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Protecting Stone Workers
Reducing Exposure to Respirable Crystalline Silica during Stone Countertop Fabrication
BY CHAOLONG QI, DREW THOMPSON, AND URSULA “ASHA” BROGAN
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Stone countertops are becoming increasingly popular among consumers, leading to an increased number of workers in the stone countertop industry. Unfortunately, stone countertop fabrication can create a lot of dust that is potentially hazardous to workers. Dust in this industry may contain high levels of respirable crystalline silica (RCS), and overexposure to RCS can cause several diseases including silicosis, lung cancer, chronic obstructive pulmonary disease, and kidney disease.
This occupational health hazard is especially pronounced for workers fabricating certain types of engineered stone that contain higher amounts of crystalline silica. In 2023, a study published in JAMA Internal Medicine reported 52 silicosis cases, including 10 fatalities, in the state of California alone, highlighting the critical need to reduce workers’ RCS exposure. Unfortunately, this hazard is not new; in 2015, OSHA and NIOSH published a hazard alert about countertop industry worker exposures to RCS, and NIOSH in a 2019 blog post further outlined concerns about an outbreak of silicosis among engineered stone countertop workers in four states.
Recent NIOSH research findings shed light on assessing RCS exposure and providing exposure controls. In this article, a team from the Engineering and Physical Hazards Branch, in the Division of Field Studies and Engineering at NIOSH, address questions that are critical to protecting workers in the stone countertop industry.
What are the main factors contributing to RCS exposures during the fabrication of stone countertops? Workers’ RCS exposures have two main determinants: the amount of respirable dust that they are exposed to and the crystalline silica content in the respirable dust. Without effective exposure controls, exposures to respirable dust, which is small enough to penetrate the deep part of the lung and cause harm, may occur during stone countertop fabrication and housekeeping tasks like grinding, polishing, cutting, edging, contouring, sweeping, or using compressed air. The crystalline silica content in the stone products turns respirable dust exposures into RCS exposures.
What are the best ways to reduce workers’ RCS exposures during the fabrication of stone countertops? The hierarchy of controls lists the preferred order of action based on how well they reduce exposure. It includes elimination, substitution, engineering controls, administrative controls, and personal protective equipment. For elimination and substitution, verify the crystalline silica content of the stone products under consideration for fabrication and, when possible, choose products with little to no crystalline silica. This could result in lower generation of RCS, as described in a 2023 NIOSH report on airborne dust generated from the grinding of natural and engineered stone products (PDF), and therefore lower RCS exposure, as explained in a NIOSH report on ventilation engineering controls for stone countertop fabrication published earlier this year (PDF). Engineering and administrative controls are measures to reduce workers’ exposure to respirable dusts by minimizing their release into the air that workers breathe. But these controls often do not remove specific chemicals, such as crystalline silica, in respirable dust.
When the four exposure control measures outlined in the previous paragraph are properly used, they are generally effective in reducing workers’ RCS exposures below OSHA’s permissible exposure limit (PEL) of 50 µg/m3 as an eight-hour time-weighted average (TWA). In some cases, including instances detailed in the 2024 NIOSH report on engineering controls, they reduce exposures to levels below the OSHA action level of 25 µg/m3. However, in cases when RCS exposures are still above the OSHA PEL, employers should continue to use control measures to reduce exposures as much as possible and implement a respiratory protection program that provides workers with appropriate respirators and follows OSHA’s respiratory protection standard.
Does dry operation cause high RCS exposure during stone countertop fabrication? Yes, dry operation consistently leads to high RCS exposures. The 2015 hazard alert from OSHA and NIOSH warned that the highest RCS exposures “come from dry cutting, grinding, edging, and contouring stone.” And according to an article published in 2012 in the Journal of Occupational and Environmental Hygiene, dry operation for even a few minutes can release “extremely high dust concentrations as gram-sized quantities of stone are aerosolized by powered tools.” Dry operation will likely lead to RCS exposures over the OSHA PEL. Additionally, the dust can stay in the air for a long time, presenting a continued risk for indoor workers. Therefore, wet operation is the first major engineering control to reduce RCS exposures.
Which tasks other than dry operation pose the highest risk for RCS exposures? Three NIOSH studies published in 2016 (see the list of resources below) found that using handheld grinding and polishing tools led to the highest RCS exposures in fabrication shops among all tasks under wet operation. During these studies, workers used traditional on-tool wet methods. However, they still had high exposures, mainly because their breathing zones were near the source of the dust when using handheld tools to work on the edges of the stone countertop. Their exposures could often be higher than the OSHA PEL when only using these traditional on-tool wet methods.
Why might workers’ RCS exposures be higher than the OSHA PEL when they operate in wet conditions for grinding and polishing tasks? The main reason is likely the ineffective wetting by traditional on-tool wet methods.
One traditional wet method is water spray wetting where a water spray is added to the tool. As shown in Figure 1(a), the water spray comes through a nozzle in a fixed spot on the tool. It is meant to wet the point of contact with the stone, but it can completely miss the target from time to time, especially when the worker changes the handheld tool’s position.
Figure 1. Ineffective wetting of the stone surface during grinding using (a) water spray and (b) center-feed. Photos by NIOSH.
Another traditional wet method is the center-feed wetting method. The center-feed design feature feeds water through a channel inside the tool, and the water is delivered from a few tiny holes near the center of the tool’s shaft where a grinding cup wheel or a polishing pad is mounted. While a center-feed feature constantly delivers water regardless of the position of the tool, the amount of water is severely restricted, leading to ineffective wetting. As illustrated in Figure 1(b), the water supply is limited, leading to dry stone surface during grinding. To see videos that further illustrate the effectiveness of different wet methods, refer to an article published in 2020 in Stone Magazine. A NIOSH study published in 2021 (PDF) that compared different wet methods found that both traditional wet methods—water spray and center-feed—were equally poor at reducing workers’ exposures during grinding. Workers’ full-shift TWA exposures to respirable dust when using only one of these two wet methods during grinding are likely near 300 µg/m3 based on the findings of a 2016 health hazard evaluation report by NIOSH (PDF). This makes it challenging to achieve the OSHA PEL for RCS exposure of 50 µg/m3 unless the workers exclusively work with stone containing low crystalline silica (in this example, crystalline silica content would need to be below 16.7 percent). Are there any effective wet methods for handheld tools, and are there other considerations for using wet methods? Yes, the 2021 NIOSH study also found that workers’ respirable dust exposures were significantly reduced to an average of about 120 µg/m3 when using a wet method combining water spray and sheet-wetting as shown in Figure 2.
Figure 2. Worker conducting a grinding task with a wetting method combining water spray and sheet-wetting. Photo by NIOSH.
Sheet-wetting supplies water from the opposite side of the working area and allows water to gently run through to the stone’s edge in a “sheet” formation (as illustrated further in a video via Stone Magazine). This wet method not only keeps the point of contact with the stone wet but also continuously washes away the sludge formed by the mix of dust and water, preventing it from drying and becoming airborne dust again.
Figure 3 provides a comparison of average respirable dust exposure under three engineering and administrative control settings.
In general, delivering water from multiple locations and augmenting traditional wet methods with additional wetting methods to ensure constant wetting at the point of contact with the stone can reduce RCS exposure significantly for tasks using handheld tools. For example, a 2017 article in Annals of Work Exposures and Health demonstrated the effectiveness of sheet-wetting via a perforated water distribution manifold built from a PVC pipe.
When using wet methods for handheld tools, it’s important to use caution when using on-tool local exhaust ventilation (LEV) at the same time. The 2017 Annals article reported that on-tool LEV—although intended to capture the dust at its point of origin—may hinder the wet method by removing water or deflecting water delivery away from the point of contact with the stone.
It’s also important to ensure the water quality for wet operation. Whenever feasible, use tap water for handheld tools and limit recycled water for use in remotely operated machines only. When recycled water is used, it should first be treated to remove solid particles and other harmful residuals.
Besides wet operation, what other engineering and administrative controls are helpful? Although on-tool LEV may interfere with wet operation, LEV systems with air filtration and recirculation capability installed at fixed locations were found effective at reducing RCS exposure in a 2024 NIOSH investigation (PDF). Such LEV systems function as hoods to capture dust near its point of origin without interfering with water delivery and provide dilution ventilation to reduce RCS concentration in the surrounding area. Occupational and environmental health and safety professionals should consider prioritizing the location of these LEV systems for high-exposure tasks—for example, where workers are using handheld grinders. The designs of the LEV systems must account for the dimensions of the operation area and provide airflow velocities that are sufficient to capture and convey dust. Workers also need proper training to effectively use these LEV systems. For instance, workers should always position themselves upstream of the dust source and adjust the operation area to keep the dust source close to the LEV system’s hood.
An example of an auxiliary engineering control that can help reduce RCS concentrations is to enhance dilution ventilation of indoor spaces by replacing dusty indoor air with clean outdoor air (or from recirculating air filtration systems). The size of the indoor space, the available capacity of the dilution ventilation system, and the capability to heat or cool replacement air to satisfy thermal comfort are all factors that impact the effectiveness of this control.
An engineering control that can help minimize the need for high-exposure tasks (for example, tasks using handheld grinders) is to improve the design of the fabrication process by moving operations to remote-controlled tools with water-spraying systems as much as possible.
Using wet sweeping or wet floor scrubbers keeps floors clean. This housekeeping practice can help prevent dust from the floor from getting back into the air after it has settled. It’s important to avoid dry sweeping or using compressed air to clean floors and work surfaces in stone countertop fabrication shops.
Combining the improved ventilation and housekeeping practices described here with water spray wetting can reduce workers’ exposure to respirable dust to an average of about 70 µg/m3 (see Figure 3). As explained earlier, engineering and administrative controls only target respirable dust, so the RCS exposure also depends on the silica content in the respirable dust. Figure 3 illustrates the effectiveness of engineering and administrative controls.
Are all engineered stones causing the same RCS exposures, and are those exposures all higher than those associated with working with natural stones? The short answer is no. The long answer is that engineered stone, also referred to as “artificial stone,” includes many different products, and the crystalline silica content in respirable dust is one of two factors that determine workers’ RCS exposures. The crystalline silica content in stone products likely influences workers’ RCS exposure in a similar way for both engineered and natural stones.
Three NIOSH studies published in 2016 (PDFs: one, two, and three) collected short-term, task-based samples from shops grinding and polishing both natural and engineered stone countertops with a traditional wetting method of water spray as the only exposure control. NIOSH researchers observed that the workers using handheld grinders had average RCS exposures of 238 µg/m3 when working exclusively with high-silica-content engineered stones and 148 µg/m3 when working with both natural and engineered stones. Researchers also found that workers using handheld polishers had average RCS exposures of 69 µg/m3 when working exclusively with high-silica-content engineered stones and 37 µg/m3 when working with both natural and engineered stones. The high-silica-content engineered stones observed in these studies contained up to 90 percent crystalline silica in a polymer resin matrix. For comparison, granite generally contains 10 to 45 percent crystalline silica, and marble generally contains less than 5 percent crystalline silica. The higher RCS exposures observed among individuals working exclusively with high-silica-content engineered stones verify that the crystalline silica content in the stone product is indeed a major factor influencing workers’ RCS exposures.
A NIOSH study published last year (PDF) identified nine representative stone products to investigate after a search of media reports and market data on engineered stone products. They included seven engineered stones containing crystalline silica in a polymer resin matrix from five major manufacturers, one engineered stone containing recycled glass in a cement matrix, and one natural stone, granite. The laboratory study found that the generation of RCS from grinding is strongly related to the silica content in the stones:
• the engineered stone containing recycled glass in a cement matrix with less than 0.2 percent crystalline silica generated an undetectable level of RCS
• the engineered stone containing less than 50 percent crystalline silica in a polymer resin matrix generated RCS at a similar level as the granite
• the engineered stones containing greater than 70 percent crystalline silica in a polymer resin matrix from various manufacturers generated the highest amount of RCS
Figure 4 shows the average crystalline silica content in the respirable dust generated from grinding the different types of stones. The engineered stone containing less than 50 percent crystalline silica in a polymer resin matrix is a relatively new product specifically designed with silica content lower than earlier engineered stone products.
Under the same working conditions, workers are likely to be exposed to lower concentrations of RCS when working with engineered stones containing no or trace crystalline silica; followed by engineered stones specifically designed with lower silica content and granite, similar to the stones examined in the 2023 NIOSH study; and, finally, engineered stones with high silica content. Choosing to work with products with little to no crystalline silica adheres to the top exposure control measures of elimination and substitution in the hierarchy of controls.
A recent NIOSH field study (PDF) illustrates these recommendations in action. During the field study, workers’ respirable dust exposures were reduced to an average of 60–70 µg/m3 for handheld grinding and polishing tasks (as shown in Figure 3 for grinding) after the implementation of improved engineering and administrative controls, including the proper use of wet methods, LEV, and floor cleaning. When these improved exposure control measures were combined with the substitution of high-silica-content engineered stones with engineered and natural stones containing lower crystalline silica, the silica content in the respirable dust was reduced to an average of 8.1 percent (from 35.8 percent in previous NIOSH field studies) and the workers’ RCS exposures were reduced to levels below the OSHA action level of 25 µg/m3.
AN ACHIEVABLE TASK Reducing workers’ RCS exposure during stone countertop fabrication to levels below the OSHA PEL is not an easy task, but NIOSH research findings show it is achievable. By following the hierarchy of controls, employers, workers, and others can take actions to further reduce RCS exposure. Going forward, we encourage manufacturers of engineered stones to develop and make new products with little to no crystalline silica (if these products do not introduce other health hazards), and fabrication shops are encouraged to work with such products. The engineering and administrative control measures summarized here can help fabrication shops reduce workers’ exposure to respirable dust and RCS, and we hope they inspire the development of new and improved technologies to further reduce exposures.
CHAOLONG QI, PhD, PE, is an engineer consultant in the Engineering and Physical Hazards Branch, Division of Field Studies and Engineering at NIOSH.
DREW THOMPSON, PhD, is a research general engineer in the Engineering and Physical Hazards Branch, Division of Field Studies and Engineering at NIOSH.
URSULA “ASHA” BROGAN, MS, is a health communication specialist in the Engineering and Physical Hazards Branch, Division of Field Studies and Engineering at NIOSH.
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RESOURCES
Annals of Work Exposures and Health: “Experimental Evaluation of Respirable Dust and Crystalline Silica Controls During Simulated Performance of Stone Countertop Fabrication Tasks With Powered Hand Tools” (July 2017).
JAMA Internal Medicine: “Silicosis Among Immigrant Engineered Stone (Quartz) Countertop Fabrication Workers in California” (July 2023).
Journal of Occupational and Environmental Hygiene: “Prevalence of Dry Methods in Granite Countertop Fabrication in Oklahoma” (July 2012).
NIOSH: “About Hierarchy of Controls.”
NIOSH: “Comprehensive Report: Characterization of Airborne Dust Generated from the Grinding of Natural and Engineered Stone Products” (PDF, October 2023).
NIOSH: “Comprehensive Report: Engineering Control of Silica Dust from Stone Countertop Fabrication and Installation – Evaluation of Wetting Methods for Grinding” (PDF, June 2021).
NIOSH: “Comprehensive Report: Investigation of Ventilation Engineering Controls for Stone Countertop Fabrication” (PDF, February 2024).
NIOSH: “Evaluation of Crystalline Silica Exposure during Fabrication of Natural and Engineered Stone Countertops” (PDF, March 2016).
NIOSH: “In-Depth Survey Report: Engineering Control of Silica Dust from Stone Countertop Fabrication and Installation” (PDF, March 2016).
NIOSH: “In-Depth Survey Report: Engineering Control of Silica Dust from Stone Countertop Fabrication and Installation” (PDF, September 2016).
NIOSH Science Blog: “Outbreak of Silicosis among Engineered Stone Countertop Workers in Four States” (October 2019).
OSHA/NIOSH: “Hazard Alert: Worker Exposure to Silica during Countertop Manufacturing, Finishing and Installation” (February 2015).
Stone Magazine: “Avoiding the Silica Dust-Up” (November 2020).