NIOSH logo and tagline

Engineering Controls Database

Best Practices for Dust Control in Metal/Nonmetal Mining – Mineral Processing Operations – Background Issues – Secondary Dust Sources

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 dust.

When a worker is overexposed to respirable dust, most often the assumption is that the dust exposure came from the worker’s primary job function. A study was performed that documented a number of occasions where this was not the case [Cecala and Thimons 1987]. When a worker obtains a high respirable dust measurement, the correct course of action is to evaluate the worker’s job function to determine the dust sources that contributed to this exposure and the magnitude of the exposure from each of these sources. Sometimes a secondary or background dust source can be the major contributor to the worker’s overall exposure. Controlling these less obvious dust sources can have a major impact on bringing levels back to acceptable concentrations.
The following are examples that demonstrate the impact that secondary dust sources on a worker’s respirable dust exposure:

• Outside dust sources traveling inside structures. When outside dust sources travel inside structures, every worker inside the structure is impacted. Most bagging operations at mineral processing plants use an exhaust ventilation system to draw the dust generated from the bagging process down into the fill hopper. It is important that the air being drawn into this exhaust ventilation system, commonly called make-up air, be clean air. At one operation, the make-up air was drawn directly from the bulk loading area outside the mill. The dust generated from this bulk loading process traveled through an open door into the mill, substantially contaminating the workers inside the mill. During periods when bulk loading was not performed, the bag operator’s dust exposure was 0.17 mg/m3. As trucks were loaded at the bulk loading area, the bag operator’s exposure increased to 0.42 mg/m3 due to this contaminated air [USBM 1986]. If outside air is used as make-up air, it must be from a location where the air is not contaminated.

• Performance of job function. During an evaluation of a dust control system at one processing plant, substantial variations existed in the dust exposures of two different workers due to differences in their work practices. A number of factors were identified that impacted these differences. One factor was the amount of time the bag operator allowed the bag to remain on the fill spout before removing it. If the bag remained on the fill spout for a few seconds after it was filled, there was less dust generated from the rooster tail of product that spewed from the bag valve and fill nozzle as the bag was removed. A second factor was the extent to which the bag valve was sealed by the bag operator. One operator did not pay attention to where he grasped the bag as he lifted it from the fill spout to transfer it onto a conveyor belt. A second operator grasped the bag at the fill spout and crimped it closed as he placed the bag on the conveyor. This substantially lowered the amount of product that spewed from the bag as it was placed on the conveyor. A third factor impacting the operator’s dust exposure was the general manner in which the operator removed the bag from the bag spout and placed it on the conveyor. More dust was generated when this was done in a forceful manner, as compared to a more continuous and gentle fashion [Cecala and Thimons 1993]. A number of modifications were tested to lower these workers’ respirable dust exposures. However, regardless of the effectiveness of the dust control system, the worker who performed his work duties in a rough and careless manner had approximately a 70% higher respirable dust exposure when compared to his coworker who performed the tasks in a conscientious and gentler manner.

• Broken bags of product. In most cases, bag breakage occurs because of flaws in the bags delivered from the bag manufacturer. At one particular operation, a bag operator’s dust exposure went from 0.07 mg/m3 before the bag break to 0.48 mg/m3 afterwards. Although the bag broke during the conveying process and not directly in front of the worker, the dust substantially contaminated the surrounding mill air, which flowed over the bag operator. Once again, this occurred because the exhaust ventilation system in the bag loading area created a negative pressure that draws background air from the mill.

For mineral processing operations to keep workers at acceptable dust levels, management must be aware of the various dust contamination sources and methods to reduce these sources. The substantial effects of the various secondary dust sources should be recognized, identified, and controlled in an effort to minimize workers’ dust exposures.

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.
Cecala AB, Thimons ED [1987]. Significant dust exposures from background sources. Pit Quarry 79(12):46–51.

Cecala AB, Thimons ED [1993]. Tips for reducing dust from secondary sources during bagging. Powder Bulk Eng 7(5):77–84.

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.

USBM [1986]. Impact of background sources on dust exposure of bag machine operator. Cecala AB, Thimons ED. Washington, DC: U.S. Department of the Interior, U.S. Bureau of Mines, IC 9089.
dust control
metal/nonmetal mining
mineral mining
mineral processing