An explosive atmosphere is an event which, in combination with sufficient oxygen and fuel in gas, mist, vapor or dust form, can cause an explosion. Potentially explosive environment forms when flammable substances exists int eh athmosphere in a given concentration; if the concentration is too low (low mixture) or too high (rich mixture) no explosion occurs, it may produce a combustion reaction, or even no reaction.
The explosion may thus occur only through a ignition source and when the concentration is between the explosive range of substances, between the minimum limit (LEL) and upper (UEL). The explosion limits depend on the environmental pressure and the oxygen ratio in the air.
For liquid spray booths, zone 2 is defined as the internal volume of the booth (including the air recirculation ducts) and the external volume around the booth up to 1 m from the permanent openings if the concentration is under 25% of the LEL (low explosion level).
For powder spray booths, zone 22 is defined as the internal volume of the booth (including the air recirculation ducts and the open powder recovery systems) where the standard requires that concentrations of flammable substances are kept below 50% of the LEL for powders through forced ventilation.