Occupational exposures to fibers and quartz at 19 crushed stone mining and milling operations

From 1979 to 1982, the National Institute for Occupational Safety and Health (NIOSH) conducted a cross-sectional exposure assessment and mortality study of selected crushed stone facilities in the United States. This study was undertaken in part to address concerns that asbestos exposures could be occurring in some crushed stone operations due to the presence of amphibole and serpentine minerals. The investigation was also designed to characterize exposures to crystalline silica and other mineral compounds. Nineteen crushed stone operations, mining limestone, granite, or traprock were surveyed to assess exposures to respirable and total dusts, mineral compounds including crystalline silica, asbestos, and mineral fibers. At the initiation of the study, crushed stone operations were selected from a Mine Safety and Health Administration (MSHA) listing of the active industry in 1978. With the exception of requiring inclusion of the traprock operation in Maryland where asbestos fibers were initially discovered, a stratified sample of operations was randomly selected by rock type (granite, limestone, traprock, or sandstone). However, because of reluctance or refusal of some companies to participate and because of the closures of some of the selected operations, replacements were randomly selected. Some replacement selections were likewise replaced due to lack of cooperation from the companies. The studied sample included only 10 of the 27 randomly selected operations in the original sample. Asbestos fibers were detected at one traprock facility, the Maryland operation where asbestos was originally found. Measured personal exposures to fibers exceeded the NIOSH Recommended Exposure Limit (REL) for two out of 10 samples. All of the samples were below the MSHA Permissible Exposure Limit (PEL), which was in effect at the time of the survey. However, due to the presence of nonasbestos mineral fibers in the environment, it could not be stated with certainty that all of the fibers counted by phase contrast microscopy were asbestos. A variety of silicate mineral fibers (other than those classified by NIOSH as asbestos) were detected in the traprock operations and at one granite operation. Crystalline silica was detected at 17 of the 19 surveyed crushed stone operations. Overexposures to crystalline silica were measured at 16 of the crushed stone operations; approximately one in seven personal-respirable dust samples (14%) exceeded the MSHA PEL for crystalline silica. Approximately 25% of the respirable dust samples exceeded the NIOSH REL for crystalline silica. Mill operators and mill laborers consistently had the highest and most frequent overexposures to crystalline silica.