30 CFR § 57.22103
Open flames (I-A, II-A, III, and V-A mines).
Open flames shall not be permitted underground except for welding, cutting,
and other maintenance operations, and for igniting underground retorts in a
Subcategory I-A mine. When using open flames in other than fresh air, or in
places where methane may enter the air current, tests for methane shall be
conducted by a competent person before work is started and every 10 minutes
until the job is completed. Continuous methane monitors with audible alarms
may be used after the initital test has been conducted as an alternative to
the ten-minute interval testing requirement. Open flames shall not be used in
atmospheres containing 0.5 percent or more methane.