Although all materials—under a specific temperature and pressure—may become gases, the materials considered gaseous in practice are those that are in a gas state at normal temperature (~20 °C) and normal atmospheric pressure (~100 kPa). In respect to fire and explosion hazards, gases may be ranked in two main groups: combustible and non-combustible gases. According to the definition accepted in practice, combustible gases are those that burn in air with normal oxygen concentration, provided that the conditions required for burning exist. Ignition only occurs above a certain temperature, with the necessary ignition temperature, and within a given range of concentration. Non-combustible gases are those that do not burn either in oxygen or in air with any concentration of air. A portion of these gases support combustion (e.g., oxygen), while the other portion inhibit burning. The non-combustible gases not supporting burning are called inert gases (nitrogen, noble gases, carbon dioxide, etc.). In order to achieve economic efficiency, the gases stored and transported in containers or transporting vessels are typically in compressed, liquefied, or cooled-condensated (cryogenic) state. Basically, there are two hazardous situations in connection with gases: when they are in containers and when they are released from their containers. For compressed gases in storage containers, external heat might considerably increase the pressure within the container, and the extreme overpressure might lead to explosion. Gaseous storage containers will typically include a vapour phase and a liquid phase. Because of changes in pressure and temperature, the extension of the liquid phase gives rise to the further compression of vapour space, while the vapour pressure of the liquid increases in proportion with the increase of temperature. As a result of these processes, critically dangerous pressure may be produced. Storage containers are generally required to contain the application of overpressure relief devices. These are capable of mitigating a hazardous situation due to higher temperatures.
If the storage vessels are insufficiently sealed, or damaged, the gas will flow out to the free air space, mix with air and depending on its quantity and the way of its flowing, may cause the formation of a large, explosive air space. The air around a leaking storage vessel can be unsuitable for breathing and may be dangerous for people nearby, partly due to the toxic effect of some gases and partly due to the diluted concentration of oxygen.
Bearing in mind the potential fire hazard due to gases and the need for safe operation, one must get detailed knowledge of the following features of gases either stored or used, especially for industrial consumers: the chemical and physical properties of gases, ignition temperature, the lower and upper limits of concentration for flammability, the hazardous parameters of the gas in the container, the risk factors of the hazardous situation caused by the gases released into the open air, the extent of the necessary safety zones and the special measures to be taken in case of a possible emergency situation connected with fire-fighting.