Probabilistic risk assessment of occupational exposure to respirable crystalline silica among ceramic workers in an industrial town in Iran: a Monte Carlo simulation approach.

Introduction
Occupational air pollution, a critical health, safety, and environment (HSE) issue, has grown with industrial expansion, exposing workers to hazardous chemicals1. The 2016 Global Burden of Disease study reported 1.53 million deaths and 76.1 million Disability Adjusted Life Year (DALY) from occupational exposures, including 9% of lung cancer cases2.
Respirable crystalline silica (RCS) is a fine mineral particle that is recognized as a significant health risk factor for workers in various industries, including mining, construction, and the ceramic industry. Due to its specific physical and chemical properties, this type of dust can cause serious and long-term health problems for exposed workers3.
RCS is linked to silicosis, lung cancer, and chronic respiratory diseases4,5. A 2021 study by Tompa et al. projected 125 lung cancer cases by 2060 from RCS exposure, with preventive measures offering a 422.13 million USD benefit6. Reducing RCS exposure could prevent 630,000 COPD cases in the EU over 25 years, highlighting economic and health benefits of intervention7. RCS inhalation triggers inflammation and fibrosis, leading to silicosis, while the IARC classifies it as a Group A1 carcinogen, with smoking amplifying lung cancer risk8–11. A SYNERGY project analysis reported an odds ratio of 1.45 for lung cancer in the highest RCS exposure quartile, confirming the dose-dependent risk12.
Most of these studies have pointed out the need to assess the carcinogenic and non-carcinogenic risks of silica exposure, especially in workers with high and long-term exposure, as well as to implement control interventions according to exposure risk level13,14.
Given the limited number of studies assessing silica exposure risks among workers in the ceramic industry particularly in western Iran, where this traditional industry remains widespread, this study aimed to address a critical gap. The workshop-based nature of the ceramic sector and the lack of adequate occupational health training for workers further underscore the need for such an investigation.
The primary objective was to quantify exposure levels to RCS across various occupational groups and evaluate the associated carcinogenic and non-carcinogenic health risks. A probabilistic risk assessment approach was employed, integrating field measurements, laboratory analysis, and statistical modeling. To address variability and uncertainty in exposure data, Monte Carlo simulation was used, ensuring a comprehensive and reliable risk characterization15.

Materials and methods

Sampling strategy and production process
This cross-sectional study was conducted to evaluate occupational exposure to respirable crystalline silica (RCS) and its associated health risks among workers in ceramic workshops. The research was carried out in an industrial town in the west of Iran. The ceramic production process involves several stages that contribute to RCS exposure: polishing, which involves cleaning ceramic surfaces using air pressure and generates fine dust; rubbing which involves in rubbing the surface of cleaned ceramic; casting, involving molding of ceramic items; and material making, where raw materials are mixed. Each stage produces airborne RCS particles due to the handling of silica-containing materials, exacerbated by inadequate ventilation and limited use of personal protective equipment (PPE) in many workshops. The studied workshops were characterized by similar production processes, raw materials (primarily quartz-based clays), equipment (e.g., manual grinding tools, kilns), and ventilation systems (mostly natural ventilation with limited mechanical extraction), ensuring comparable exposure conditions across the sampled occupational groups. Inclusion criteria required participants to be aged 18–60 years and have at least 2 years of experience in their current roles to ensure a sufficient exposure history for meaningful risk assessment. A total of 24 air samples were collected, with 6 samples per occupational group.
To provide a clear overview of the study design, the technical roadmap of the research, including sampling, laboratory analysis, and risk assessment steps, is presented in Fig. 1.

Materials and equipment
Personal air sampling was performed using SKC 224-44MTX air sampling pumps equipped with 25-mm PVC filters (5 μm pore size) to capture RCS particles and HD cyclones to isolate respirable fractions (< 10 μm aerodynamic diameter), which are most relevant for lung deposition. Filters were weighed before and after sampling using an analytical balance (Sartorius) with microgram precision, allowing gravimetric analysis to determine the mass of collected dust. For laboratory analysis, a Specac hydraulic press with a capacity of up to 2 tons was used to prepare KBr pellets by compressing samples with potassium bromide (KBr) powder, which is essential for infrared spectroscopy due to its transparency in the IR spectrum. RCS concentrations were quantified using a PerkinElmer Spectrum Two FTIR spectrometer, a high-sensitivity instrument capable of detecting crystalline silica phases like quartz through characteristic absorption bands. All equipment was calibrated prior to use, with pumps adjusted using electronic flow calibrators to ensure consistent flow rates, and the FTIR spectrometer calibrated with standard reference materials to maintain analytical accuracy.

Data collection methods
Personal air sampling was conducted following the NIOSH 7602 method to measure time-weighted average (TWA) RCS concentrations. Sampling pumps were attached to workers’ breathing zones (within 30 cm of the nose and mouth) to capture inhalable RCS levels accurately, operating at a calibrated flow rate of 2.2 L/min. Initially, the sampling volume was set at 400 L per sample, as recommended by NIOSH, but high RCS concentrations in the workshops led to filter overloading, reducing analytical sensitivity. After consultation with NIOSH, the volume was adjusted to 150–200 L to prevent overloading while maintaining TWA accuracy, ensuring the collected dust mass remained within the optimal detection range of the FTIR spectrometer. Samples were collected over multiple days to account for daily variations in production intensity and environmental conditions. Demographic and occupational data, including age, years of work experience, and smoking status, were gathered using structured questionnaires administered during face-to-face interviews at the workshops, ensuring comprehensive worker profiles for risk analysis. All data collection adhered to standardized protocols, with pumps recalibrated before and after each sampling session to ensure reliability.

Laboratory analysis
Air samples collected on PVC filters were analyzed using Fourier Transform Infrared (FTIR) spectroscopy with a PerkinElmer Spectrum Two instrument, following the NIOSH 7602 method. Filters were first weighed to determine the total dust mass, then combined with KBr powder and pressed into transparent pellets using a Specac hydraulic press. These pellets were analyzed in the FTIR spectrometer, which identifies RCS phases (e.g., quartz) by detecting characteristic vibrational absorption bands at specific wavelengths (e.g., 798 cm−1 for quartz). The concentration of RCS was calculated by comparing absorption intensities to calibration curves established with certified reference materials (CRM). Quality control measures included analyzing duplicate samples and using blanks to account for background interference to confirm the presence of crystalline silica, despite FTIR’s limitations at very low concentrations (below 0.02 mg/m3).

Statistical and risk assessment analysis
Statistical analysis was performed using SPSS (version 27) to evaluate RCS concentrations across occupational group. Descriptive statistics, including mean, standard deviation, and range, were calculated to summarize exposure levels, while the Kolmogorov-Smirnov test assessed data normality.
In order to evaluate the potential non-carcinogenic and carcinogenic health risks associated with occupational exposure to respirable crystalline silica (RCS), two metrics were calculated: the Hazard Quotient (HQ) and the Incremental Lifetime Cancer Risk (ILCR). HQ and ILCR were selected as the primary risk indicators in accordance with the USEPA Risk Assessment Guidance for Superfund (RAGS) methodology16. These metrics are globally recognized for quantifying the health risks of hazardous air pollutants These indicators were estimated using a probabilistic Monte Carlo Simulation (MCS) approach with 10,000 iterations implemented in Python (NumPy and Pandas libraries). Carcinogenic and non-carcinogenic risk were assessed using the Hazard Quotient (HQ) and incremental Lifetime Cancer Risk (ILCR) based on the following formulae:
Where:

C is the airborne concentration of crystalline silica (mg/m3),

IR is the inhalation rate (m3/hour),

ET is the exposure time (hours/day),

EF is the exposure frequency (days/year),

ED is the exposure duration (years),

CSF is the cancer slope factor [(mg/kg-day)−1],

BW is the body weight (kg),

AT is the averaging time (days),

RFC is the reference concentration for non-carcinogenic effects (mg/m3), and.

1.14 × 10−4 is a unit conversion factor to account for exposure frequency per year.

Both formulae were incorporated into a Monte Carlo simulation framework to account for variability and uncertainty in input parameters. All input variables were defined using appropriate probability distributions derived from measured field data and literature sources. While the fundamental intake equations are mathematically linear, the use of Monte Carlo Simulation (MCS) accounts for the non-linear variability and uncertainty inherent in human exposure parameters. By fitting probability distributions (Lognormal and Triangular) to input variables (e.g., body weight, inhalation rate, and concentration), the model generates a non-linear probability density of risk rather than a single deterministic point estimate.

Results and discussion
The study included workers from four occupational groups: Polishers, Rubbers, Material Makers, and Casters. These occupational groups reflect distinct tasks within the ceramic production process, ranging from raw material handling to surface cleaning and shaping.
All participants were male (100%), This gender distribution reflects the actual workforce composition in the ceramic workshops of this region, where heavy manual labor in production lines is exclusively performed by male workers due to socio-cultural and industrial norms with a mean age of 31.63 ± 11.63 years and average work experience of 7.4 ± 1.85 years. In terms of educational background, 75% of participants had completed high school, while the remaining individuals had completed either primary school or junior high school (each 12.5%).
The smoking rate among participants was high, with 62.5% reporting active tobacco use—an important risk-amplifying factor in the context of respirable crystalline silica (RCS) exposure9. However, none of the workers reported any prior respiratory disease at the time of the study.

RCS concentration and occupational exposure levels
respirable crystalline silica (RCS) concentrations measured among ceramic workers across four occupational groups: polisher (cleaning ceramic surfaces with air pressure, generating fine dust), rubber (smoothing ceramic surfaces), caster (molding ceramic items), and material maker (handling raw materials). Results summarized in Table 1.

The mean TWA values were: polisher (2.7559 mg/m3), rubber (0.6221 mg/m3), caster (0.4101 mg/m3), and material maker (0.2739 mg/m3), Exceeding the OSHA PEL by 55.1, 12.4, 8.2, and 5.5 times, respectively.

Figure 2 provides a box-and-whisker plot of the TWA RCS concentrations across the four occupational groups. The visualization clearly demonstrates that while all groups exceed the Iran OEL the Polisher group exhibits the most significant exposure intensity and spread, with outliers reaching nearly 100 times the permissible limit.
Table 2 compares mean TWA values of silica in different occupational groups.

The results of Kruskal-Wallis Test in confirms that there is a statistically significant difference in RCS exposure levels between occupational groups.
Polishers exhibited the highest TWA exposure to respirable crystalline silica (RCS) among all job groups. This elevated exposure is primarily attributed to their use of compressed air for cleaning ceramic surfaces, a practice that generates substantial airborne dust, particularly in the absence of effective local exhaust ventilation. Additionally, these tasks are often performed in enclosed or poorly ventilated areas, which further concentrates airborne particulates and increases the risk of inhalation.
In contrast, Material Makers experienced the lowest exposure levels. This can be explained by the nature of their work, which is often conducted outdoors or in semi-open spaces with natural ventilation. Furthermore, a significant portion of material handling in this group is mechanized or automated, reducing direct contact with dust sources. These environmental and operational differences likely contribute to their relatively lower exposure.
Rubbers and Casters showed intermediate levels of exposure. Rubbers, who are involved in smoothing surfaces, work with previously cleaned ceramics and may still be exposed to residual dust, especially if dry methods are used. Casters are primarily involved in molding operations, which may generate less airborne RCS compared to polishing, but still involve contact with raw ceramic materials.
Compared to Fubini et al., who reported 0.1–1.5 mg/m3 in Italian ceramic workshops17, polisher exposure (2.7559 mg/m3) is significantly higher, possibly due to weaker safety standards and lack of effective ventilation in the workshops but other groups such as material maker’s 0.2739 mg/m3 aligns with their range18–20. Chen et al. reported 0.8 mg/m3 in Chinese foundries21, comparable to rubber (0.6221 mg/m3) and caster (0.4101 mg/m3) but lower than polishers, reflecting differences in work practices and dust control. The dry climate of Iran may enhance dust dispersion, and seasonal factors like poor winter ventilation and increased summer production further elevate exposure, underscoring the need for improved dust control22.

Health risk assessment using Monte Carlo simulation
Due to variability in crystalline silica exposure levels and uncertainty in individual and environmental parameters, a probabilistic approach was adopted to assess occupational health risks. This method allows for the generation of a full risk distribution rather than relying on single-point estimates, providing a more comprehensive understanding of potential health outcomes.
The simulation was conducted using input parameters described in Table 3, These parameters were informed by direct field measurements and reputable toxicological references. Each parameter was assigned a suitable distribution, such as lognormal or triangular, to reflect its real-world variability.

The results obtained from the Monte Carlo simulations, which were conducted using the specified input distributions and parameters for each occupational group, are comprehensively summarized in Table 4; Figs. 3 and 4.

When evaluating ILCR, it was found that all occupational groups exceeded the USEPA’s negligible risk level of 1.00E-06. The Polisher group exhibited the highest mean ILCR (5.66E-04), which is 566 times greater than the acceptable threshold. The 95th percentile reached 1.52E-03, exceeding the upper bound of the USEPA’s acceptable range (1.00E-04), indicating a clear and urgent need for risk mitigation in this group. The ILCRs for Rubbers (1.28E-04), Casters (8.47E-05), and Material Makers (5.64E-05) were also significantly above the acceptable range, though lower than in Polishers.
Regarding non-carcinogenic risk, HQ values revealed that none of the occupational groups were within safe limits. The Polisher group again showed the most concerning results, with a mean HQ of 114 and a 95th percentile of 271 — levels associated with high probabilities of developing conditions like silicosis and chronic respiratory disorders28. The HQ values for Rubbers (25.8), Casters (17.1), and Material Makers (11.3) were also far above the USEPA’s acceptable level of 1, highlighting systemic exposure issues across all tasks. These outcomes are detailed in Table 4.
The results of the 10,000-iteration Monte Carlo simulation are visualized in Figs. 3 and 4. The Probability Density Function (PDF) in Fig. 3 reveals the lognormal nature of the Incremental Lifetime Cancer Risk (ILCR), highlighting the ‘tail risk’ that standard mean calculations often overlook. Furthermore, the Complementary Cumulative Distribution Function (CCDF) in Fig. 4 illustrates that for the Polisher group, over 95% of the simulated population exceeds the maximum acceptable cancer risk threshold of 10E-04.
The simulation results clearly illustrate a risk gradient across occupational groups, directly tied to their respective exposure levels (TWA). For example, Polishers, with the highest TWA RCS level (2.76 mg/m3), had ILCR values 4.4 times higher than Rubbers, 6.7 times higher than Casters, and 10 times higher than Material Makers. The same pattern held for HQ values, where Polishers had a risk ratio 4.44× that of Rubbers, 6.70× of Casters, and 10.09× of Material Makers.
This trend aligns with the results obtained in other studies. For instance, Salamon et al. (2021) reported HQ values between 2 and 12 in engineered stone workers in Italy29, while our findings for Polishers (HQ = 114) substantially exceed this range but still similar to material maker. Likewise, Chen et al. (2012) reported ILCR values from 5.00E-06 to 2.00E-05 in similar occupational settings21, which align with our lower-exposure groups but fall well below the values seen in Polishers. The findings also echo Leung et al. (2012), who noted that silicosis may emerge in its accelerated form within 5–10 years of high exposure—a timeline that may apply to Polishers in this study, particularly considering their high HQ and current average work history of 4 years30.
The distributions generated in this simulation were lognormal, consistent with the multiplicative nature of exposure-related parameters. In the Polisher group, for example, the ILCR ranged from 6.84E-06 to 8.20E-03, with a high standard deviation (5.05E-04), reflecting extreme variability in potential outcomes—especially under longer exposure durations such as 20–30 years.
These findings underscore a pressing need for improved occupational safety measures in small-scale ceramic workshops as Excessive exposure to respirable crystalline silica (RCS) can lead to serious health effects, most notably silicosis, a progressive and irreversible lung disease caused by inhalation of fine silica particles28. Silicosis may present in chronic, accelerated, or acute forms depending on the intensity and duration of exposure. In highly exposed groups, symptoms may emerge within 5 to 10 years30.
Prolonged exposure also increases the risk of chronic obstructive pulmonary disease (COPD) and lung infections such as tuberculosis6, particularly when combined with smoking—a factor present in over 60% of workers in this study. Additionally, autoimmune diseases and kidney dysfunction have been associated with long-term silica exposure9.
Given these risks, a range of workplace interventions is urgently needed. Engineering controls such as local exhaust ventilation and wet processing techniques should be prioritized to reduce airborne dust. Installing general HVAC systems can help control exposure, especially during colder months when natural ventilation is limited.

Conclusion
This study revealed significant health risks associated with occupational exposure to respirable crystalline silica (RCS) in small-scale ceramic workshops in Iran. Through probabilistic risk assessment using Monte Carlo simulation, all occupational groups were found to exceed acceptable thresholds for both cancer and non-cancer risks. Polishers, in particular, experienced the highest exposure levels and risk values due to their dust-generating tasks and lack of effective protective measures.
The findings emphasize the urgent need for regulatory reforms and implementation of engineering controls, such as local exhaust ventilation and wet processing. Mandatory use of respiratory protection, worker training programs, and regular medical surveillance are also critical to safeguarding worker health. Special attention should be given to high-risk groups and compounding factors like smoking.
In conclusion, the integration of probabilistic modeling and detailed visualization in this study confirms that the risk of silicosis and lung cancer among ceramic workers is a distribution-based phenomenon heavily influenced by specific high-intensity tasks. Future interventions must prioritize the ‘Polisher’ group and replace dry-cleaning methods with vacuum or wet-suppression systems to effectively mitigate the extreme ‘tail risks’ identified in our simulation.
In summary, RCS exposure in traditional ceramic production presents a clear and preventable occupational hazard. Immediate, coordinated efforts involving policy, technology, and education are essential to reduce exposure, prevent disease, and improve long-term outcomes for ceramic workers.