Metal and nonmetal miners and coal miners exposed to respirable crystalline silica (RCS) in respirable dust can develop lung diseases such as chronic obstructive pulmonary disease, and various forms on pneumoconiosis, such as silicosis, coal workers’ pneumoconiosis, progressive massive fibrosis, and rapidly progressive pneumoconiosis. These lung diseases are irreversible and may ultimately be fatal. MSHA’s existing standards limit miners’ exposures to RCS in respirable dust. MSHA’s metal and nonmetal mine standards are based on the 1973 American Conference of Governmental Industrial Hygienists (ACGIH) Threshold Limit Values formula: 10 mg/m³ divided by the percentage of silica plus 2. MSHA’s existing coal mine standards evaluate miners’ exposure to quartz in relation to the respirable dust standard. When respirable coal mine dust samples are analyzed for quartz and exceed 0.1 mg/m³ (100 micrograms per cubic meter of air or µg/m³), the Agency reduces the applicable respirable dust standard for sections of the mine represented by the sample data.  MSHA will publish a proposed rule to address miners' exposure to RCS.