What is the upper limit of flammability in air, as a percent of vapor in air/gas mixture?

The upper limit of flammability refers to the maximum concentration of a vapor in the air above which combustion cannot occur. If the concentration of the flammable vapor is higher than this upper limit, there isn't enough oxygen present to support combustion.

In the context of many common flammable gases, the upper flammability limit (UFL) is precisely defined. For example, gases like methane and propane have specific UFLs that are understood and documented in safety information. The correct choice indicates a specific level at which a mixture is no longer capable of igniting due to surplus of fuel relative to oxygen.

In general, knowledge of these limits is crucial for safety in handling flammable materials, both in industrial settings and everyday practices. Being aware of the upper limits helps in risk assessment and in implementing appropriate safety measures.