At what concentration of methane must people be withdrawn from a part of a mine that is required to be ventilated?

• A. 1.0%
• B. 3.0%
• C. 2.5%
• D. 5.0%

Answer: (C)

Managing methane in a ventilated zone hinges on having a clear withdrawal threshold so workers aren’t exposed to unsafe gas levels. When methane in the air reaches 2.5% by volume, the area that must be ventilated is considered unsafe for personnel to remain in, so everyone is withdrawn to a safer area while gas control actions are taken. This gives time to reduce the concentration through increased ventilation or gas drainage and to re-check that the air is safe before re-entry. The 2.5% level is chosen as a practical safety margin between normal operation and the higher risks that come with more gas, including asphyxia and the possibility of ignition if an ignition source is present. Keeping workers out at that threshold helps ensure that gas levels can be brought down in a controlled way rather than allowing entry to continue into an area where conditions could rapidly deteriorate. A higher threshold would raise the risk by delaying necessary control actions, while a lower threshold would cause unnecessary withdrawal and disruption.