At what methane concentration must hot work be stopped if underground?

• A. 1.0%
• B. 0.5%
• C. 2.0%
• D. 0.1%

Answer: (B)

The main idea is managing ignition risk from methane during hot work. Methane forms an explosive mixture with air in the range around 5% to 15%, so precautions are needed even when readings are well below that range. In underground coal mines, a practical safety rule is to stop hot work when methane reaches 0.5% by volume. At or above this level, you stop, ventilate, re-test, and only resume once the concentration has dropped below that threshold and all other controls are in place. This gives a safety margin to account for potential fluctuations and ensures ignition sources from hot work don’t ignite methane.

The other options set the stopping point at higher or lower levels than this standard, which would not align with the typical safety practice.