Labor Code § 7974
CHAPTER 2. Gassy and Extrahazardous Tunnels
In any tunnel classified as gassy by the division, ventilation shall include continuous exhausting of fumes and air, unless an alternative ventilation plan which is as effective or better is approved by the division. Fans for this purpose shall be located at the surface, and shall be reversible from a single switch at the portal or shaft. These requirements shall not preclude the use of auxiliary fans to supply more air or greater exhaust to a tunnel or underground mine.