What does the upper flammable limit (UFL) refer to?

The upper flammable limit (UFL) defines the maximum concentration of fuel vapor in the air that can sustain combustion. When the concentration of the fuel vapor exceeds this limit, there is not enough oxygen present to support the combustion process. This characteristic is crucial for understanding fire behavior, as it helps determine the conditions under which a flammable substance can ignite and burn effectively. In scenarios where the concentration exceeds the UFL, the mixture is too rich in fuel for combustion to occur, illustrating the vital role of maintaining an appropriate ratio of fuel to air for effective ignition and burning. Therefore, the answer clearly reflects this key aspect of fire dynamics.