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.