Atlanta, GA: U.S. Department of Health and Human Services, Public Health Service, Centers for Disease Control and Prevention, National Institute for Occupational Safety and Health, R01-OH-009271, 2009 Dec; :1-19
Background and Objectives - Concrete surface grinding exposes workers to unacceptable levels of airborne crystalline silica dust, known to cause diseases such as silicosis and lung cancer. The effectiveness of existing dust control methods and major confounding factors influencing the exposure levels of silica dust were examined by simulating field concrete surface grinding in an enclosed workplace laboratory. Approach - Personal, area, and background air monitoring was performed during 201 concrete grinding sessions ranging in duration from 5-90 minutes with approximately 80% grinding and 20% rest time per session. A variety of grinders, diamond grinding cups and accessories were used on concrete slabs, containing 24 +/- 9% crystalline silica, oriented in either horizontal or inclined positions. Dust control methods included general ventilation (GV), three types of local exhaust ventilation (LEV) and wet-grinding compared to conventional uncontrolled concrete grinding. Key Findings - Task-specific mean concentrations of silica dust (mg/m3 ) obtained by personal monitoring during LEV-, wet- and uncontrolled grinding, while GV was on (off) were 0.135 (0.571), 0.874 (3.48), and 8.89 (33.1), respectively. Levels of silica dust for 4-5 inch and 7 inch cup grinding were 1.14 (5.16) and 3.58 (13.1), respectively, with no significant difference among levels when grinding with 4, 4.5, and 5 inch cups. Silica dust levels were significantly lower for (1) GV "on" (75.4%) vs. "off' and (2) LEV-grinding (98.4%) or wet-grinding (89.4%) vs. uncontrolled grinding. Task-specific mean concentrations of RSP (mg/m3) obtained by personal monitoring during LEV-, wet- and uncontrolled-grinding while GV was on (off) were 1.32 (6.15), 6.46 (24.1), and 59.4 (207), respectively. RSP levels for 4-5 inch and 7 inch cup grinding were 8.02 (32.4) and 24.3 (88.1), respectively, with no significant difference among levels when grinding with 4, 4.5, and 5 inch cups. RSP levels were significantly lower for (1) GV "on" (74.0%) vs. "off' and (2) LEV grinding (97.3%) or wet-grinding (88.3%) vs. uncontrolled grinding. Levels of silica dust and RSP were significantly lower for (1) smaller grinding cup sizes vs. larger, and (2) samples collected on the left shoulder vs. the right shoulder for the right-handed operator. Factors that did not result in significantly different levels included: (1) orientation of grinding surfaces, (2) water flow rates for wet-grinding, or (3) task-specific sampling periods. None of the control methods lowered the 8-hour TWA exposure levels of silica dust to below the ACGIH recommended criteria of 0.025 mg/m3, requiring further refinement in engineering control and the use of administrative control or respirator. Recommendations - Relative to the confounding factors and the dust control methods that represented the parameters of this study, the most effective conditions for controlling exposure to silica dust and RSP during concrete surface grinding would be: (1) the use of a smaller size grinding cup (4 to 5 inch diameter), (2) in a smaller size (4.5 to 6 inch diameter) grinder that is either manufactured as a shrouded concrete grinder or is an angle grinder retrofitted with the durable urethane dust shroud, (3) attached to the HEPA tank vacuum or HEPA Cyclone vacuum, (4) in a well-ventilated workspace. With a work-rest regime of 25-75%, the operator may still need a half-mask respirator to reduce silica dust and RSP exposures to a level below recommended criteria. The establishment of uniform guidelines for the manual concrete grinding in industry is urgently needed.