California Code, Labor Code - LAB § 7955

(a) Nongassy, which classification shall be applied to tunnels or underground mines where there is little likelihood of encountering gas during the construction of the tunnel or operation of an underground mine. Such tunnels shall be constructed or underground mines operated under regulations, rules, and orders developed by the division and board and approved by the board. This subdivision shall not prohibit the division chief or his representatives from establishing any special orders that they feel are necessary for safety.

(b) Potentially gassy, which classification shall be applied to tunnels or underground mines where there exists a possibility gas will be encountered.

(c) Gassy, which classification shall be applied to tunnels or underground mines where it is likely gas will be encountered. Special safety measures, including those set forth in Sections 7965 to 7976, inclusive, those established by the division and board and adopted by the board, or special orders written by the chief or his representatives shall be observed in construction of gassy tunnels in addition to regular rules, orders, special orders, or regulations.

(d) Extrahazardous, which classification may, when the division finds that there is a serious danger to the safety of the employees, be applied to tunnels or underground mines where gas or vapors have caused an explosion or fire, where the likelihood of encountering petroleum vapors exists, or where tests show, with normal ventilation, a concentration of hydrocarbon petroleum vapors in excess of 20 percent of the lower explosive limit within three inches of the roof, face, floor, or walls of any open workings. Construction in extrahazardous tunnels or operation in extrahazardous underground mines shall conform to safety measures set forth in Sections 7977 to 7985, inclusive, any rules, regulations, orders, or special orders of the division, or any special rules, orders, or regulations adopted by the board.

The division shall not be required to reclassify any tunnel or underground mine that is shut down seasonally, when such tunnel or underground mine is put back into operation in not less than six months after date of the shutdown.