The Low Explosion Limit (LEL) defines the lowest concentration of a combustible in air that allows the explosion. There could be a high explosion risk even with very small gas concentrations, in fact the great part of flammable gases and vapours have the LEL figure below 5 % of volume.
The Upper Explosion Limit (UEL) defines the maximum concentration of fuel in air that may cause explosion. Concentrations of the combustible above the UEL cannot generate explosion because of the insufficient concentration of the burner (typically Oxygen).
E.g. Methane (CH4) L.E.L. = 5 % (=50000 ppm) and U.E.L. = 15 % (150000 ppm). 20% of LEL = 1 % (= 10000 ppm).
Methane (CH4 ) L.E.L. = 5 % ( = 50000 ppm), U.E.L. = 15 %
20 % of L.E.L. = 1 % ( = 10000 ppm)
Propane (C3 H8 ) L.E.L. = 2,1 % ( = 21000 ppm), U.E.L. = 9,5 %
20 % L.E.L. = 0,42 % ( = 4200 ppm)