Cincinnati, OH: U.S. Department of Health and Human Services, Public Health Service, Centers for Disease Control and Prevention, National Institute for Occupational Safety and Health, EPHB-347-15a, 2011 Oct; :1-37
Background: Workplace exposure to respirable crystalline silica can cause silicosis, a progressive lung disease marked by scarring and thickening of the lung tissue. Quartz is the most common form of crystalline silica. Crystalline silica is found in several construction materials, such as brick, block, mortar and concrete. Construction tasks that cut, break, grind, abrade, or drill those materials have been associated with overexposure to dust containing respirable crystalline silica. Highway construction tasks that can result in respirable crystalline silica exposures include breaking pavement with jackhammers, concrete sawing, milling pavement, clean-up using compressed air, and dowel drilling. Dowel drilling machines are used to drill horizontal holes in concrete pavement so that dowels can be inserted to transfer loads across pavement joints. NIOSH scientists are conducting a study to assess the effectiveness of dust control systems sold by dowel drill manufactures by measuring exposures to workers operating dowel drills with and without dust controls installed. This site visit was part of that study. Assessment: NIOSH staff visited the Fred Weber Company site at the Springfield-Branson National Airport on August 1-4, 2011, and performed industrial hygiene sampling on August 2 and 4, 2011. The sampling measured exposures to respirable dust among two workers that operated five-gang dowel drills to drill holes in a new concrete runway. One worker operated a rented drill, while the other ran a drill owned by the paving contractor. The NIOSH scientists who visited the site also monitored the wind speed and direction, and collected data about the dust controls and the work process in order to understand the conditions that led to the measured exposures. Results: Air sampling for respirable dust found concentrations that ranged from 1.1 mg/m3 to 3.3 mg/m3, 8-hr TWA. For the actual sampling times, TWA respirable dust exposures ranged from 1.7 mg/m3 to 6.0 mg/m3. Those actual TWA respirable dust data were assumed to follow a log-normal distribution, with a geometric mean of 3.0 mg/m3, and a geometric standard deviation of 1.9. Unfortunately, a laboratory error occurred during the analysis of the air samples for respirable crystalline silica. Due to this error, it was not possible to compare the air sampling results to the NIOSH Recommended Exposure Limit for crystalline silica or the OSHA Permissible Exposure Limit for respirable dust that contains greater than 1% quartz (because that limit varies with the quartz content measured in the airborne dust samples). The quartz content in bulk concrete dust samples collected on August 2 and 4, 2011, ranged from 2.2 to 12 percent by weight, with an arithmetic mean quartz content of 6.4 percent. Video exposure monitoring revealed that the practices of reversing air flow through the dust collection system and kneeling near the drills to mark the pavement may have contributed to the measured respirable dust exposures. The air flows measured at the drills' dust collectors were 1.5 m3/min (53 cfm) for the rental drill and 1.6 m3/min (56 cfm) for the company-owned drill. Those flow rates would have resulted in duct velocities of 12 and 13 meters/second (m/sec)(2400 and 2600 feet per minute (fpm)), respectively, excluding the friction losses due to the corrugated duct (the measurements were made with the duct disconnected from the dust collector). Conclusions and Recommendations: The ACGIH® industrial ventilation manual recommends a transport velocity of 3500 to 4000 fpm for "average industrial dust" (e.g., granite or limestone dust, brick cuttings, silica flour). The observed slower flow rate in the dust control systems may explain the tendency for dust to settle in the corrugated hose and the need to periodically purge the dust collection system with the reverse-pulse to maintain performance. The purging process resulted from a pulse of reverse high-pressure air flow that blasted clogged concrete dust back out through the hood inlets as well as through any other gaps in the system. The dust clouds that result from the periodic purging of the system seem to defeat the purpose of an industrial ventilation system - to reduce exposures by capturing the contaminant. In other words, it does little good to capture the concrete dust during drilling only to re-aerosolize a portion of it during the purging process. According to the drill manufacturer, the reverse pulse system was not used as intended. The reverse pulse is only designed to remove the dust cake from the filter in the dust collector. Newer models of the same drill are programmed with a 1-second automatic pulse for this purpose. Options that may help to improve the performance of the dust collection system include increasing the air flow through the system to achieve the recommended transport velocity, using smooth-bore flexible duct, minimizing the use of flexible duct to the extent possible (using rigid duct for long horizontal runs, for example), and emptying the dust collection receptacles more frequently. The length of duct and number of elbows, bends, and sags should be kept to a minimum. In addition, the drill operator should be trained to mark the pavement when the drills are not running or be provided with a long-handled marking device that eliminates the need to bend or kneel to mark the pavement.
Region-7; Control-technology; Engineering-controls; Construction; Construction-equipment; Construction-industry; Construction-materials; Construction-workers; Machine-operation; Machine-operators; Equipment-design; Equipment-operators; Respirable-dust; Dusts; Dust-collection; Dust-collectors; Dust-control; Dust-control-equipment; Control-equipment; Control-systems; Concretes; Road-construction; Road-surfacing; Hydraulic-equipment; Measurement-equipment; Silica-dusts; Silicosis; Quartz-dust; Respiratory-system-disorders; Lung-disease; Pulmonary-system-disorders; Lung-disorders; Exposure-assessment; Cutting-tools; Grinding-equipment; Air-quality-measurement; Air-sampling; Air-sampling-equipment; Sampling; Exposure-levels; Exposure-limits; Permissible-limits; Respirators; Hazardous-materials; Work-practices; Respiratory-protective-equipment; Emission-sources; Exhaust-ventilation; Ventilation-equipment; Airports; Jack-hammers;
Author Keywords: Highway, Street, and Bridge Construction; silica; crystalline silica; dowel drill; concrete; respirable dust; rock drill
National Institute for Occupational Safety and Health, Division of Applied Research and Technology, Engineering and Physical Hazards Branch, Mail Stop R-5, 4676 Columbia Parkway, Cincinnati, OH 45226-1998