The U.S. Department of Labor's Occupational Safety and Health Administration (OSHA) has issued a hazard alert on protecting workers from silica exposure during hydraulic fracturing operations. From the related press release: "The alert states that employers must ensure that workers are properly protected from overexposure to silica. The alert describes how a combination of engineering controls, work practices, protective equipment and product substitution, where feasible, along with worker training, can protect workers who are exposed to silica. *** According to the alert, transporting, moving and refilling silica sand into and through sand movers, and along transfer belts and into blender hoppers, can release dust into the air containing up to 99 percent silica that workers breathe."
You can find a copy of alert here.