The minimum concentration of a substance in the air that is required for ignition is called the:

The concept of the Lower Explosive Limit (LEL) is crucial when discussing flammability and potential hazards associated with various substances. The LEL defines the minimum concentration of a flammable vapor or gas in the air that is necessary to support combustion. If the concentration of the substance is below this level, the mixture is too "lean" to ignite and burn effectively, which means it lacks sufficient fuel for combustion to occur.

Understanding the LEL is vital for ensuring safety in environments where flammable materials are present, as workers must be aware of the concentrations that pose risks of fire or explosion. Awareness of this threshold helps inform safe handling practices and necessary precautions when working with hazardous materials, ensuring informed decision-making when assessing air quality and flammability risks in a workplace setting.

In contrast, the Upper Explosive Limit (UEL) pertains to the maximum concentration of a substance in air that can ignite, and the explosive range refers to the range between the LEL and UEL where combustion can occur. Time-Weighted Average (TWA), on the other hand, relates to permissible exposure limits over time and does not pertain to ignition or flammability directly.