MSHA - Code of Federal Regulations - 30 CFR 77.201-2
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MSHA - Title 30 CFR
30 CFR § 77.201-2
Methane accumulations; change in ventilation.

If, at any time, the air in any structure, enclosure or other facility contains 1.0 volume per centum or more of methane changes or adjustments in the ventilation of such installation shall be made at once so that the air shall contain less than 1.0 volume per centum of methane.