LOC is defined as which of the following?

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

Answer: (B)

Limiting Oxygen Concentration is the minimum oxygen level required to sustain a flame. If the atmosphere’s oxygen falls below this limit, combustion cannot continue even if fuel is present and heat exists. This idea helps explain why inerting a space with nitrogen can extinguish fires by dropping the oxygen concentration enough to be below the LOC for the fuel involved. It’s different from the lower and upper explosive limits, which describe fuel concentration ranges in air for ignition; autoignition temperature, which is the temperature needed for spontaneous ignition without a spark, and ignition energy, which is the energy needed to ignite a vapor cloud.