MSHA - Code of Federal Regulations - 30 CFR 75.400-1
30 CFR § 75.400-1
(a) The term coal dust means particles of coal that can pass
a No. 20 sieve.
(b) The term float coal dust means the coal dust consisting
of particles of coal that can pass a No. 200 sieve.
(c) The term loose coal means coal fragments larger in size
than coal dust.