2.1 A gas is a substance which:

1. at 50°C has a vapour pressure greater than 300 kPa; or
2. is completely gaseous at 20°C at a standard pressure of 101.3 kPa.

2.2. The transport condition of gas is described according to its physical state as:

1. compressed gas: a gas which when packaged under pressure for transport is entirely gaseous at - 50°C; this category includes all gases with a critical temperature less than or equal to - 50°C;
2. liquefied gas: a gas which when packaged under pressure for transport is partially liquid at temperatures above - 50°C . A distinction is made between: high pressure liquefied gas: a gas with a critical temperature between - 50°C and + 65°C , and low pressure liquefied gas: a gas with a critical temperature above + 65°C ;
3. refrigerated liquefied gas: a gas which when packaged for transport is made partially liquid because of its low temperature;
4. dissolved gas: a gas which when packaged under pressure for transport is dissolved in a liquid phase solvent;
5. adsorbed gas : a gas which when packaged for transport is adsorbed onto a solid porous material resulting in an internal receptacle pressure of less than 101.3 kPa at 20°C and less than 300kPa at 50 °C.

2.3 The class comprises compressed gases, liquefied gases, dissolved gases, refrigerated liquefied gases, absorbed gas, mixtures of one or more gases with one or more vapours of substances of other classes, articles charged with a gas, aerosols and chemicals under pressure.

2.4 Class subdivisions

Class 2 is subdivided further according to the primary hazard of the gas during transport;

Note: For UN 1950 AEROSOLS see also the criteria in special provision 63. For chemicals under pressure of UN Nos. 3500 to 3505, see also special provision 362. For UN 2037 RECEPTACLES, SMALL, CONTAINING GAS (GAS CARTRIDGES), see also special provision 303.

RECEPTACLES, SMALL, CONTAINING GAS (GAS CARTRIDGES) see also special provision 303.

2.4.1 Class 2.1 Flammable gases

Gases which at 20°C and a standard pressure of 101.3 kPa:

1. are ignitable when in a mixture of 13% or less by volume with air; or
2. have a flammable range with air of at least 12 percentage points regardless of the lower flammable limit. Flammability shall be determined by tests or calculation in accordance with methods adopted by the International Organization for Standardization (see ISO 10156:2017). Where insufficient data are available to use these methods, tests by a comparable method recognized by a national competent authority may be used.

2.4.2 Class 2.2 Non-flammable, non-toxic gases

Gases which :

1. are asphyxiant - gases which dilute or replace the oxygen normally in the atmosphere; or
2. are oxidizing - gases which may, generally by providing oxygen, cause or contribute to the combustion of other material more than air does; or
3. do not come under the other classes.

2.4.3 Class 2.3 Toxic gases

Gases which:

1. are known to be so toxic or corrosive to humans as to pose a hazard to health; or
2. are presumed to be toxic or corrosive to humans because they have a LC50 value (as defined in 2.6.2.1 of IMDG Code) equal to or less than 5,000 mL/m3 (ppm).

Note: Gases meeting the above criteria owing to their corrosivity are to be classified as toxic with a subsidiary corrosive hazard.