Evaluation and control of respirable silica exposure during lateral drilling of concrete.

Silica exists in both crystalline and noncrystalline forms, with crystalline silica being the hazard of concern. Silica, or silicon dioxide, exposure is associated with many occupations, including construction workers, laborers, miners, foundry workers, glassmakers, drywall finishers, bricklayers, and tuckpointers.(1) Crystalline silica occurs most commonly in the form of quartz, cristobalite, and tridymite, and occupational exposure limits (OELs) have been established for these three structures.(2,3) Quartz is the most common, both in the environment and industry, and is found in many natural materials, including granite, slate, limestone, sandstone, and manufactured materials such as concrete and masonry units (e.g., bricks, blocks, paving stones). Prolonged or repeated exposure to respirable (particles less than 10 μm in aerodynamic diameter) silica can result in silicosis or other silicarelated diseases, including pulmonary tuberculosis, lung cancer, silicoproteinosis,(4) rheumatoid arthritis, sarcoidosis, and scleroderma. Silicosis is a fibrotic lung disease resulting from the deposition of crystalline silica in the alveolar region of the lungs. Symptoms associated with silicosis include shortness of breath, fatigue, chest pains, and susceptibility to infection. While preventable, silicosis can be a fatal lung disease. The Occupational Safety and Health Administration (OSHA), National Institute for Occupational Safety and Health (NIOSH) and ACGIH R © have published OELs (Table I) based on the respirable fraction of the silica aerosol, e.g., particles less than 10 μm in aerodynamic diameter.(2,5–7) Several construction trades are exposed to silica dust through the cutting, grinding, chipping, and drilling of concrete. A number of studies have measured silica dust generated by construction tasks involving mechanical disturbance of concrete, and some have also evaluated various types of dust reduction methods.(8–12) However, few studies have evaluated exposures due to drilling. Studies sampling respirable silica exposure during vertical drilling into concrete with pneumatic rock drills found geometric mean concentration of 0.20 mg/m3 (GSD = 5.2),(13,14) suggesting that drilling may lead to exposures above OELs when controls are not used. There were approximately 7.2 million workers employed in construction in 2008. The two segments of the industry most likely to utilize rock drills of the type used in this case study – (1) highway, street, and bridge construction; and (2) foundation, structure, and building exterior construction – employ 328,900 and 987,800 workers, respectively.(15) Exposure to dust can be reduced through one or more of the following methods: reduced dust generation or dispersion, dilution, isolation from the worker, and control or capture. Of these, the preferred method is reduction of dust generation followed by dust control or capture. The primary means of dust exposure reduction methods used in construction are source dust collection and/or wet dust suppression. Source