Developing the Silica Control Tool
A Model for Respirable Crystalline Silica Exposures at Construction Work Sites
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The Silica Control Tool is a web-based tool developed by the British Columbia Construction Safety Alliance (BCCSA) to help construction employers manage occupational exposure to respirable crystalline silica (RCS). The BCCSA is a not-for-profit association with a mandate to provide health and safety resources and services to the local construction industry. Construction companies are primarily small businesses, and many do not have access to occupational hygiene data or expertise. The Silica Control Tool uses a database of RCS exposure measurements from construction work sites to help employers generate RCS exposure estimates for their work activities and environments. The tool aims to connect employers with exposure data and help them use that data to create an effective exposure control plan (ECP). RCS BACKGROUND AND REGULATIONS Crystalline silica, or silicon dioxide, is a naturally occurring mineral that is ubiquitous in the earth’s crust and has been a known occupational health hazard for centuries. Crystalline silica is commonly found at construction work sites in rocks, sand, and earth and is also present in construction materials, including concrete, cement, brick, mortar, asphalt, and gypsum. Construction work activities that disturb and fracture these materials generate dust containing respirable crystalline silica, which may be inhaled by workers. Inhalation of RCS can cause serious health impacts primarily on the respiratory system, such as silicosis, an irreversible and sometimes fatal form of pulmonary fibrosis. RCS has been classified by the International Agency for Research on Cancer as a Group 1 carcinogenic agent, meaning there is strong evidence that it causes cancer in humans. Exposure to RCS has also been linked to an increased risk of chronic obstructive pulmonary disease, tuberculosis, and kidney disease. ACGIH has set a time-weighted average (TWA) threshold limit value (TLV) of 0.025 mg/m3 of RCS. This TLV reflects the potential for RCS to cause serious impacts even at low levels of exposure. In 2016, OSHA announced updated silica exposure regulations that reduced the agency’s TWA permissible exposure limit (PEL) for RCS from 0.1 mg/m3 to 0.05 mg/m3. The rule also requires employers to use engineering and administrative controls to control exposure to RCS and to use respiratory protection when other controls cannot reduce exposure levels below the PEL. A construction standard for RCS was published as part of the final rule, and enforcement began in 2017. In Canada, occupational health and safety is regulated by provincial governments. The regulatory authority in British Columbia, WorkSafeBC, has adopted the ACGIH TLV of 0.025 mg/m3 as the occupational exposure limit. Under a substance-specific requirement for RCS added to the regulation in 2017, employers must create a written exposure control plan (ECP) for any workplace where workers are or may be exposed to RCS at levels that exceed 0.025 mg/m3. The ECP must specify the exposure controls used to control RCS levels and employers must use air monitoring data to demonstrate that their selected controls can reduce exposure levels below 0.025 mg/m3. CHALLENGES FOR RCS MONITORING Monitoring silica levels in workers’ breathing zones is necessary for selecting controls and verifying their effectiveness. But conducting air monitoring in the construction industry is challenging due to the variable nature of construction work. On-site hazards and conditions change from day to day, and construction projects often employ multiple trades who come and go at short notice. There are no direct-reading RCS measurement methods. In the days required to obtain analytical results from RCS monitoring, a project may move on to other tasks and exposure scenarios. Moreover, in BC, as in many other jurisdictions, more than 90 percent of construction employers have fewer than 20 employees. Employers typically do not have industrial hygienists on staff, and there are insufficient consultants available to frequently monitor RCS exposure levels. Recognizing the challenges that construction employers face to collect air monitoring data, the WorkSafeBC regulation permits employers to use data from “equivalent work operations” to demonstrate compliance. The OSHA standard for RCS in the construction industry (29 Code of Federal Regulations 1926.1153) includes a similar clause allowing the use of “objective data.” However, most construction employers and their staff lack the training or experience to identify or access peer-reviewed studies or published exposure monitoring reports, evaluate the equivalence of work operations, and determine whether data were collected using valid sampling and analysis methods. Alternative methods exist that permit employers to achieve compliance without collecting and evaluating air monitoring data. For example, OSHA’s construction industry standard for RCS specifies combinations of controls employers can use to achieve regulatory compliance for common construction activities. Modeling tools are another option. In 2013, researchers at the Université de Montréal and the Institut de Recherche Robert-Sauvé en Santé et en Sécurité du Travail in Quebec, Canada, published their analysis of a database of RCS exposure measurements from construction work sites. This database was assembled from measurements reported in scientific journals, research reports, and government and institutional databases. The team’s analysis demonstrated that “task-based” samples, as opposed to full-shift sampling, could be used to develop a model that predicted RCS exposure levels. Their statistical model used work task, construction sector, construction project type, work environment (for example, indoor or outdoor workplaces), presence of general ventilation, and use of engineering controls to estimate RCS exposure. The final model explained 60 percent of the variability in RCS exposure data in the database. In Europe, several tools have been developed to assist exposure modeling for regulatory compliance, including the Advanced REACH Tool (ART), the Estimation and Assessment of Substance Exposure (EASE) model version 2.0, Stoffenmanager, and the ECETOC Targeted Risk Assessment (TRA) tool. There are several benefits to using a modeling approach for exposure assessment and risk assessment. First, a model can be calibrated to regulations and requirements used by other jurisdictions. Second, a model can be used to demonstrate the effect of exposure controls on exposure levels. It can also be updated with new measurements to respond to changes in work practices and control technologies. Collecting exposure measurements from employers can encourage industry engagement with exposure monitoring. Finally, exposure measurements collected by industry can be used beyond a one-time compliance assessment to inform the whole industry, rather than an individual employer.

Conducting air monitoring in the construction industry is challenging due to the variable nature of construction work.
DEVELOPMENT OF THE SILICA CONTROL TOOL WorkSafeBC proposed an update to the RCS regulation in 2013 and opened a period of public consultation. In response to implementation concerns from the construction industry, the BCCSA partnered with industrial hygiene researchers from the University of British Columbia (UBC) to develop a tool for creating effective ECPs backed by high-quality industrial hygiene data. The tool was developed with funding from the BCCSA’s research, development, and opportunity fund, with oversight from a steering committee comprised of WorkSafeBC representatives, UBC researchers, web developers, construction industry stakeholders, and BCCSA management.
Work on the model began with engaging a focus group of the BCCSA’s construction employer members to create a list of common work processes that can result in RCS exposure and common engineering control methods. We obtained the RCS exposure database assembled by the Quebec research team and supplemented it with an updated literature review to form the basis of the Silica Control Tool Database. Canadian researchers, employers, and government agencies were contacted to share data. Data that represent the common silica processes listed by the industry focus group were identified in the database. We conducted targeted task-based exposure monitoring at BC construction work sites to collect data on any common silica processes that were missing from the database and increase the amount of data from BC within the database.
The first version of the Silica Control Tool model was developed using 4,550 observations that met quality control criteria. The majority (62 percent) were from the U.S. Under quality control criteria, data were restricted to personal samples of RCS exposure collected using standardized sampling and chemical analysis methods and industrial hygiene best practices, such as use of flow rate calibration and field blanks. The model is a linear regression model that estimates RCS exposure levels based on: • common silica process, such as drilling concrete with an electric hammer drill or milling asphalt with a milling machine • engineering control used, such as no controls, local exhaust ventilation, or wetting • construction industry sector • construction project type, such as renovation, demolition, or new build • work environment, such as indoor or outdoor environments • region, such as province, state, or country • sampling duration
The model uses uncertainty analysis to estimate a 95th percent confidence interval around the estimated geometric mean exposure level. The exposure estimate used in the Silica Control Tool is the upper limit of this confidence interval. These conservative estimates of exposure ensure that users appropriately account for statistical uncertainty when they make decisions.
USE OF THE SILICA CONTROL TOOL The project team developed a website that integrates the predictive model into an interface allowing the user to select their work conditions from a series of drop-down menus. The user’s selections populate the model variables and create a personalized exposure estimate.
The Silica Exposure Tool walks users through a process of seven steps, starting with identification of the silica process at the work site and ending with generation of an ECP. First, users enter information about the work site and environment. They then receive an estimate of exposure without the use of engineering controls. Users are then guided through the hierarchy of controls, beginning with elimination and substitution of the task or product. If neither elimination nor substitution is possible, the tool presents users with options for engineering controls, followed by administrative controls.
The engineering control selections generate an updated controlled exposure estimate. For most common silica processes, the model can estimate the exposure level for work done with and without engineering controls. This allows the user to see the effects of controls on the exposure level and reinforces the benefits of using them. If the controlled exposure estimate is above 0.025 mg/m3, the tool advises users on appropriate respiratory protection.
Finally, the tool creates an ECP compliant with WorkSafeBC requirements, using the exposure scenario and control options selected by the user. WorkSafeBC regulatory guidelines cite the Silica Control Tool as a source of objective monitoring data that may be used in place of workplace air monitoring. The Silica Control Tool’s user interface also includes educational information and FAQs on relevant topics, including health hazards associated with RCS, industrial hygiene best practices, and proper implementation of control strategies. As of May 2017, the Silica Control Tool has been available at no cost to all employers in British Columbia.
Since its launch, the Silica Control Tool has received an average of 720 new users per year. As of December 2023, 4,890 users had registered accounts, and the tool had been used to complete 11,270 ECPs. The number of ECPs generated from the tool per year increased from 634 in 2017 to 2,674 in 2023. These figures do not include instances in which users employed the tool’s exposure estimates to create their own ECPs.
In 2022, the most recent year for which complete data are available, the common silica process selected most often by Silica Control Tool users was drilling concrete with an electric hammer drill. Other common silica processes frequently selected by users were: • chipping concrete • cutting concrete with a saw • mixing and pouring cementitious material • coring concrete with a coring machine • breaking concrete with a jackhammer • grinding concrete with a surface grinder • sweeping construction dust • mechanized moving of small rocks, soil, and so on • grinding concrete with an angle grinder
ONGOING AND FUTURE DEVELOPMENTS The Silica Control Tool model is administered by the BCCSA and updated annually to incorporate new air monitoring data. These include data identified in literature reviews, volunteered by construction employers, and collected in the BCCSA’s ongoing silica monitoring campaign. If a silica-generating process or control option is not represented in the database, the tool cannot estimate an exposure level for that process or control, and the tool prompts users to contact the BCCSA. This information is used to select priority processes and controls for targeted sampling and to identify and recruit companies to participate.
Since 2017, 540 new exposure measurements have been added to the database. We welcome data submissions from hygienists and employers and would be grateful to colleagues who can help identify sources of RCS exposure data.
Although Ontario has not yet updated their regulations and occupational exposure limit for RCS, the Occupational Health Clinics for Ontario Workers (OHCOW) and the Ontario Occupational Illness Prevention Steering Committee have encouraged reducing the RCS exposure limit and supported bringing the Silica Control Tool to Ontario since 2018. The Ministry of Labour, Immigration, Training, and Skills Development (MLITSD) requested public comment on a proposal to lower the Ontario exposure limit to 0.025 mg/m3 in 2021.
In 2023, OHCOW led the Occupational Illness Prevention Steering Committee with the MLTSD Prevention Division, in partnership with the BCCSA, the Infrastructure Health and Safety Association, and the Canadian Centre for Occupational Health and Safety. All the Ontario Occupational Health and Safety system partners adopted a version of the Silica Control Tool modified for Ontario as a best practice tool. Construction workers, workplaces, and hygiene consultants in the province now have access to a version of the Silica Control Tool that uses the same exposure model as the BC tool.
In the province of Alberta, members of the Alberta Roadbuilders and Heavy Construction Association also have access to a version of the tool that reflects Alberta regulations.
The adoption of the Silica Control Tool by users in British Columbia and other Canadian jurisdictions demonstrates the potential for mobilizing industrial hygiene data to develop tools that can be used directly by employers to assess and manage the risk of health hazards. A similar approach is possible in other industries and with other hazards. The BC Ministry of Energy, Mines, and Low Carbon Innovation is currently working with the BCCSA to develop an RCS database, model, and tool that can be used at sand, gravel, and aggregate mining operations. Two of the authors of this article are collaborating with Victoria Arrandale, PhD, at the University of Toronto to develop an exposure measurement database to assess the feasibility of developing a risk assessment tool for welding fume exposure.
The Silica Control Tool and similar initiatives are not intended to replace exposure monitoring but to address some of its challenges, while demonstrating the importance and impact of using monitoring data in risk assessment and decision-making. Although it is not yet available in the Silica Control Tool interface, the authors have developed a Bayesian version of the model that will allow users to enter their own monitoring data to develop personalized exposure estimates harnessing the large body of evidence available in the Silica Control Tool database. The Bayesian model uses the original Silica Control Tool linear regression model as a “prior” that can be updated with the user’s own data.
Exposure monitoring is essential to industrial hygiene practice. The Silica Control Tool demonstrates an approach that engages industry in exposure monitoring and data sharing to support worker protection—not only at individual workplaces and companies, but across the industry.
MELANIE GORMAN NG, PHD, CIH, is a health and exposure scientist at BC Construction Safety Alliance and an adjunct professor at the University of British Columbia School of Population and Public Health.
HUGH W. DAVIES, PHD, CIH, is a professor at the University of British Columbia School of Population and Public Health.
JÉRÔME LAVOUÉ, PHD, is a professor at the University of Montreal School of Public Health.
KIMBERLY O’CONNELL, CIH, ROH, CRSP, is the executive director at Occupational Health Clinics for Ontario Workers.
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Annals of Occupational Hygiene: “Silica Exposure during Construction Activities: Statistical Modeling of Task-Based Measurements from the Literature” (December 2012).
Annals of Work Exposures and Health: “Determinants of Respirable Crystalline Silica Exposure in Construction in Western Canada” (June 2023).
Frontiers in Public Health: “Development of a Web-Based Tool for Risk Assessment and Exposure Control Planning of Silica-Producing Tasks in the Construction Sector” (August 2020).
IRSST: “Construction Workers’ Exposure to Crystalline Silica: Use of a Database Taken from the Literature” (2013).
OSHA: Safety and Health Regulations for Construction, Toxic and Hazardous Substances, Respirable Crystalline Silica.
OSHA: “Silica, Crystalline.”
The Silica Control Tool.
WorkSafeBC: “Silica.”