Gases are mostly carried under pressure to reduce their volume, and hence to save space in transport and storage.
This pressure itself creates a danger if it is released suddenly. Pressure never dissipates, unlike heat, which is transferred to or from the surroundings until a uniform temperature prevails.
If a cylinder valve gets knocked off, the escaping gas is concentrated into a powerful jet which makes the cylinder take off like a rocket, and do dire damage.
Most gases are heavier than air. They can cause suffocation if they displace or dilute air in confined spaces.
Applying pressure to gases will reduce their volume, but if they turn to liquid under pressure, the volume is reduced much further, several hundred times. Some gases liquefy under pressure at normal temperatures, e.g. the liquefied petroleum gases, chlorine, ammonia. But some, the permanent gases, will only liquefy if they are also refrigerated down to very low (critical) temperatures, e.g.. As low as - 269° C for liquid helium.
These include oxygen, nitrogen, hydrogen, helium, neon, and argon. Once liquefied, they have to be contained in special heavily-insulated containers to prevent them warming up. The extreme cold clearly creates a danger if cold metal, etc, is accidentally touched without protection. Also, an escape of very cold-gas creates a breathing danger, causing direct damage to the lungs, or to local oxygen starvation.
So far these have all been physical dangers. But gases also present chemical dangers, e.g. flammable gases such as butane, acetylene, and toxic gases such as chlorine, ammonia. The Class therefore has three Divisions:
| Class 2.1
| flammable gases
| Class 2.2
| non-flammable, non-toxic gases
| Class 2.3
| toxic gases