The highest concentration that will burn when an ignition source is present is called the Upper Explosive Limit (UEL) or Upper Flammable Limit (UFL).

• A. Upper Explosive Limit (UEL) or Upper Flammable Limit (UFL)
• B. Lower Explosive Limit (LEL) or Lower Flammable Limit (LFL)
• C. Minimum Ignition Concentration (MIC)
• D. Ignition Window

Answer: (A)

The main idea is the flammable range of a vapor in air. A mixture will ignite only if its concentration lies between two limits: the lower limit is the minimum needed to support ignition, and the upper limit is the maximum concentration at which ignition can still occur. The largest concentration that can still burn when an ignition source is present is the upper explosive limit, also known as the upper flammable limit. Above this limit the mixture is too rich to burn. The lower explosive limit describes the opposite end of the range, and while some people might hear “ignition window,” the precise and widely used terms for these limits are LEL/ULE (or LFL/UFL).