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Current MSHA standards limit exposures to quartz (crystalline silica) in respirable dust. Overexposure to crystalline silica can result in some miners developing silicosis, an irreversible but preventable lung disease, which ultimately may be fatal. The metal and nonmetal mining industry standard is based on the 1973 American Conference of Governmental Industrial Hygienists (ACGIH) Threshold Limit Values formula: 10 mg/m3 divided by the percentage of quartz plus 2. The formula is designed to limit exposures to 0.1 mg/m3 (100 ug/m3) of silica. The National Institute for Occupational Safety and Health (NIOSH) recommends a 50 ug/m3 exposure limit for respirable crystalline silica. MSHA continues to review to determine an appropriate next step that would address miners' exposure to respirable crystalline silica.