Crystalline silica, in the form of sand, can cause silicosis (an incurable but preventable lung disease) when inhaled by workers. Sand is a main ingredient used in the fracking process. The National Institute for Occupational Safety (NIOSH) collected air samples from 11 fracking sites around the country. All 11 sites exceeded relevant occupational health criteria for exposure to respirable crystalline silica. In 31% of the samples, silica concentrations exceeded the NIOSH exposure limit by a factor of 10, which means that even if workers were wearing proper respiratory equipment, they would not be adequately protected.