MSHA - Code of Federal Regulations - 30 CFR 57.22217
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MSHA - Title 30 CFR
30 CFR § 57.22217
Seals and stoppings (I-A, I-B, and I-C mines).

All seals, and those stoppings that separate main intake from main return airways, shall be of substantial construction and constructed of noncombustible materials, except that stoppings constructed of brattice materials may be used in face areas.