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 of each mine is exposed, as measured with an approved sampling device and expressed in terms of an equivalent concentration, at or below:

(a) 2.0 milligrams of respirable dust per cubic meter of air (mg/m3).

(b) 1.5 mg/m3 as of August 1, 2016.