A worker stands on top of a storage bin on July 27, 2011, at a drilling operation in Claysville, Pa. The dust is from powder mixed with water for hydraulic fracturing. Keith Srakocic/AP hide caption

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A worker makes a cut in the side of a sandstone block at the Cleveland Quarries facility in Vermilion, Ohio, earlier this month. The legal limit on the amount of silica that workers can inhale was set decades ago. Ty Wright/Bloomberg via Getty Images hide caption

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