The lowest concentration that will burn when an ignition source is present is called the Lower Explosive Limit (LEL) or Lower Flammable Limit (LFL).

• A. Lower Explosive Limit (LEL) or Lower Flammable Limit (LFL)
• B. Upper Explosive Limit (UEL) or Upper Flammable Limit (UFL)
• C. Lower Flammable Limit (LFL) or Lower Explosive Limit (LEL)
• D. Ignition Threshold (IT)

Answer: (A)

The minimum concentration of a flammable gas or vapor in air that will burn when an ignition source is present is the lower explosive limit, also called the lower flammable limit. This marks the lower bound of the flammable range. If the concentration is below this limit, the mixture is too lean to sustain combustion; with an ignition source, a flame will not propagate. Within the interval from the LEL to the upper explosive limit (UEL), mixtures can ignite, and above the upper limit the mixture is too rich to burn. So, identifying the LEL/LFL as the lowest concentration that will burn is correct. The upper limit is described by the upper explosive limit (UEL) or upper flammable limit (UFL), and the term ignition threshold isn’t a standard descriptor for this concept.