Engineering Controls Database
Best Practices for Dust Control in Metal/Nonmetal Mining –
Controlling Respirable Silica at Surface Mines – Enclosed Cab Filtration Systems
Respirable crystalline silica dust exposure has long been known to be a serious health threat to workers in many industries and occupations. Workers with high exposure to crystalline silica include miners, sandblasters, tunnel workers, silica millers, quarry workers, foundry workers, and ceramics and glass workers Overexposure to respirable crystalline silica dust can has been associated with development of silicosis, lung cancer, pulmonary tuberculosis, and airways disease.
The International Agency for Research on Cancer (IARC) reviewed the published experimental and epidemiologic studies of cancer in animals and workers exposed to respirable crystalline silica and concluded that there was sufficient evidence to classify silica as a human carcinogen [IARC 1997]. Silicosis is also a fibrosing disease of the lungs caused by the inhalation, retention, and pulmonary reaction to the crystalline silica. When silicosis becomes symptomatic, the primary symptom is usually dyspnea (difficult or labored breathing and/or shortness of breath), first noted with activity or exercise and later, as the functional reserve of the lung is also lost, at rest. Once contracted, there is no cure for silicosis. The goal, therefore, is to limit worker exposure to respirable dust to prevent development of these diseases.
Silica refers to the chemical compound silicon dioxide (SiO2), which occurs in a crystalline or noncrystalline (amorphous) form [NIOSH 2002]. Silica is a common component of rocks. Ore goes through a number of crushing, grinding, cleaning, drying, and product-sizing sequences as it is mined and processed into a marketable commodity. Because these operations are highly mechanized, they are able to process high tonnages of ore and generate high levels of silica.
Surface mining operations present dynamic and highly variable silica dust sources. Most of the dust generated at surface mines is produced by mobile earth-moving equipment such as drills, bulldozers, trucks, and front-end loaders excavating silica-bearing rock and minerals. Four practical areas of engineering controls designed to mitigate exposure of surface mine workers to all airborne dusts, including silica, are enclosed cab filtration systems, drill dust collection systems, controlling dust on unpaved haulage roads, and controlling dust at the primary hopper dump.
Enclosed cab filtration systems are one of the mainstay engineering controls for reducing mobile equipment operators’ exposure to airborne dust at surface mines. Enclosed cabs with heating, ventilation, and air conditioning (HVAC) systems are typically integrated into the drills and mobile equipment to protect the operator from the outside environment. Air filtration is often part of the HVAC system as an engineering control for airborne dusts. Surface mining dust surveys conducted by NIOSH on drills and bulldozers have shown that enclosed cabs can effectively control the operator’s dust exposure, but cab performance can vary [Organiscak and Page 1999]. The enclosed cab protection factors (outside ÷ inside dust concentration) measured on rotary drills ranged from 2.5 to 84, and those measured on bulldozers ranged from 0 to 45. NIOSH also conducted field studies of upgrading older equipment cabs to improve their dust control effectiveness. These studies involved retrofitting older enclosed cabs with air-conditioning, heating, and air filtration systems to demonstrate the effectiveness of upgrading older mine equipment cabs. During these retrofits, cab enclosure cracks, gaps, or openings were sealed with silicone and closed cell foam tape. Varying degrees of enclosure integrity were achieved. NIOSH conducted controlled laboratory experiments to examine the key design factors of enclosed cab dust filtration systems. The key performance factors for effective enclosed cab dust filtration systems are summarized below.
Key Performance Factors for Enclosed Cab Filtration Systems
Ensure good cab enclosure integrity to achieve positive pressurization against wind penetration into the enclosure. Significant improvements in cab protection factors were achieved in the field studies when cab pressures exceeded 0.01 inches of water gauge. This corresponded to wind velocity equivalents (an indicator of cab wind velocity resistance) greater than 4.5 miles per hour. The cab enclosures with greater than 0.01 inches of water gauge pressure were of close-fitted construction and their integrity could be readily improved by sealing cab enclosure cracks, gaps, or openings with silicone and closed cell foam tape. The loosely fitted cab construction on one of the drills and the truck were difficult to seal, which limited the amount of cab pressure that could be attained.
Use high-efficiency respirable dust filters on the intake air supply into the cab. Filter efficiency performance specifications used in the field were 95% or greater on respirable-sized dusts [Checkan and Colinet 2003; Cecala et al. 2004, 2005; Organiscak et al. 2004]. Laboratory experiments showed an order of magnitude increase in cab protection factors when using a 99%-efficient filter versus a 38%-efficient filter on respirable-sized particles [NIOSH 2007].
Use an efficient respirable dust recirculation filter. All the cab field demonstrations used recirculation filters that were 95% efficient or better in removing respirable-sized dusts [Checkan and Colinet 2003; Cecala et al. 2004, 2005; Organiscak et al. 2004]. Laboratory experiments showed an order of magnitude increase in cab protection factors when using an 85%- to 94.9%-efficient filter as compared to no recirculation filter [NIOSH 2007]. Laboratory testing also showed that when using a recirculation filter the time for interior cab concentration to decrease and reach stability after the cab door is closed was cut by more than half.
Minimize dust sources in the cab. Use good housekeeping practices and move heater outlets that blow across soiled cab floors. Dust levels were shown to increase from 0.03 to 0.26 mg/m3 by turning on a floor heater inside the cab [Cecala et al. 2005]. The floor heater was removed and cab heating was discharged down from the ceiling HVAC system, reducing dust entrainment in the cab during colder winter months [Cecala et al. 2005]. Another method of reducing entrainment of dust from a soiled cab floor is placing a gritless (without sand added) sweeping compound on the floor during the working shift. Most commercial sweeping compounds have petroleum-based oils or wax added to the cellulose material. However, people sensitized to petroleum distillates could have allergic reactions to these sweeping compounds if used in enclosed cabs. A few companies offer nonpetroleum-base sweeping compounds that use either a natural oil or chemical additive for dust adhesion [NIOSH 2001]. It is also recommended to cover the floor with rubber matting instead of carpeting for easy cleaning. More frequent cleaning of heavily soiled floors may be a more straightforward alternative than using sweeping compounds to minimize this type of dust entrainment.
Keep doors closed during equipment operation. On one drill operation, the respirable dust concentrations inside the cab averaged 0.09 mg/m3 with the door closed and averaged 0.81 mg/m3 when the door was briefly opened to add drill steels [Cecala et al. 2007]. Although this occurred after drilling stopped and the visible dust dissipated, opening the door, even briefly, produced a ninefold increase in respirable dust concentrations inside the cab during many drill steel changes made over a working shift.
NIOSH . Information circular 9517. Best practices for dust control in metal/nonmetal mining. Morgantown, WV: U.S. Department of Health and Human Services, Centers for Disease Control and Prevention, National Institute for Occupational Safety and Health, DHHS (NIOSH) Publication No. 2010-132.
Cecala AB, Organiscak JA, Heitbrink WA, Zimmer JA, Fisher T, Gresh RE, Ashley JD . Reducing enclosed cab drill operator’s respirable dust exposure at surface coal operation with a retrofitted filtration and pressurization system. In: SME Transactions 2003, Vol. 314. Littleton, CO: Society for Mining, Metallurgy and Exploration, Inc., pp. 31–36.
Cecala AB, Organiscak JA, Zimmer JA, Heitbrink WA, Moyer ES, Schmitz M, Ahrenholtz E, Coppock CC, Andrews EH . Reducing enclosed cab drill operator’s respirable dust exposure with effective filtration and pressurization techniques. J Occup Environ Hyg 2:54–63.
Cecala AB, Organiscak JA, Zimmer JA, Moredock D, Hillis M . Closing the door to dust when adding drill steels. Rock Prod October:29–32.
Chekan GJ, Colinet JF . Retrofit options for better dust control. Aggregates Manag 8(9):9– 12.
IARC . IARC monographs on the evaluation of carcinogenic risks to humans: silica, some silicates, coal dust and para-aramid fibrils. Vol 68. Lyon, France: World Health Organization, International Agency for Research on Cancer.
NIOSH . Technology News 487: Sweeping compound application reduces dust from soiled floors within enclosed operator cabs. U.S. Department of Health and Human Services, Centers for Disease Control and Prevention, National Institute for Occupational Safety and Health.
NIOSH . NIOSH hazard review: health effects of occupational exposure to respirable crystalline silica. Cincinnati, OH: U.S. Department of Health and Human Services, Centers for Disease Control and Prevention, National Institute for Occupational Safety and Health, DHHS (NIOSH) Publication No. 2002-129.
Organiscak JA, Page SJ . Field assessment of control techniques and long-term dust variability for surface coal mine rock drills and bulldozers. Int J Surf Min Reclam Environ 13(4):165–172.
Organiscak JA, Cecala AB, Thimons ED, Heitbrink WA, Schmitz M, Ahrenholtz E . NIOSH/industry collaborative efforts show improved mining equipment cab dust protection. In SME Transactions 2003, Vol. 314. Littleton, CO: Society for Mining, Metallurgy and Exploration, Inc., pp. 145–152.