in the 1990's, msha warned the industry about silica exposure, but no rules were adopted.the industry'sning biggest lobbying group -- successfully blocked broader regulatory measures. in 2014, the obama administration passed a rule limiting overall coal dust, but stopped short of regulating silica. >> regulations on coal mining have always had pushback from the industry. the industry's interested in running as much coal as possible, and regulations slow that down. william: rebecca shelton is the director of policy at the appalachian citizens law center. she says, when silica levels are high, the best way to protect workers is to increase the spraying of water to tamp down the dust, and boosting ventilation in the mines. but the industry has argued that giving miners additional respirator masks would be enough. in a statement to the newshour, the national mining association said the “new proposed rule specifically indicates that the use of respiratory protection equipment cannot be used as a method of compliance. we believe this is a mistake.” >> we have represented many mi