Lower flammable limit (LFL) or Lower Explosive Limit (LEL) is minimum vapor concentration in air which a mixture will burn when an ignition source is present.
Upper flammable limit (UFL) or Upper Explosive Limit (UEL) is maximum vapor concentration in air which a mixture will burn when an ignition source is present.
Concentration of mixture of vapor in air below LFL/LEL (too lean) or above UFL/UEL (too rich), mixture will not burn even an ignition source is present. Therefore, flammable range or explosive range is concentrations between LFL/UFL and UFL/UEL.
LFL/LEL and UFL/UEL for some common gases are indicated in table below. Some of the gases are commonly used as fuel in combustion processes.
Note : The limits indicated are for component and air at 20oC and atmospheric pressure.