30 CFR § 75.402
All underground areas of a coal mine, except those areas in which the dust is
too wet or too high in incombustible content to propagate an explosion, shall
be rock dusted to within 40 feet of all working faces, unless such areas are
inaccessible or unsafe to enter or unless the Secretary or his authorized
representative permits an exception upon his finding that such exception will
not pose a hazard to the miners. All crosscuts that are less than 40 feet
from a working face shall also be rock dusted.