Coal Mine Rescue Field Test 5 Taken from the 2018 Kentucky River Mine Rescue Contest held in Hazard, Kentucky Progress Indicator: Question 4 of 10 4.  When quartz is present in a respirable coal mine dust sample in an amount greater than __________, the respirable dust standard may be lowered by MSHA. Choices: 10% 1 mg/m3 or 1000 µg/m3 0.1 mg/m3 or 100 µg/m3 5%