The concentration of gas in the air that is most likely to lead to a fire or explosion. The lower level of this range is called the Lower Explosive Limit (LEL). Concentrations less than the LEL cannot combust because there is not enough fuel. The upper level of this range is called the Upper Explosive Limit (UEL). Concentrations greater than the UEL cannot combust because there is not enough oxygen. Each gas has a distinct flammable range with widely varying LEL and UEL concentrations.