What percentage indicates the lower limit of flammability in air?

The lower limit of flammability, often referred to as the lower explosive limit (LEL), indicates the minimum concentration of a gas or vapor in air that can ignite and sustain combustion. For propane, which is commonly discussed in the context of LP gas, this lower limit is approximately 2.15%. This means that if the concentration of propane in the air is below this percentage, there will not be enough propane present to support combustion, regardless of the presence of an ignition source.

Understanding the LEL is crucial for safety in various applications involving flammable gases. It helps in assessing when a gas-air mixture could pose a fire hazard. The other percentages provided in the options represent concentrations that are higher than the actual LEL for propane, which would not effectively indicate the point at which flammability begins. Therefore, 2.15% is the correct indication of the lower limit of flammability in air for propane.