Gas characteristics and hazards

Describes the characteristics of and hazards associated with main gases. Safety management using gas detectors is important because even gases generally considered safe can pose hazards in certain situations.

Oxygen (O2)

Oxygen is vital to life on our planet. It’s also essential for a wide range of industries that support daily life, including steel manufacturing, chemicals, paper manufacturing, and medicine. However, it also poses certain hazards. Here we’ll present some potentially life-saving facts about oxygen.

Composition of air

Accounting for approximately 21 % of Earth’s atmosphere, oxygen is essential for humans and other creatures.

Atmospheric gas component Percentage by volume Partial pressure
Oxygen 20.93 % 151.1 mmHg
Nitrogen 78.09 % 593.5 mmHg
Carbon dioxide 0.03 % 0.2 mmHg
Argon and other rare gases 0.95 % 7.2 mmHg
Total 100.00 % 760.0 mmHg

Definition of anoxia (oxygen deficiency)

Anoxia is the condition in which the concentration of oxygen in the air is below 18 %.
As defined in Article 2 of the Ordinance on Prevention of Anoxia, etc. (in Japan)

Data on industrial accidents involving oxygen deficiency (1990 - 2020) (in Japan)

Extract from Ministry of Health, Labour and Welfare Basic Industrial Safety Notification No. 0707-3

- Main causes of oxygen deficiency -

Oxygen consumption
Oxidation of metals, ore, and subsoil iron, human respiration, respiration by cereals, fruits, vegetables, and timber, oxidation of paints and solvents

Influx of inert gases or coolants
Influx of N2 gas (including liquid nitrogen), argon gas, or CO2, influx or buildup of coolants such as fluorocarbons

Release of combustible gases
Release of methane gas during excavation, gas leaks from fuel gas pipes

Excessive oxygen

Higher oxygen concentrations will increase the size and temperature of flames, lower the ignition temperature of combustible materials, and increase combustion speed. All this results in an increased risk of fire.
For humans, continuous inhalation of high oxygen concentrations can lead to convulsions, loss of consciousness, and, in the worst case, death.

Toxic gases (CO, H2S etc.)

Gases that are harmful to humans in the event of inhalation or exposure are called toxic gases. While these gases are dangerous, a wide range of industries depend on their particular characteristics. Stringent legal standards regulate the manufacture, transportation, use, and management of gases. Here, we’ll present information on the types and characteristics of toxic gases, which may prove useful in the unlikely event of an accident or disaster.

Definition of toxic gas

While the definition of toxic gas was revised through revision of the ministerial ordinance, according to the Exemplified standards relating to the Regulation on Safety of General High Pressure Gas (in Japan), the alarm setpoints for gas leak detection and alarm equipment remain unchanged, and must be threshold limit values or lower.

Toxic gas (in Japan)

Acrylonitrile, acrolein, sulfur dioxide, arsine, monosilane, monomethylamine, hydrogen sulfide (33 gases listed)

and other gases specified as toxic substances in Article 2-1 of the Poisonous and Deleterious Substances Control Act

*Exemplified standards relating to the Regulation on Safety of General High Pressure Gas

Toxic gas (in Japan)(Comparison before and after the 2013 revision)

Previously: 33 listed gases
+ maximum allowable concentration (= threshold limit value) not exceeding 200 ppm

After revision: 33 listed gases
+ toxic substances stipulated in Article 2-1 of Poisonous and Deleterious Substances Control Act
* The only semiconductor material gas classified as a toxic substance is HF.

Threshold limit value (TLV) In accordance with ACGIH (American Conference of Government Industrial Hygienists) recommended values

Workplace air concentrations below which it has been determined most workers suffer no ill effects

・TWA (time weighted average: threshold limit value for workers normally working eight hours a day, 40 hours per week)
・STEL (short-term exposure limit: value that the 15-minute mean cannot exceed)
・C (ceiling value: upper value that cannot be exceeded)

Toxic gas list

Carbon monoxide (CO) gas

This combines with the hemoglobin in red blood cells, impeding the ability to supply oxygen throughout the body. Exposure results in symptoms of toxicity. These symptoms include headache, nausea, dizziness, tinnitus, perspiration, and general lethargy.

CO chemical properties

Colorless, odorless gas
Slightly soluble in water
Molecular weight: 28
Specific gravity: 0.97, virtually identical to air
TLV: 25 ppm (ACGIH: American Conference of Government Industrial Hygienists)
Lower explosive limit (LEL): 12.5 vol%

CO concentration, exposure duration, and symptoms

CO concentration Exposure duration and symptoms
25 ppm
0.02 % Mild headache within 2 - 3 hours
0.04 % Frontal headache within 1 - 2 hours, rear headache within 2.5 - 3.5 hours
0.08 % Headache, dizziness, nausea within 45 minutes, loss of consciousness within 2 hours
0.16 % Headache, dizziness, nausea within 20 minutes, loss of consciousness within 2 hours
0.32 % Headache, dizziness within 5 - 10 minutes, death within 30 minutes
0.64 % Headache, dizziness within 1 - 2 minutes, death within 10 - 15 minutes
1.28 % Death within 1 - 3 minutes

From Work Environment Measurement Handbook

Hydrogen sulfide (H2S) gas

This gas irritates the skin and mucous membranes due to its high water solubility and reduces oxygen partial pressure in the lungs by inhibiting the action of cytochrome oxidase, a transmembrane molecule, thereby causing breathing difficulties (cellular level oxygen deficiency). ⇒ This is accelerated at higher concentrations, potentially resulting in collapse/loss of consciousness.

H2S chemical properties

Colorless, transparent gas with rotten egg odor
Highly soluble in water (0.25 g/100 ml at 40 °C)
Molecular weight: 34
Specific gravity: 1.190; heavier than air
TLV: 1 ppm (ACGIH: American Conference of Government Industrial Hygienists)
Lower explosive limit: 4 vol% 

- Hydrogen sulfide poisoning -

This refers to the condition in which symptoms are observed due to the inhalation of air containing hydrogen sulfide at concentrations of more than 10 parts per million (= 10 ppm).
* As defined in Article 2 of the Ordinance on Prevention of Anoxia, etc.

Hydrogen sulfide concentration, effects, and toxicity

Concentration (ppm) Effects and toxicity
0.025 Threshold at which odor can be detected. This varies from individual to individual.
0.3 Odor clearly detectable
3 - 5 Unpleasant odor of moderate strength
10 Lower limit of irritation to eye membranes. Threshold limit value.
20 - 40 Strong but not unbearable odor. Lower limit of irritation to lung membranes.
100 Sense of smell dulled after 2 - 15 minutes Irritation of eyes and airways within 1 hour.
Death may result after continuous exposure for 8 - 48 hours.
170 - 300 Threshold for avoiding serious health damage after 1 hour exposure.
400 – 700 Risk to life after exposure for
30 minutes - 1 hour
800 - 900 Rapid loss of consciousness,
respiratory failure, death
1,000 Immediate loss of consciousness, death

Combustible gases (CH4, H2 etc.)

Gases that burn in air or oxygen are called combustible gases. These gases are used in spray cans and other products, both in industry and in the home. They encompass a wide range of types and properties. Since these substances can cause gas explosions and other hazardous incidents, various regulations govern their use. Here, we’ll discuss the types and characteristics of combustible gases.

Definition of combustible gas (in Japan)

The Regulation on Safety of General High Pressure Gas defines combustible gases as follows:

・Gases for which the lower explosive limit (explosive limit when mixed with air; the same hereinafter) is 10 % or less

・Gases for which the difference between the upper and lower explosive limits is 20 % or greater

Lower explosive limit (LEL): Concentration in air above which explosions may occur
Upper explosive limit (UEL):Concentration in air below which explosions may occur

Example: Explosive range of hydrogen

・%LEL is the concentration expressed as a percentage of the lower explosive limit
・The lower explosive limit varies for different gases. (For example, for methane the LEL is 5 vol%; for isobutane, the LEL is 1.8 vol%)

Combustible gas list

Methane (CH4) gas

- Methane (CH4) gas properties and characteristics -

・Readily combustible (relatively narrow combustion range)
 Combustion range: 5 vol% - 15 vol%
・Will not readily or spontaneously ignite
 Ignition point (spontaneous ignition temperature: 537 °C
・Lighter than air
 Molecular weight: 16.04; specific gravity (compared to air): 0.6
・The main constituent of natural gas (including shale gas and methane hydrate)
 Widely used as fuel gas
・Colorless, odorless, and not harmful to humans
・Has a global warming coefficient approximately 25 times greater than CO2, and is claimed to be a cause of global warming due to
 its presence in gases emitted by ruminant animals such as cows, sheep and goats.
 Research is underway in numerous countries into ways to reduce emissions to protect the global environment.

Hydrogen (H2) gas

Hydrogen (H) is the lightest and most abundant element found on Earth.
Hydrogen is a combustible gas with a low minimum ignition energy and a broad explosive range when mixed with air. Since it does not generate the greenhouse gas CO2 when burned, unlike other typical combustible gases such as methane and propane, it has become the focus of attention in recent years as a potential clean energy source.

Typical combustible gases: CH4 + 2O2 → 2H2O + CO2 (greenhouse gas) emission
Hydrogen gas: H2 + 1/2O2 → H2O

- Hydrogen (H2) gas properties and characteristics -

・Readily combustible (wide combustible range)
 Combustible range: 4 vol% - 75 vol%
・Will not readily or spontaneously ignite
 Ignition point (spontaneous ignition temperature): 570 °C
・The lightest of all gases on Earth
 Molecular weight: 2.017; specific gravity (compared to air): 0.07
・Colorless, odorless, and not harmful to humans
・Does not generate CO2 when burned
 Low environmental load
・Plentiful resource; most abundant element on Earth
 Manufacturing method: Electrolysis of water (e.g., seawater), extraction from fossil fuels, byproduct gas from steel manufacturing,

- Topic: Technology enabling a hydrogen society -

While hydrogen is a combustible gas related with risk of explosion, it is seen as potentially playing a major role in preventing global warming, as it does not generate CO2 emissions, unlike other fossil fuels.

- Hydrogen usage examples -

Power to Gas

Technology for manufacturing hydrogen and synthetic methane using power from renewable energy (e.g., solar and wind power). Research and demonstration testing is currently underway around the world to establish an environmentally friendly low-carbon society.


Technology for synthesizing methane (CH4), the main constituent of natural gas, from CO2 and H2. CH4 synthesized using CO2 as a raw material can be considered a carbon-neutral fuel. In contrast to H2 directly, synthesized CH4 would allow use of the existing natural gas infrastructure.

FCVs (fuel cell vehicles)

Vehicles that run on electricity generated by a chemical reaction between hydrogen and oxygen in a fuel cell to drive a motor It is anticipated as an environmentally friendly zero-emission technology, since it generates no CO2 is emitted, and all that is with water its only emission.

Organic hydride technology

Technology facilitating storage and transportation using MCH (methylcyclohexane), with a volume approximately 1/500th of hydrogen, produced through a catalytic reaction between hydrogen and toluene This will allow storage of hydrogen for extended periods and safe, simple hydrogen transport in greater volumes.

Other gases (e.g., CO2, etc.)

Other hazardous gases are also used in various industrial and domestic settings. Here, we’ll explain the terms specific gravity of vapor and vapor pressure, key factors in detecting leaks of CO2 gas - something highlighted as one of the SDGs.

The effects of CO2 on the human body

Carbon dioxide has low toxicity and rarely causes respiratory symptoms. In high concentrations, it produces anesthetic effects and can cause death by suffocation.

CO 2 chemical properties

Colorless, odorless gas
Soluble in water Solubility: 90.1 mL/100 mL, 20 °C
Molecular weight: 28 Specific gravity: 1.5
TLV: 5,000 ppm (ACGIH: American Conference of Government Industrial Hygienists)
Not defined as a toxic gas in the Regulation on Safety of General High Pressure Gas

Act on Maintenance of Sanitation in Buildings
(abbreviation: Building Maintenance Act)
(in Japan)

The owners, occupants, and managers of specified buildings exceeding a specific total area (e.g., department stores, offices, schools, hotels) must maintain buildings in accordance with standards stipulated by government ordinances (Article 4)

Building Environmental Hygiene Management Standards (Government Ordinance Article 2)
Air conditioning equipment is required to supply air purified so that carbon dioxide concentrations do not exceed 1,000 ppm.

CO2 concentration and symptoms

CO2 concentration symptoms
0.1 % (1,000 ppm) Building hygiene management standard
0.5 % TLV-TWA threshold at which harmful effects to the body are caused by 8 hours exposure daily
1 - 2 % Causes discomfort
3 - 4 % Irritation of the respiratory center resulting in symptoms such as increased respiratory rate, increased pulse and blood pressure, headache, and dizziness
6 % Respiratory difficulties
7 - 10 % Loss of consciousness within a few minutes, cyanosis, and death

Aging of CO2 gas concentration

The following graph shows changes in concentrations of CO2 gas in the atmosphere in Japan between 1987 and 2020.
The concentration of CO2 gas is increasing annually, with levels of 410 ppm or more of CO2 in the atmosphere recorded in recent years.

Based on Meteorological Agency measurement data from Ryori, Ofunato, in Iwate Prefecture

Carbon dioxide (CO2) is known as one of the greenhouse gases that cause global warming. Signatory nations of the 2016 Paris Agreement* are pushing ahead with decarbonization measures to reduce CO2 emissions.
* International initiative to tackle climate change issues from 2020 onward at the Conference of the Parties to the United Nations Framework Convention on Climate Change (known as “COP”) held in Paris in 2015.

Other references: Covid-19 cluster prevention measures

Details on ventilation methods for improving poorly ventilated enclosed spaces (leaflet) published on the Ministry of Health, Labour and Welfare website (revised April 3, 2020) indicate that promoting indoor ventilation in accordance with the Building Maintenance Act standard (not more than 1,000 ppm carbon dioxide) constitutes one measure for preventing the risk of infection clusters.

Specific gravity of vapor

This indicates the ratio of the density of a particular gas with respect to the density of air under standard conditions. This is one of the factors that must be taken into consideration when considering where to install gas detectors.

For gases heavier than air (specific gravity of vapor > 1)

If gas leaks occur, gas is likely to accumulate at lower points
⇒ Gas detectors should be installed at lower points.

For gases lighter than air (specific gravity of vapor < 1)

If gas leaks occur, gas is likely to accumulate at higher points
⇒ Gas detectors should be installed at higher points.

Vapor pressure

This refers to the pressure at which a liquid changes into a gas at a specific temperature. If vapor pressure data is available from documents such as SDS (safety data sheets) for VOCs or other organic solvents, this enables calculation to determine the concentration at which the substance will vaporize (at a given temperature).

Example calculation:
Ethyl alcohol Vapor pressure: 5.8 kPa/20 °C
If atmospheric pressure is 101.3 kPa
Vapor pressure concentration (%) = Vapor pressure ÷ Atmospheric pressure = 5.8 ÷ 101.3 ≒ 5.73 vol%
Thus, the vapor pressure concentration of ethyl alcohol at 20 °C is approximately 5.73 vol%