An asphyxiant gas, also known as a simple asphyxiant, is a nontoxic or minimally toxic

gas Gas is a state of matter that has neither a fixed volume nor a fixed shape and is a compressible fluid. A ''pure gas'' is made up of individual atoms (e.g. a noble gas like neon) or molecules of either a single type of atom ( elements such as ...

which reduces or displaces the normal

oxygen Oxygen is a chemical element; it has chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen group (periodic table), group in the periodic table, a highly reactivity (chemistry), reactive nonmetal (chemistry), non ...

concentration in breathing

air An atmosphere () is a layer of gases that envelop an astronomical object, held in place by the gravity of the object. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A stellar atmosph ...

. Breathing of oxygen-depleted air can lead to death by

asphyxia Asphyxia or asphyxiation is a condition of deficient supply of oxygen to the body which arises from abnormal breathing. Asphyxia causes generalized hypoxia, which affects all the tissues and organs, some more rapidly than others. There are m ...

tion (suffocation). Because asphyxiant gases are relatively inert and odorless, their presence in high concentration may not be noticed, except in the case of

carbon dioxide Carbon dioxide is a chemical compound with the chemical formula . It is made up of molecules that each have one carbon atom covalent bond, covalently double bonded to two oxygen atoms. It is found in a gas state at room temperature and at norma ...

(

hypercapnia Hypercapnia (from the Greek ''hyper'', "above" or "too much" and ''kapnos'', "smoke"), also known as hypercarbia and CO2 retention, is a condition of abnormally elevated carbon dioxide (CO2) levels in the blood. Carbon dioxide is a gaseous pro ...

Toxic Toxicity is the degree to which a chemical substance or a particular mixture of substances can damage an organism. Toxicity can refer to the effect on a whole organism, such as an animal, bacterium, or plant, as well as the effect on a subst ...

gases, by contrast, cause death by other mechanisms, such as competing with oxygen on the cellular level (e.g.

carbon monoxide Carbon monoxide (chemical formula CO) is a poisonous, flammable gas that is colorless, odorless, tasteless, and slightly less dense than air. Carbon monoxide consists of one carbon atom and one oxygen atom connected by a triple bond. It is the si ...

) or directly damaging the

respiratory system The respiratory system (also respiratory apparatus, ventilatory system) is a biological system consisting of specific organs and structures used for gas exchange in animals and plants. The anatomy and physiology that make this happen varies grea ...

(e.g.

phosgene Phosgene is an organic chemical compound with the formula . It is a toxic, colorless gas; in low concentrations, its musty odor resembles that of freshly cut hay or grass. It can be thought of chemically as the double acyl chloride analog of ...

). Far smaller quantities of these are deadly. Notable examples of asphyxiant gases are

methane Methane ( , ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The abundance of methane on Earth makes ...

nitrogen Nitrogen is a chemical element; it has Symbol (chemistry), symbol N and atomic number 7. Nitrogen is a Nonmetal (chemistry), nonmetal and the lightest member of pnictogen, group 15 of the periodic table, often called the Pnictogen, pnictogens. ...

argon Argon is a chemical element; it has symbol Ar and atomic number 18. It is in group 18 of the periodic table and is a noble gas. Argon is the third most abundant gas in Earth's atmosphere, at 0.934% (9340 ppmv). It is more than twice as abu ...

helium Helium (from ) is a chemical element; it has chemical symbol, symbol He and atomic number 2. It is a colorless, odorless, non-toxic, inert gas, inert, monatomic gas and the first in the noble gas group in the periodic table. Its boiling point is ...

, butane and propane. Along with trace gases such as carbon dioxide and

ozone Ozone () (or trioxygen) is an Inorganic compound, inorganic molecule with the chemical formula . It is a pale blue gas with a distinctively pungent smell. It is an allotrope of oxygen that is much less stable than the diatomic allotrope , break ...

, these compose 79% of

Earth's atmosphere The atmosphere of Earth is composed of a layer of gas mixture that surrounds the Earth's planetary surface (both lands and oceans), known collectively as air, with variable quantities of suspended aerosols and particulates (which create weathe ...

Asphyxia hazard

Asphyxiant gases in the breathing air are normally not hazardous. Only where elevated concentrations of asphyxiant gases displace the normal oxygen concentration does a hazard exist. Asphyxiant gases are marked SA in the

NFPA 704 "NFPA 704: Standard System for the Identification of the Hazards of Materials for Emergency Response" is a standard maintained by the United States, U.S.-based National Fire Protection Association. First "tentatively adopted as a guide" in 1960, ...

standard. Examples of hazards are: * Environmental gas displacement ** Confined spaces, combined with accidental gas leaks, such as mines,Terazawa K, Takatori T, Tomii S, Nakano K. ''Methane asphyxia. Coal mine accident investigation of distribution of gas.'' Am J Forensic Med Pathol. 1985 Sep;6(3):211-4.

submarines A submarine (often shortened to sub) is a watercraft capable of independent operation underwater. (It differs from a submersible, which has more limited underwater capability.) The term "submarine" is also sometimes used historically or info ...

refrigerator A refrigerator, commonly shortened to fridge, is a commercial and home appliance consisting of a thermal insulation, thermally insulated compartment and a heat pump (mechanical, electronic or chemical) that transfers heat from its inside to ...

s,Gill JR, Ely SF, Hua Z.''Environmental gas displacement: three accidental deaths in the workplace.'' Am J Forensic Med Pathol. 2002 Mar;23(1):26-30. or other confined spaces **

Fire extinguisher A fire extinguisher is a handheld active fire protection device usually filled with a dry or wet chemical used to extinguish or control small fires, often in emergencies. It is not intended for use on an out-of-control fire, such as one which ha ...

systems that flood spaces with

inert gases An inert gas is a gas that does not readily undergo chemical reactions with other chemical substances and therefore does not readily form chemical compounds. Though inert gases have a variety of applications, they are generally used to prevent u ...

, such as computer

data center A data center is a building, a dedicated space within a building, or a group of buildings used to house computer systems and associated components, such as telecommunications and storage systems. Since IT operations are crucial for busines ...

s and sealed vaults ** Large-scale natural release of gas, such as during the

Lake Nyos disaster On 21 August 1986, a limnic eruption at Lake Nyos in northwestern Cameroon killed 1,746 people and 3,500 livestock. The eruption triggered the sudden release of about 100,000–300,000 tons of carbon dioxide (CO2). The gas cloud initially r ...

in which volcanically-released carbon dioxide killed 1,800 people. ** Release of

boiled off by the energy released in a

magnet quench A superconducting magnet is an electromagnet made from coils of superconducting wire. They must be cooled to cryogenic temperatures during operation. In its superconducting state the wire has no electrical resistance and therefore can conduct much ...

such as the

Large Hadron Collider The Large Hadron Collider (LHC) is the world's largest and highest-energy particle accelerator. It was built by the CERN, European Organization for Nuclear Research (CERN) between 1998 and 2008, in collaboration with over 10,000 scientists, ...

or a

magnetic resonance imaging Magnetic resonance imaging (MRI) is a medical imaging technique used in radiology to generate pictures of the anatomy and the physiological processes inside the body. MRI scanners use strong magnetic fields, magnetic field gradients, and ...

machine. ** Climbing inside an inflatable balloon filled with helium * Direct administration of gas ** Inadvertent administration of asphyxiant gas in

respirators A respirator is a device designed to protect the wearer from inhaling hazardous atmospheres including lead, lead fumes, vapors, gases and particulate matter such as dusts and airborne pathogens such as viruses. There are two main categories o ...

** Use in

suicide Suicide is the act of intentionally causing one's own death. Risk factors for suicide include mental disorders, physical disorders, and substance abuse. Some suicides are impulsive acts driven by stress (such as from financial or ac ...

and

erotic asphyxiation Erotic asphyxiation (variously called asphyxiophilia, hypoxyphilia or breath control play) is the intentional restriction of oxygen to the brain for the purposes of sexual arousal. With a partner (or alone), the act often involves strangulatio ...

Risk management

The risk of breathing asphyxiant gases is frequently underestimated leading to fatalities, typically from breathing helium in domestic circumstances and nitrogen in industrial environments. The term asphyxiation is often mistakenly associated with the strong desire to breathe that occurs if breathing is prevented. This desire is stimulated from increasing levels of carbon dioxide. However, asphyxiant gases may displace carbon dioxide along with oxygen, preventing the victim from feeling short of breath. In addition the gases may also displace oxygen from cells, leading to

loss of consciousness Unconsciousness is a state in which a living individual exhibits a complete, or near-complete, inability to maintain an awareness of self and environment or to respond to any human or environmental stimulus. Unconsciousness may occur as the re ...

and death rapidly.

United States

The handling of compressed asphyxiant gases and the determination of appropriate environment for their use is regulated in the

United States The United States of America (USA), also known as the United States (U.S.) or America, is a country primarily located in North America. It is a federal republic of 50 U.S. state, states and a federal capital district, Washington, D.C. The 48 ...

by the

Occupational Safety and Health Administration The Occupational Safety and Health Administration (OSHA; ) is a regulatory agency of the United States Department of Labor that originally had federal visitorial powers to inspect and examine workplaces. The United States Congress established ...

(OSHA). The

National Institute for Occupational Safety and Health The National Institute for Occupational Safety and Health (NIOSH, ) is the List of United States federal agencies, United States federal agency responsible for conducting research and making recommendations for the prevention of work-related occ ...

(NIOSH) has an advisory role. OSHA requires employers who send workers into areas where the oxygen concentration is known or expected to be less than 19.5% to follow the provision of the Respiratory Protection Standard 9 CFR 1910.134 Generally, work in an oxygen depleted environment requires an

SCBA A self-contained breathing apparatus (SCBA) is a respirator worn to provide an autonomous supply of breathable gas in an atmosphere that is immediately dangerous to life or health from a gas cylinder. They are typically used in firefighting a ...

or airline respirator. The regulation also requires an evaluation of the worker's ability to perform the work while wearing a respirator, the regular training of personnel, respirator fit testing, periodic workplace monitoring, and regular respirator maintenance, inspection, and cleaning." Containers should be labeled according to OSHA's Hazard Communication Standard 9 CFR 1910.1200 These regulations were developed in accordance with the official recommendations of the

Compressed Gas Association The Compressed Gas Association (CGA) is an American trade association for the industrial and medical gas supply industries. The CGA publishes standards and practices that codify industry practices. In cases where government regulation is inspec ...

(CGA) pamphlet P-1. The specific guidelines for prevention of asphyxiation due to displacement of oxygen by asphyxiant gases is covered under CGA's pamphlet SB-2, ''Oxygen-Deficient Atmospheres.'' Specific guidelines for use of gases other than air in back-up respirators is covered in pamphlet SB-28, ''Safety of Instrument Air Systems Backed Up by Gases Other Than Air.''

Odorized gas

To decrease the risk of asphyxiation, there have been proposals to add warning odors to some commonly used gases such as nitrogen and argon. However, CGA has argued against this practice. They are concerned that odorizing may decrease worker vigilance, not everyone can smell the odorants, and assigning a different smell to each gas may be impractical. Another difficulty is that most odorants (e.g., the

thiol In organic chemistry, a thiol (; ), or thiol derivative, is any organosulfur compound of the form , where R represents an alkyl or other organic substituent. The functional group itself is referred to as either a thiol group or a sulfhydryl grou ...

s) are chemically reactive. This is not a problem with natural gas intended to be burned as fuel, which is routinely odorized, but a major use of asphyxiants such as nitrogen, helium, argon and

krypton Krypton (from 'the hidden one') is a chemical element; it has symbol (chemistry), symbol Kr and atomic number 36. It is a colorless, odorless noble gas that occurs in trace element, trace amounts in the Earth's atmosphere, atmosphere and is of ...

is to protect reactive materials from the atmosphere.

In mining

The dangers of excess concentrations of nontoxic gases has been recognized for centuries within the

mining industry Mining is the extraction of valuable geological materials and minerals from the surface of the Earth. Mining is required to obtain most materials that cannot be grown through agricultural processes, or feasibly created artificially in a la ...

. The concept of

black damp Blackdamp (also known as stythe or choke damp), sometimes found in enclosed environments such as mines, sewers, wells, tunnels and ships' holds, is an asphyxiant, reducing the available oxygen content of air to a level incapable of sustaining huma ...

(or "stythe") reflects an understanding that certain gaseous mixtures could lead to death with prolonged exposure. Early mining deaths due to mining fires and explosions were often a result of encroaching asphyxiant gases as the fires consumed available oxygen. Early self-contained

respirator A respirator is a device designed to protect the wearer from inhaling hazardous atmospheres including lead, lead fumes, vapors, gases and particulate matter such as dusts and airborne pathogens such as viruses. There are two main categories o ...

s were designed by mining engineers such as

Henry Fleuss Henry Albert Fleuss (13 June 1851 – 6 January 1933) was a pioneering diving engineer, and Master Diver for Siebe, Gorman & Co. of London. Fleuss was born in Marlborough, Wiltshire in 1851. In 1878 he was granted a patent which improved rebr ...

to help in rescue efforts after fires and floods. While canaries were typically used to detect carbon monoxide, tools such as the

Davy lamp The Davy lamp is a safety lamp used in flammable atmospheres, invented in 1815 by Sir Humphry Davy.Geordie lamp The Geordie lamp was a safety lamp for use in flammable atmospheres, invented by George Stephenson in 1815 as a miner's lamp to prevent explosions due to firedamp in coal mines. Origin In 1815, Stephenson was the engine-wright at the Killingw ...

were useful for detecting

and

, two asphyxiant gases. When methane was present, the lamp would burn higher; when carbon dioxide was present, the lamp would gutter or extinguish. Modern methods to detect asphyxiant gases in mines led to the

Federal Mine Safety and Health Act of 1977 The Federal Mine Safety and Health Act of 1977 (Public Law 95-164) amended the Coal Mine Safety and Health Act of 1969. It can be found in the United States Code under Title 30, Mineral Lands and Mining, Chapter 22, Mine Safety and Health. The ...

in the United States which established ventilation standards in which mines should be "ventilated by a current of air containing not less than 19.5 volume per centum of oxygen, not more than 0.5 volume per centum of carbon dioxide".

References