MSHA - Code of Federal Regulations - 30 CFR 90.100
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MSHA - Title 30 CFR
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 90 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).