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Gas Groups - Grouped by Risk
Gases and vapours are divided into
Groups of similar risk. Group I is for gases found below ground e.g. in mines.
Group II gases are those found in chemical and process plants and are further
subdivided into IIA, IIB and IIC in ascending order of danger. The factors used
in grouping gases include the ability of a gas to spread into the atmosphere and
its ease in achieving its lowest explosive concentration. When equipment is
suitable for IIC gases, it is automatically suitable for IIB and IIA gases also.
Hydrogen is regarded as one of the most dangerous gases and is an example of a
Group IIC gas.