What is the term for the lowest concentration of a vapor or gas that can produce a flash of fire when an ignition source is present?

The term that describes the lowest concentration of a vapor or gas that can lead to a flash of fire in the presence of an ignition source is known as the lower explosive limit (LEL). This threshold is crucial in hazardous materials and fire safety because it indicates the minimum amount of flammable vapors necessary for combustion to occur.

When the concentration of gas or vapor is below this limit, the mixture is too lean to ignite, meaning there isn't enough fuel present for a fire to start, even if an ignition source is available. Understanding this concept helps responders evaluate the potential risks in a hazardous materials incident, as knowing the LEL allows for better decision-making regarding safety measures and necessary response actions.

In contrast, terms like upper explosive limit (UEL) refer to the highest concentration of vapor or gas in the air that can ignite, while the flammable range describes the entire spectrum between the LEL and UEL. Ignition threshold is not a standard term used in this context, further distinguishing why the LEL is the correct answer.