In a controlled ventilation area, a CH4 concentration threshold of what percentage makes the area dangerous?

• A. 1%
• B. 2.5% CH4
• C. 5%
• D. 10%

Answer: (B)

In mine ventilation, gas levels are used to determine when extra safety measures must kick in. Methane is flammable, with the mixture becoming explosive at about five percent in air (the lower explosive limit). But in a controlled ventilation area, the designated danger threshold is set lower—two and a half percent. Reaching this level signals a significant gas presence with enough risk of pockets, rapid concentration rises, or ignition sources to warrant immediate safety actions like increasing ventilation, stopping work, or evacuating. The other options are either below this safety trigger or lie at/above the flammable limit, but the established rule for this context uses two and a half percent as the trigger for a dangerous atmosphere.