Engineering Controls Database
Best Practices for Dust Control in Metal/Nonmetal Mining – Mineral Processing Operations – Background Issues – Open-Structure Design
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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. |
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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; and; throughout the mineral processing cycle, mined ore goes through a number of crushing, grinding, cleaning, drying, and product-sizing sequences as it is processed into a marketable commodity. Because these operations are highly mechanized, they are able to process high tonnages of ore, potentially liberating large amounts of silica.. Many different types of structures and materials have been used to build mineral processing facilities through the years. Although structure type and building material were not viewed as significant factors affecting the health of employees in these facilities when they were built, a recent study was performed that compared dust levels with three different building types: masonry, an open-structure design, and a steel-sided design [Cecala et al. 2007]. Respirable dust measurements were taken within these structures to evaluate and compare levels. When considering the data, the most effective structural design of these three building types from a dust standpoint was an open-structure design. Respirable dust concentrations were significantly lower in the open structure because the natural environment acts as the best source of ventilation to dilute and carry away dust generated and liberated within the structure. If any visible dust plumes are present, source dust controls must be implemented to address the problem. |
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Figure 1 shows a conceptual drawing of a typical walled processing facility, then an identically sized facility with an open-structure design and a roof. Obviously, a roof would provide more protection from the natural elements than a totally open design. When building new facilities, the open-structure design is more cost-effective because there are lower material and construction costs involved. Some companies may also want to consider modifying their existing structures with a more open design to further reduce dust levels. If an open-structure design is considered for an operation, a number of issues need to be addressed: • Safety railings, guards, and/or chain-link wall containments must be installed to minimize the potential for any personnel or objects falling from the structure. • Equipment and personnel must be protected from environmental elements such as rain, snow, sleet, and hail. One possibility to minimize this concern would be to design a structure with a sufficient overhang. • An open-structure design must be considered a secondary design. The first approach to lower dust exposures in any structure is to have an effective primary dust control plan that captures major dust sources at their point of origin, before they are allowed to liberate into the plant and contaminate workers.  NOTE: The above information is taken directly from the following publication: NIOSH [2010]. 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. |
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Cecala AB, Rider JP, Zimmer JA, Timko RJ [2006]. Lower respirable dust and noise exposure with an open structure design. 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. 2007-101. Cecala AB, Rider JP, Zimmer JA, Timko RJ [2007]. Dial down dust and noise exposure. Aggregates Manag 12(7):50–53. IARC [1997]. 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 [2002]. 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. |
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dust control metal/nonmetal mining mineral mining mineral processing miners |
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| In a NIOSH study, when production levels were normalized, respirable dust levels at the open structure were more than four times lower than at the masonry structure and over 1,000 times lower than at the steel-sided structure [Cecala et al. 2006]. |