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.
The force contained in a high-pressure gas cylinder can be very great. If an air cylinder used in a breathing set had both ends cut off, and was then slit lengthways and rolled out flat, the area might be 50 cms x 40 cms = 2000 cms². The internal air pressure could be as high as 200 kg per cm², so that the total force trying to escape through the cylinder wall would be 400 tonnes.
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
In spite of these definitions, pure oxygen in Class 2.2 can be very dangerous, as it reacts readily with any hydrocarbon or other combustible material, and will ignite it rapidly. Valves in oxygen lines have to be operated grease-free. Recall the near-disastrous explosion on the rocket Apollo 13 on its way to the Moon in the 1970s.