30 CFR § 71.100
Respirable dust standard.
Each operator shall continuously maintain the average concentration of
respirable dust in the mine atmosphere during each shift to which each miner
in the active workings is exposed at or below 2.0 milligrams of respirable
dust 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 §71.206
sampling devices; equivalent concentrations).
For further information: See MSHA's Program Policy Manual