If flammable gas in excess of 1.0% by volume is detected in the air not less than 12 inches from the back, face, and rib of an underground working place, or in air returning from a working place or places, adjustments shall be made in the ventilation immediately so that the concentration of flammable gas in such air is reduced to 1.0% or less.
Ariz. Admin. Code § R11-1-1827