of a scuba set. It provides gas to the scuba diver through the demand valve of a diving regulator....

 pressures of 200 bar (2,900.8 psi) from lower pressure banks of oxygen and helium cylinders.

Because sound
Sound is a mechanical wave that is an oscillation of pressure transmitted through a solid, liquid, or gas, composed of frequencies within the range of hearing and of a level sufficiently strong to be heard, or the sensation stimulated in organs of hearing by such vibrations.-Propagation of...

 travels faster in heliox than in air, voice
Human voice
The human voice consists of sound made by a human being using the vocal folds for talking, singing, laughing, crying, screaming, etc. Its frequency ranges from about 60 to 7000 Hz. The human voice is specifically that part of human sound production in which the vocal folds are the primary...

Formants are defined by Gunnar Fant as 'the spectral peaks of the sound spectrum |P|' of the voice. In speech science and phonetics, formant is also used to mean an acoustic resonance of the human vocal tract...

s are raised, making divers' speech very high-pitched and hard to understand to people not used to it. Surface personnel often employ a piece of communications equipment called a "helium de-scrambler," which electronically lowers the pitch of the diver's voice as it is relayed through the communications gear, making it easier to understand.

Trimix is a slightly less expensive alternative to heliox for deep diving. Trimix is often used in commercial diving and in technical diving
Technical diving
Technical diving is a form of scuba diving that exceeds the scope of recreational diving...


See also

  • Argox
  • Nitrox
  • Hydreliox
    Hydreliox is an exotic breathing gas mixture of helium, oxygen and hydrogen.It is used primarily for research and scientific deep diving, usually below . Below this depth, extended breathing of heliox gas mixtures may cause high pressure nervous syndrome . Two gas mixtures exist that attempt to...

  • Hydrox
    Hydrox (breathing gas)
    Hydrox, a gas mixture of hydrogen and oxygen, is used as a breathing gas in very deep diving. It allows divers to descend several hundred metres....

  • Trimix
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