what is the percentage of gas in underground mines? | Underground mines me gas kitna hona chahiye?

Ø  CMR 2017 ke regulation no. – 153 , ke anusaar mines ke return airway Ke GBA(general body of air) me gas 0.75% se adhik nhi hona chahiye or mines ke other GBA me 1.25% se adhik nhi hona chahiye.