MSHA - Code of Federal Regulations - 30 CFR 57.4533
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MSHA - Title 30 CFR
30 CFR § 57.4533
Mine opening vicinity. Surface buildings or other similar structures within 100 feet of mine openings used for intake air or within 100 feet of mine openings that are designated escapeways in exhaust air shall be--

(a) Constructed of noncombustible materials; or

(b) Constructed to meet a fire resistance rating of no less than one hour; or

(c) Provided with an automatic fire suppression system; or

(d) Covered on all combustible interior and exterior structural surfaces with noncombustible material or limited combustible material, such as five-eighth inch, type "X" gypsum wallboard.

For further information: See MSHA's Program Policy Manual