(a) Each operator shall continuously maintain the average concentration of respirable quartz dust in the mine atmosphere during each shift to which each miner in the active workings of each mine is exposed at or below 0.1 mg/m3 (100 micrograms per cubic meter or µg/m3) as measured with an approved sampling device and expressed in terms of an equivalent concentration.
(b) When the equivalent concentration of respirable quartz dust exceeds 100 µg/m3, the 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 as measured with an approved sampling device and expressed in terms of an equivalent concentration at or below the applicable dust standard. The applicable dust standard is computed by dividing the percent of quartz into the number 10. The application of this formula shall not result in an applicable dust standard that exceeds the standard established by § 70.100(a).
Assume the sampled MMU or DA is on a 1.5-mg/m3 dust standard. Suppose a valid representative dust sample with an equivalent concentration of 1.12 mg/m3 contains 12.3% of quartz dust, which corresponds to a quartz concentration of 138 µg/m3. Therefore, the average concentration of respirable dust in the mine atmosphere associated with that MMU or DA shall be maintained on each shift at or below 0.8 mg/m3 (10/12.3% = 0.8 mg/m3).