LEL stands for "Lower Explosive Limit" and is the lowest concentration of a particular gas that has the potential to be flammable or combustible. In other words, it is the minimum amount of gas that will catch fire or explode when an ignition source is present. The LEL is the lowest concentration of gas mixed with air that can ignite, while the UEL (Upper Explosive Limit) is the highest concentration. The LEL varies from gas to gas, but for most flammable gases, it is less than 5% by volume in air. If the concentration of gas in the air is lower than the LEL, gas mixtures are "too lean" to burn. LEL gas detectors are used to detect hazardous levels of combustible gases and to trigger an alarm before the LEL is exceeded.