30 CFR § 70.101
Respirable dust standard when quartz is present.
When the respirable dust in the mine atmosphere of the active workings
contains more than 5 percent quartz, 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 at or below
a concentration of respirable dust, expressed in milligrams per cubic meter
of air as measured with an approved sampling device and in terms of an
equivalent concentration determined in accordance with §70.206
(Approved sampling devices; equivalent concentrations), computed
by dividing the percent of quartz into the number 10.
The respirable dust associated with a mechanized mining unit
or a designated area in a mine contains quartz in the amount of 20%.
Therefore, the average concentration of respirable dust in the mine
atmosphere associated with that mechanized mining unit or designated area
shall be continuously maintained at or below 0.5 milligrams of respirable
dust per cubic meter of air (10/20=0.5 mg/m(super)3).
For further information: See MSHA's Program Policy Manual