After the 20th 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, as measured with an approved sampling device and expressed in terms of an equivalent concentration, at or below:

(a) 1.0 milligrams of respirable dust per cubic meter of air (mg/m 3).

(b) 0.5 mg/m 3 as of August 1, 2016.