UFL describes which of the following?

• A. The minimum oxygen concentration required to propagate a flame
• B. The energy required to ignite a mixture
• C. The highest concentration of a substance in air that will sustain combustion
• D. The lowest temperature at which a material will spontaneously ignite

Answer: (C)

UFL stands for the upper flammable limit. It is the highest concentration of a vapor or gas in air at which a flame can still be sustained. If the fuel concentration in air rises above this limit, there isn’t enough oxygen relative to fuel for a flame to propagate, so ignition cannot be maintained. The flammable range is defined from the lower flammable limit to the upper flammable limit, with ignition possible somewhere inside that range. The other options describe different fire-hazard concepts: minimum oxygen level for flame propagation, the energy needed to ignite a mixture, and the temperature at which a material will spontaneously ignite.