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Combustion is a rapid oxidation process which chemically changes combustible substances and produces heat. It is commonly associated with a luminous phenomenon, the flame. In order to take place, combustion needs a fuel, a combustive agent (oxygen) and a heat source. As soon as one of these three factor is missing, the combustion process is interrupted. To stop a fire and avoid its propagation, therefore, one needs to operate on the heat source, through heat reduction, or on the combustive agent, through suffocation, or parting fuel and combustive agent, this way letting the fire extinguish itself by depletion. Another action is possible: inhibition. Indeed, some materials act directly on the chemical process, increasing the activation energy that is necessary to the fuel, acting as a negative catalyst. Often, extinguishing agents combine more than one of these actions.

Let us see the combustion process in detail. The fuel, any oxidisable substance, be it solid, liquid or gaseous, must reach a minimum temperature to be able to emit flammable vapours. The combustive agent, oxygen or an oxidant, must stay inside its flammability limits, this means that the ratio between fuel and combustive agent must not exceed the given limits. The heat source is necessary to bring the blend of fuel and combustive agent to the ignition temperature, that is to say the minimum temperature for the gas produced by the fuel to spontaneously burn in the presence of air, and start this way the combustion process. This process realeases energy, as heat and light. The fuel reaches the combustion temperature, that is that temperature that is theoretically reached during the combustion of a given material with the minimum quantity of air. The combustion produces solid and liquid particles as well, like carbon monoxide, carbon dioxide, hydrochloric acid and hydrogen cyanide.

Other by-products are rapid gas explosions, gas or smoke accumulations, O2, flashovers (the entirety of fuel that is in an enclosed space catches fire at the same time), and backdrafts.