What does the term "lower flammable limit" (LFL) refer to?

The term "lower flammable limit" (LFL) specifically refers to the concentration of a gas in the air below which combustion cannot occur. This means that if the concentration of the flammable gas is below this limit, there is not enough fuel present to support the combustion process, regardless of the presence of an ignition source.

Understanding the LFL is critical in laboratory safety and handling of flammable gases. It helps in assessing the risks associated with the storage and use of these gases. Maintaining concentrations above the LFL ensures that a flammable gas-air mixture is within a range that can become ignitable when exposed to an ignition source, but being below the LFL indicates a safety zone where the risk of combustion is eliminated.

The other options, while relating to combustibility and safety measures, do not accurately define what LFL means in terms of combustion dynamics and thresholds.