Which statement about LFL is correct?

• A. LFL refers to the highest concentration in air that will prevent combustion
• B. LFL refers to the lowest energy required to ignite
• C. LFL refers to the lowest concentration of a substance in air that will sustain combustion
• D. LFL refers to the temperature for ignition

Answer: (C)

In flammability, there are limits that define when a vapor can burn. The lower flammable limit is the minimum concentration of vapor in air that can sustain a flame once ignition occurs. If the vapor is below this level, the mixture is too lean to burn, even if an ignition source is present. That’s why the statement describing the lowest concentration in air that will sustain combustion is the best fit.

The other ideas mix up different concepts: ignition energy is a separate measure called minimum ignition energy, and ignition temperature refers to how hot a surface or gas needs to be to ignite, not to the fuel concentration. The notion of a “highest concentration that will prevent combustion” isn’t how flammable limits are described—the relevant idea is the range between the lower and upper limits where ignition and flame propagation are possible.