What is the intake air limit for methane (CH4)?

• A. Intake Air 0.20%
• B. Intake Air 0.30%
• C. Intake Air 0.27%
• D. Intake Air 0.25%

Answer: (D)

The key idea here is safety margins in ventilation: the intake air methane limit is a conservation threshold used to keep the air entering the workings extremely lean in methane. Methane is explosive in air, but the risk is dramatically reduced when its concentration in the intake air is kept very low. A limit of 0.25% methane in intake air provides a substantial buffer below the flammable range and gives room for dilution, mixing, and detection should any small source release methane. This value is a common standard in NSW practice, reflecting a cautious approach to prevent accumulation of methane in the mine atmosphere. The other percentages do not align with the standard intake air limit used for methane in NSW underground coal mines.