Published December 7, 2023 at 400 × 287 in New Rules: Vote on Silica ETS This Month Of the 152 employees showing overexposure to silica, 25% had exposures “well above the PEL” of 50 micrograms per cubic meter of air. Of the 152 employees showing overexposure to silica, 25% had exposures “well above the PEL” of 50 micrograms per cubic meter of air.