May 18, 2023 / Jenny Houlroyd

Engineered Stone, Silica, and the Precautionary Principle

Editor's note: The opinions expressed in this post are the author's and not necessarily those of AIHA, The Synergist, or SynergistNOW.

Since becoming an industrial hygienist with the Georgia Tech OSHA Consultation Program in 2005, I have periodically performed inspections and conducted respirable crystalline silica (RCS) sampling at stone countertop fabrication shops in the state. Over the years, I came to know what exposure ranges to expect: if the employer used wet methods and ventilation for most tasks, the results typically would be within the OSHA permissible exposure limit (PEL) for silica, even after the publication of the silica standard in 2016, which reduced the PEL for RCS to 50 µg/m3.

But something changed in 2017. Suddenly, for some similar exposure groups (SEGs), air sampling results for respirable silica started coming back from stone fabrication as multiple times the OSHA PEL, even though the company had been using wet methods or ventilation. If fabrication was conducted dry, results returned well over 40 times the PEL (our highest result for RCS was 2,300 µg/m3). Our conversations with employers went from "This is how to reduce exposure" to "We need to research additional controls, but in the meantime, you must protect the employees. These are dangerously high levels of exposure."

So, what changed? According to employers, it was the introduction of a new product called quartz countertops or engineered stone, which is marketed as a durable, scratch-resistant surface that is less likely to stain and requires no sealing or resealing. Unlike natural stone, engineered stone provides consistency and uniformity, and the nonporous design makes it less prone to bacterial growth. Engineered stone is manufactured using primarily quartz rock, an abundant material, and because it is not entirely made of stone, it is lighter than natural stone products. Moreover, once installed, an engineered stone countertop can become a permanent, nontoxic fixture in the home.

However, engineered stone may contain as much as 90–97 percent crystalline silica content (PDF)—the basis of quartz stone—bound together with a polymer resin, resulting in much higher exposure concentrations to workers who fabricate, sand, or polish the stone than countertops produced from natural stone. By contrast, granite contains only 10–45 percent silica content (PDF), and marble typically has less than 5 percent silica. Engineered stone slabs are made by crushing, blending, and compacting a unique ratio of quartz, polyester resin, and other additives. Because the quartz has already been crushed once, by the time fabricators polish or cut these slabs, the particles generated are much smaller than those of natural stone. Research indicates that some particles may even be nanosized. Beyond the use of ventilation and wet methods, if the housekeeping at the facility is not extensive, any dust that settles on surfaces or dries from use of wet methods can become airborne again due to forklift traffic and general movement around the fabrication area. Outbreaks of silicosis cases and other work-related conditions among employees involved in manufacturing and fabricating engineered stone countertops have been identified in Italy, Spain, Israel, Australia, the United States, and China. These cases are noteworthy not only because of their volume but because silicosis is being diagnosed in much younger workers after shorter than usual exposure intervals compared to other industries with workers who are exposed to crystalline silica.

In some regions of Australia, the outbreak is so severe that one in four benchtop fabricators have been diagnosed with silicosis, and Safe Work Australia, the agency that develops national policy on workplace health and safety, has sought input on whether to ban engineered stone. In response, the Australian Institute of Occupational Hygienists (AIOH) submitted its consultation (PDF), which clearly outlines the nature of the issue, explores potential variations of a ban on engineered stone (such as a 40 percent silica content cutoff), and discusses the various emissions produced by engineered stone in addition to silica, the importance of product stewardship by the manufacturers of the slabs, and the compliance practices of the industry. Based on its analysis, AIOH argues that the evidence warrants a ban on engineered stone and that "a percentage [of silica content] that is protective of worker health, or 'safe,' cannot be determined." According to AIOH, if the industry were to choose a percentage, it would result in a high level of required regulation and oversight by the government. AIOH also focuses on the other emissions from engineered stone fabrication, which include amorphous silica, resins, volatile organic compounds, pigments, and metals; reducing crystalline silica content for compliance reasons might change the formulation of these other components and thus change the toxicology of exposure in a way not yet assessed. Having considered the hierarchy of controls and recognizing that only elimination will halt the growing number of young workers developing silicosis from silica exposure generated during the production of this product, AIOH is taking a bold approach consistent with the precautionary principle.

The stone fabrication industry here in the U.S. primarily comprises vulnerable workers, many of whom have precarious work arrangements and are sometimes hired as subcontractors or private consultants to much larger corporations. The workers I have met are primarily Hispanic men from Mexico or Central America with language barriers that inhibit training. I worry about their ability to seek and receive medical treatment if they begin to show symptoms of silicosis, even though their employers must enroll them in medical surveillance under the OSHA silica standard. In my experience, full and proper implementation of medical surveillance is rare at stone fabrication companies.

In March 2022, our program had the opportunity, using Susan Harwood grant funding provided by OSHA, to interview a former stone fabrication employee diagnosed with silicosis, thanks to the help of several occupational health physicians from California. The trip to California and the subsequent time we spent developing training for stone fabrication workers about the hazards of exposure to silica from engineered stone reminded me of our ethical duty as industrial hygienists to speak out when we see something dangerous. It also proved that Susan Harwood funding from OSHA is meeting its intent of reaching those workers who are most at risk.

In December 2022, Jim Morris and Leslie Berestein Rojas reported, as part of a collaboration between the investigative news nonprofit Public Health Watch, the radio station KPCC/LAist 89.3, and the television network Univision, about an outbreak affecting at least 30 men in Los Angeles. These men have been diagnosed with silicosis since 2016 as a result of exposure to engineered stone dust. Are we about to see an explosion of cases similar to what has been observed in Australia? Based on my observations and exposure assessments, company after company I work with struggles to reduce exposures below the PEL even after implementing typical engineering controls. I worry that the workers I am monitoring now may die within the decade because of a completely preventable occupational health illness. In the absence of dramatic improvements in the methods of controlling exposures or a move to fully automated fabrication processes, which is financially not feasible for most small fabrication companies, should we be asking whether this product can be processed safely in a way that would prevent the development of silicosis and other exposure-related diseases?

In February 2015, OSHA and NIOSH issued a joint hazard alert (PDF) about worker exposure to silica during countertop manufacturing, finishing, and installation. A little more than a year later, NIOSH published a health hazard evaluation (PDF) of a countertop manufacturer stating that engineering controls would not be enough to control the exposures. Information about these publications was reported in The Synergist. But now is not the time for more information; now is the time to act. Adhering to the hierarchy of controls requires industrial hygienists to consider elimination or substitution first. I hope that AIOH's policy recommendations and statements will serve as an example and that AIHA will support AIOH's stance on the hazards of exposure to engineered stone dust.

This post was updated July 24, 2023, to reflect the correct name of the Georgia Tech OSHA Consultation Program.


Australian Department of Health: The National Dust Disease Taskforce's Final Report (PDF, 2021).

Australian Institute of Occupational Hygienists: "AIOH President Tracey Bence – Statement on Silicosis Prevention" (April 2023).

International Journal of Environmental Research and Public Health: "Characterization of Silica Exposure During Manufacturing of Artificial Stone Countertops" (June 2022).

LAist: "Ancient Lung Disease Strikes Countertop Cutters in LA" (December 2022).

NIOSH: "Evaluation of Crystalline Silica Exposure During Fabrication of Natural and Engineered Stone Countertops" (PDF, March 2016).

Occupational and Environmental Medicine: "Correspondence on 'Demographic, Exposure, and Clinical Characteristics in a Multinational Registry of Engineered Stone Workers With Silicosis' by Hua et al." (September 2022).

OSHA: "Hazard Alert: Worker Exposure to Silica During Countertop Manufacturing, Finishing, and Installation" (PDF, February 2015).

Safe Work Australia: "Closing Soon—Consultation on a Prohibition on the Use of Engineered Stone" (March 2023).

Safe Work Australia: "Prohibition on the Use of Engineered Stone" (April 2023).

Jenny Houlroyd

Jenny Houlroyd, MSPH, CIH, has worked as an industrial hygienist with the Georgia Tech OSHA Consultation Program for over 18 years and currently serves as the manager of the Occupational Health Group.


more IH data needed

Great insight and anecdotal data but I disagree on one point, I think we do need more data before we act. I have been intimately involved with silica sampling at several US fabrication locations and when I look at the results I get and your GA Tech OSHA Consultation results I was alarmed. The Georgia sampling is much higher than what I have seen. I have been gathering much information, some of it surprising, with a real-time dust detection system (RTDDS) that I have deployed in one of the shops as a pilot test. A stone fabrication shop is a very complicated and complex ecosystem with several factors that weigh heavily on the exposure results. In other words, it's not so simple to tie sampling data to SEGs. This is a critical component to understand, why are employee samples different and what drives the differences. I don't think OSHA knows how to handle the medical surveillance decision when it can be triggered by the action level, how do you determine who is exposed more than 30 days? again getting SEGs is not simple and I would argue a RTDDS makes that heavy lift a lot easier. A few other points to make. 1. There is natural stone like Quartzite that seems to be popular these days that is >90% silica. 2. The place to start is looking at the RCS and Respirable Dust in tandem. I have made a distinction between a sample near or above the action level, creating two categories, too much dust, Respirable Dust, (controls not adequate or not used effectively) and too high a silica percentage. 3. Working with ES does not automatically increase a sample silica percentage but certainly it is a factor. 4. Lastly and provocatively are the theories that there is something intrinsic with the ES that increases risk. It is after all a mixture and we don't have a PEL for that mixture.

By Corey Bender on May 24, 2023 3:37pm

Thanks for sharing this important information.

By Doris on May 18, 2023 5:52pm

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