(a) Each operator shall continuously maintain the average concentration of respirable quartz dust in the mine atmosphere during each shift to which a part 90 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 mine atmosphere of the active workings where the part 90 miner performs his or her normal work duties exceeds 100 µg/m3 of respirable quartz dust, the operator shall continuously maintain the average concentration of respirable dust in the mine atmosphere during each shift to which a part 90 miner is exposed as measured with an approved sampling device and expressed in terms of an equivalent concentration at or below the applicable standard. The applicable standard is computed by dividing the percent of quartz into the number 10. The application of this formula shall not result in an applicable standard that exceeds the standards specified in §90.100.

Example: Assume the part 90 miner is on a 0.5 mg/m3 dust standard. Suppose a valid representative dust sample with an equivalent concentration of 0.50 mg/m3 contains 25.6% of quartz dust, which corresponds to a quartz concentration of 128 µg/m3. Therefore, the average concentration of respirable dust in the mine atmosphere associated with that part 90 miner shall be maintained on each shift at or below 0.4 mg/m3 (10/25.6% = 0.4 mg/m3).