Three components are necessary for an atmosphere to become flammable: fuel, oxygen, and a source of ignition.
Some confined spaces my contain solvents, fuel oil, gasoline, kerosene, etc. which provide the fuel for combustion. In order for an atmosphere to become flammable, it must have the proper mixture of fuel and oxygen. If the concentration of a specific gas is below the lower explosive limit (LEL) it is too lean to burn. If the concentration is above the upper explosive limit (UEL) it is too rich to burn.