Cal. Lab. Code § 7967

Whenever gas levels in excess of 10 percent of the lower explosive limit are encountered initially in a tunnel or underground mine classified as gassy, the division shall be notified immediately by telephone or telegraph. The chief of the division or his authorized representative may waive subsequent notification for gas readings less than 20 percent of the lower explosive limits upon a finding that adequate ventilation and other safety measures are provided to assure employee safety.