30 CFR § 57.22237
Actions at 2.0 to 2.5 percent methane in bleeder systems (I-A and III mines).
If methane reaches 2.0 percent in bleeder systems at the point where a
bleeder split enters a main return split, mining shall not be permitted on
ventilation splits affected by the bleeder system. If methane has not been
reduced to less than 2.0 percent within 30 minutes, or if methane levels
reach 2.5 percent, all persons other than competent persons necessary to
take corrective action shall be withdrawn from affected areas.