Cal. Code Regs. tit. 8, § 7099 – Mechanical Ventilation.

(a) When the Division considers it necessary for the protection of employees, mechanically produced and positively controlled air currents shall be provided.
(b) For maximum accessibility and to isolate them from underground hazards, the main fan or fans shall be installed on the surface, unless permission in writing is secured from the Division for their underground installation.
Fans shall be so arranged that the mine entrances can be used for rescue or other purposes.
(c) For the protection of employees, the main fan or fans shall be installed so that the ventilating current can be quickly reversed in direction.
(d) Auxiliary fans underground shall be located so as to provide the best air available to the working place. Recirculation of air by auxiliary fans shall be avoided as far as practical.
(e) (5-22) All surface fans, casings, and air ducts shall be constructed wholly of noncombustible materials.
(f) Every fan house and every building within 50 feet of a fan house shall be constructed wholly of noncombustible material, or shall be made fire resistant by the use of gunite, cement plaster, metal sheathing, or by other equally effective means.
(g) Compressed air that does not contain smoke, harmful gases, or excessive oil may be used for ventilating purposes.
(h) Where underground fans are operated by electricity, all combustible material in the immediate vicinity shall be removed or made fire resistant.