MSHA - Code of Federal Regulations - 30 CFR 57.22211
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MSHA - Title 30 CFR
30 CFR § 57.22211
Air flow (I-A mines).

The average air velocity in the last open crosscut in pairs or sets of developing entries, or through other ventilation openings nearest the face, shall be at least 40 feet per minute. The velocity of air ventilating each face at a work place shall be at least 20 feet per minute.