Section 7972 Of Chapter 2. Gassy And Extrahazardous Tunnels From California Labor Code >> Division 5. >> Part 9. >> Chapter 2.

. If the level of gas in any tunnel or underground mine reaches 20 percent of its lower explosive limit at any time all men shall be removed, the division notified immediately by telephone or telegram, and no one shall reenter the tunnel or underground mine until approval is given by the division.