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Engineering Controls Database

Best Practices for Dust Control in Metal/Nonmetal Mining – Mineral Processing Operations – Clothes Cleaning 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; 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. This in turn can generate large quantities of dust, often containing elevated levels of respirable crystalline silica, which can be liberated into the work environment.

One significant area of respirable dust exposure to workers at mineral processing operations is from contaminated work clothing. For the mineral processing industry, a U.S. Bureau of Mines report documented two cases where a tenfold increase in a worker’s respirable dust exposures came from dusty work clothes [USBM 1986]. This study indicated that respirable dust levels liberated from the soiled clothes were elevated to the extent that workers could be over their permissible exposure limit (PEL) in less than two hours. As the individuals performed their work duties, dust was continually emitted from their clothing, and the only way to eliminate this dust source was to clean or change their work clothing. Although disposable coveralls have been in use for many years, as well as some new and improved clothing material less susceptible to dust capture, the vast majority of workers continue to wear clothing similar to what they have worn for years. In addition, dirty work clothing, if it is not cleaned or changed at the end of the shift, can also contaminate personal vehicles and expose family members [Langenhove and Hertleer 2004; Hartsky et al. 2000; Salusbury 2004].
The Mine Safety and Health Administration (MSHA)-approved method of cleaning work clothing involves a filtered vacuuming system, which is both difficult and time-consuming for the worker. Because of this, workers sometimes use a single compressed-air hose to blow dust from their clothing, even though this is not an approved practice. This technique creates a significant dust cloud which increases the worker’s respirable dust exposure and contaminates the work area.

To address these problems, NIOSH and Unimin Corporation developed a clothes cleaning system that is able to quickly, effectively, and safely remove dust from a worker’s clothing without exposure to the worker, the work environment, or coworkers during the cleaning process [NIOSH 2005]. This system has been shown to be significantly more effective than the vacuuming or single air hose technique, while being performed in a fraction of the time.

The clothes cleaning system consists of four major components. Figure 1 shows the various components of the clothes cleaning system.
Figure - 1 -  Clothes cleaning system design.

Figure - 1 - Clothes cleaning system design.

• Cleaning booth. The cleaning booth used for testing has a base dimension of 48 inches by 42 inches and provides a safe and controlled area to perform the clothes cleaning process. All intake air enters the cleaning booth through a 24-inch cutout on the roof. The air flows directly down and over the worker in the booth before flowing through expanded metal grating on the floor and exiting through a return air plenum at the bottom and back of the booth. All dust and product cleaned from the worker’s clothing is contained within the booth and then exits via the exhaust ventilation system.

• Air spray manifold. The air spray manifold is composed of 26 spray nozzles, spaced 2 inches apart, to remove the dust and product from the worker’s clothing [Pollock et al. 2005]. These spray nozzles are regulated to limit the operating pressure to a maximum of 30 pounds per square inch (psi). The top 25 spray nozzles are flat-fan air nozzles and are used to clean the clothing. The bottom nozzle is a circular design and is used for cleaning the individual’s work boots.

• Air reservoir. The air reservoir supplies the required air volume necessary for the air nozzles used in the air spray manifold. A 240-gallon unit reservoir is recommended for the system because it allows for multiple cleanings to be performed one after another. The air reservoir is located next to the cleaning booth and hard-piped to the air spray manifold located inside the booth.

• Exhaust ventilation system. Exhaust ventilation is used to keep the cleaning booth under negative pressure throughout the entire clothes cleaning process. This exhaust ventilation system can be tied into a local exhaust ventilation system or directed outside the plant and exhausted from an elevated stack. Testing on this system has verified that an exhaust volume of 2,000 cubic feet per minute (cfm) is required to maintain a negative pressure throughout the entire clothes cleaning cycle [Pollock et al. 2006]. This exhaust ventilation system should only be operated when a worker enters the booth to perform a clothes cleaning cycle and can be de-activated once the process is completed and the worker exits the booth.

During the development of the clothes cleaning system, a matrix of tests was performed to evaluate the effectiveness of this technique in comparison to that of the high-efficiency particulate air (HEPA) filter vacuuming and the single compressed-air hose approach. For this testing, both 100% cotton and cotton/polyester blend coveralls were soiled with dust before a worker entered the cleaning booth. The new clothes cleaning technique was proven to be approximately 40% and 50% more effective than the vacuuming and single compressed-air hose technique, respectively [Cecala et al. 2007a]. The clothes cleaning system was also superior in its ability to uniformly remove dust from all areas of the worker’s clothing. Another major benefit was that the complete cleaning process was performed in a fraction of the time. The average cleaning times were 317 seconds for vacuuming, 178 seconds for the air hose, and 18 seconds for the clothes cleaning system. This test also indicated that polyester/cotton blend coveralls were cleaned more effectively than the 100% cotton coveralls. Figure 2 shows the effectiveness of the technique before and after a worker performed the clothes cleaning process while wearing polyester/cotton blend coveralls.
Figure - 2 - Test subject before and after using the clothes cleaning booth

Figure - 2 - Test subject before and after using the clothes cleaning booth

All workers performing the cleaning process are required to wear a half-mask, fit-tested respirator with N100 filters, hearing protection, and eye protection. To perform the clothes cleaning process, the worker enters the booth wearing his/her personal protective equipment (PPE), pushes the start button, slowly spins in front of the air spray manifold (18 seconds), and exits the booth with clean clothing. This clothes cleaning system provides a quick and effective method for workers to clean dusty clothes during the workday without risk to the worker, coworkers, or the work environment [Cecala et al. 2005b]. MSHA recognized the benefits of this system and has approved Petitions for Modification so that the system can be used in place of HEPA vacuuming.
NIOSH [2010]. Information circular 9517. Best practices for dust control in metal/nonmetal mining. Pittsburgh, PA: 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, Zimmer JA, Heitbrink WA, Moyer ES, Schmitz M, Ahrenholtz E, Coppock CC, Andrews EH [2005a]. Reducing enclosed cab drill operator’s respirable dust exposure with effective filtration and pressurization techniques. J Occup Environ Hyg 2:54–63.
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.

Hartsky MA, Reed KL, Warheit DB [2000]. Assessments of the barrier effectiveness of protective clothing fabrics to aerosols of chrysotile asbestos fibers. In: Nelson C, Henry N, eds. Performance of protective clothing: issues and priorities for the 21st century. Vol. 7. West Conshohocken, PA: American Society for Testing and Materials, pp. 141–154.

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.

Langenhove LV, Hertleer C [2004]. Smart clothing: a new life. Int J Cloth Sci Tech 16(1,2):63– 72.

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.

NIOSH [2005]. Technology news 509: a new method to clean dust from soiled work clothes. 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. 2005136.

Pollock DE, Cecala AB, O’Brien AD, Zimmer JA, Howell JL [2005]. Dust off. Rock Prod March.

Pollock DE, Cecala AB, Zimmer JA, O’Brien AD, Howell JL [2006]. A new method to clean dust from soiled work clothes. In: Proceedings of the 11th U.S./North American Mine Ventilation Symposium. University Park, PA: Pennsylvania State University, pp. 197–201.

Salusbury I [2004]. Tailor-made-self-cleaning clothing, chemical warfare suits that trap toxins. Materials World August:18–20.

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