Combustible gases concentration must be less than what percent?

• A. 10%
• B. 5%
• C. 15%
• D. 20%

Answer: (A)

The safety limit hinges on knowing the lower explosive limit (LEL) for combustible gases. The LEL is the smallest concentration of gas in air that can ignite if an ignition source is present. To maintain a safe working environment, you keep the gas concentration well below that ignition point. The standard rule in this context is to limit combustible gas concentration to less than 10 percent of the LEL. That 10% threshold provides a comfortable safety margin so that, even with ventilation changes or a potential ignition source, a flammable mixture won’t form. Readings at higher fractions of the LEL (like 15% or 20%) indicate a significantly greater fire or explosion risk and require corrective action. Even though a lower fraction (for example 5% of the LEL) is safer, the commonly cited criterion for this scenario is 10% of the LEL.