MSHA - Code of Federal Regulations - 30 CFR 57.22220
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MSHA - Title 30 CFR
30 CFR § 57.22220
Air passing unsealed areas (I-A, II-A, III, and V-A mines).

Air that has passed by or through unsealed abandoned or unsealed inactive areas and contains 0.25 percent or more methane shall--

(a) Be coursed directly to a return airway;

(b) Be tested daily for methane by a competent person; and

(c) Not be used to ventilate work places.