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Engineering Controls Database

Best Practices for Dust Control in Metal/Nonmetal Mining – Mineral Processing Operations – Background Issues – Overhead Air Supply System

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; 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.
A successful control technique to reduce respirable dust exposures at mineral processing operations when workers are at stationary positions is with Overhead Air Supply Island System (OASIS). The OASIS air cleaning device is suspended over a worker and provides a flow of filtered air over the work station. Mill air is drawn into the system and passed through a primary cartridge filter. This primary filter is self cleaning, automatically using the reverse pulse technique when excessive filter pressure is sensed. The air can then pass through a heating or cooling chamber, which is optional depending on the mill air temperature, and from there into a distribution manifold, which also serves as a secondary filter (Figure 1). The resulting filtered air flows down over the worker at an average velocity of approximately 400 feet per minute (fpm), which restricts mill air from entering the clean air zone.
Figure - 1 - Overhead air supply island system.

Figure - 1 - Overhead air supply island system.

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.
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.

Volkwein JC, Engle MR, Raether TD [1988]. Dust control with clean air from an overhead air supply island (OASIS). Appl Ind Hyg J 3(8):236–239.
dust control
metal/nonmetal mining
mineral mining
mineral processing
During development, OASIS systems were installed over bag operators at two different mineral processing operations. Testing at these two operations showed 98% and 82% reductions in the bag operators’ respirable dust exposures. An additional benefit with this system was a 12% reduction in general dust levels throughout the mill building as a result of the OASIS cleaning the mill air [Volkwein et al. 1988].