30 CFR § 57.22216
Separation of intake and return air (I-C mines).
The main intake and return air currents in single shafts shall be separated
by ventilation tubing, curtain walls, or partitions. Ventilation tubing shall
be constructed of noncombustible material. Curtain walls or partitions shall
be constructed of reinforced concrete or other noncombustible equivalent, and
provided with pressure-relief devices.