30 CFR § 90.100
Respirable dust standard.
After the twentieth calendar day following receipt of notification from MSHA
that a Part 90
miner is employed at the mine, the
operator shall continuously maintain the average concentration of respirable
dust in the mine atmosphere during each shift to which the Part
miner in the active workings of the mine is exposed
at or below 1.0 milligrams per cubic meter of air. Concentrations shall be
measured with an approved sampling device and expressed in terms of an
equivalent concentration determined in accordance with §90.206
(Approved sampling devices; equivalent concentrations).