medical breathing (oxygen therapy) at medical facilities and at home (high pressure cylinder)
breathing at altitude in aviation, either in a decompression emergency, or constantly (as in unpressurized aircraft), usually in high pressure cylinders
oxygen first aid sets, in small portable high pressure cylinders[1]
Contrary to popular belief most scuba divers do not carry oxygen tanks. The vast majority of divers breathe air or nitrox stored in a diving cylinder. A small minority breathe trimix, heliox or other exotic gases. Some may carry pure oxygen for accelerated decompression or as supply gas to a rebreather. Some shallow divers, particularly naval combat divers, use oxygen rebreathers, and they use a small oxygen cylinder to provide the gas.[citation needed]
Oxygen is rarely held at pressures higher than 20 megapascals (3,000 psi), due to the risks of fire triggered by high temperatures caused by adiabatic heating when the gas changes pressure when moving from one vessel to another. Medical use liquid oxygen airgas tanks are typically 2.4 MPa (350 psi).[citation needed]
All equipment coming into contact with high pressure oxygen must be "oxygen clean" and "oxygen compatible", to reduce the risk of fire.[3][4] "Oxygen clean" means the removal of any substance that could act as a source of ignition. "Oxygen compatible" means that internal components must not burn readily or degrade easily in a high pressure oxygen environment.
In some countries there are legal and insurance requirements and restrictions on the use, storage and transport of pure oxygen.[citation needed] Oxygen tanks are normally stored in well-ventilated locations, far from potential sources of fire and concentrations of people.[citation needed]