A Review of Respirable Crystalline Silica Dust Characteristics and Toxicity in Metal and Nonmetal Mines

1. Introduction

Workers in the mining industry face exposure to a range of occupational hazards, including physical hazards (such as high temperatures and excessive noise), chemical and toxic agents, and airborne pollutants like diesel particulate matter, and harmful fumes [1]. Among these, respirable dust generated during activities such as drilling, blasting, loading, haulage, and comminution poses a particularly serious health risk [2,3]. The problem is especially severe in underground mines, where confined spaces and limited ventilation often lead to higher concentrations of airborne dust.

A primary source of respiratory health concerns in mining environments is exposure to naturally occurring silica and silicate minerals, which are among the most abundant constituents of the Earth’s crust [4,5]. Dust containing respirable crystalline silica (RCS), a form of crystalline silicon dioxide, has been strongly associated with a range of adverse health outcomes, including pneumoconiosis, silicosis, lung cancer, and other non-malignant respiratory disorders [6]. These silica-related diseases remain a major occupational health concern across both surface and underground metal and nonmetal (MNM) mining operations [7,8].

Silicosis is one of the most well-documented and serious health outcomes resulting from occupational exposure to RCS [9,10]. It is a progressive and potentially fatal pulmonary disease characterized by inflammation and scarring of lung tissue caused by RCS inhalation [11,12,13]. Furthermore, exposure to silica has been linked to a range of other health complications, including chronic silicosis, tuberculosis, mycobacterial, fungal and bacterial lung infections, arthritis, and kidney disorders [6,14].

Effective control of respirable crystalline silica (RCS) exposure in metal and nonmetal (MNM) mines depends on continuous monitoring, systematic sampling, and regulatory enforcement. Regular monitoring and assessment of silica dust concentrations are crucial in preventing respiratory diseases and identifying hazardous exposure levels in the workplace [15]. Surveillance programs play a critical role in establishing the casual link between silica exposure and respiratory diseases like silicosis. Data collected from these programs can identify high-risk jobs and specific areas within mines that may require additional monitoring and control measures [16].

The importance of continuous exposure monitoring in mining environments has been highlighted by the Mine Safety and Health Administration's (MSHA) new dust rule, which mandates regular monitoring, mitigation strategies, and medical surveillance for mine workers. Regular exposure measurements for individual workers provide valuable insights into exposure variability, temporal trends, and a more accurate estimation of cumulative exposure risk. These insights help more precise identification of jobs and mine locations with a higher likelihood of disease, and support the implementation of engineering controls, administrative practices, and health surveillance aimed at reducing silica-related disease burdens [15].

The wide-ranging health impacts of RCS exposures, along with recent updates to MSHA’s silica regulations, highlight the complex concerns related to RCS dust exposure in the mining industry. In response, this comprehensive review explores the following key questions:

• What analytical techniques are currently employed to characterize silica dust in MNM mines?
• How does the toxicity of silica dust correlate with the prevalence of lung diseases among MNM mine workers?
• How effective have past regulatory measures been in reducing silica exposure, and what potential impact might the revised MSHA standards have on future occupational health outcomes?

By integrating perspectives from epidemiology, toxicology, occupational medicine, and engineering, this review aims to advance understanding of silica-related health risks and inform strategies that enhance worker protection and promote sustainable mining practices

2. The Laws in Effect in Metal and Nonmetal Mines

2.1. A Brief History of Silica Regulations in the United States

The Federal Mine Safety and Health Act of 1977 created a unified set of safety rules for all miners, regulated by the Department of Labor. It requires the Secretary to inspect mines when specific safety complaints are filed, sets standards for training mine workers in health and safety, and authorizes the Director of the Office of Management and Budget to manage the transfer of enforcement and administrative responsibilities. The Act also covered how interest on civil penalties is calculated and enforces health and safety regulations for surface mining projects[17].

Historically, however, regulatory focus was predominantly directed toward coal mining, leaving MNM mines comparatively under-regulated. Between 1961 and 1977, the Mine Safety Board implemented inspection criteria for MNM mines. These standards included safety protection measures that these mines were required to provide for their personnel during the operations. A more structured regulatory approach emerged in 1999, when MSHA implemented formal safety and health standards for MNM mines in response to inspection findings. These included mandatory annual inspections for underground MNM mines, authority for federal inspectors to issue violations and removal orders and expanded state-level enforcement authority under state plan systems. In addition, education and training programs were strengthened to improve compliance and awareness among mine workers [18].

2.2. Monitoring and Assessment of Respirable Dust Exposure

Two principal approaches are used to monitor the risks associated with respirable particle exposure in mining environments. The first approach involves on-site personal airborne sampling, which captures activity from distinct stages of mining operations. The collected samples are subsequently analyzed to determine their elemental composition and metal concentrations, using techniques such as inductively coupled plasma-atomic emission spectroscopy (ICP-AES) [19]. Respirable crystalline silica analysis for MNM samples, is typically conducted using X-ray diffraction (XRD) techniques in accordance with established protocols, including MSHA P-2, NIOSH 7500, and OSHA ID-142. Each of these protocols can identify and quantify the three polymorphs of silica [15].

The second monitoring approach, which is not mandated by law, involves biological monitoring. This method assesses the presence of metallic substances or their metabolites in miners' biological samples to determine the level of damage, physical consequences, and levels of absorption. On-the-spot biological sampling offers valuable information about inhalation exposure, can complement airborne measurements. It is also important to consider additional exposure pathways, such as ingestion, which can pose further health risks beyond inhalation.

The American Conference of Governmental Industrial Hygienists (ACGIH) has developed Biological Exposure Indices (BEIs) to assist in interpreting biomonitoring results, while the Occupational Safety and Health Administration (OSHA) has set the action limits for various metals. BEI values are typically reported in micrograms per deciliter (µg/dL) of creatinine in urine samples [20]. Current exposure regulations in the United States are based on Threshold Limit Values (TLVs) for chemical substances in workplace air, as originally adopted by ACGIH in 1973 [16].

2.3. The Permissible Exposure Limits (PEL) for Respirable Crystalline Silica in MNM Mines

Permissible Exposure Limits (PELs) are critical tools in the mining industry for regulating and monitoring occupational exposure to airborne pollutants. Mining companies employ various strategies to maintain pollutant concentrations below these limits, thereby protecting workers’ health. Historically, the PEL for respirable dust containing crystalline silica was determined using the formula derived from the 1973 TLV established by ACGIH[21,22]. According to this method, the permissible concentration of respirable dust decreased as the percentage of crystalline silica in respirable dust increased. Specifically, the sample-specific PEL for quartz-containing dust was determined using the equation:

$$\mathrm{PEL}(\mathrm{mg}/{\mathrm{m}}^3)={\displaystyle \frac{10}{(\%quartz+2)}}$$

When rare polymorphs of silica, such as cristobalite or tridymite, were present, the calculated PEL was further reduced by 50% [16].

In addition to MSHA's standards for the mining sector, the Occupational Safety and Health Administration (OSHA) established a PEL for RCS of 50 µg/m³ for general industry, construction, and maritime sectors in 2016, and an action level of 25 µg/m³ time-weighted average during an 8-hour work shift [15].

Internationally, PELs for silica vary, although many countries have adopted similar standards (Table 1). For example, Canada, Britain, and the European Union maintain a PEL of 0.1 mg/m³, while Australia and the United States set stricter limits at 0.05 mg/m³. India’s exposure threshold is 3 mg/m³ for respirable dust with less than 5% silica content, but stricter limits apply for higher silica percentages [23]. In China, PEL vary between 0.07 mg/m³ and 0.35 mg/m³ depending on the silica content [24].

In April 2024, MSHA implemented updated regulations for MNM mines, aligning the silica exposure standards with those adopted in other industries. The new rule establishes a PEL for respirable crystalline silica at 50 µg/m³ (0.05 mg/m³) and an action level of 25 µg/m³, both based on an 8-hour TWA for all mining operations. The PEL defines the maximum allowable exposure, whereas the action level serves as a proactive measure to prompt preventive steps before reaching dangerous levels [15].

• To enforce compliance with the new PEL, MSHA has outlined a standardized procedure for sampling and analyzing respirable dust in MNM mines: Sample Collection and Preparation: Respirable dust sample collection is performed by MSHA inspectors at the mine. They select certain miners and a specific working area for sampling. The task is performed by using gravimetric samplers, which collect air from both the breathing zone of the miners and that of the specific work area selected for the entire shift.
• Quartz Content Analysis: Samples that are damaged, ripped, or appear wet, based on visual inspection, are disposed of. The accepted mine samples are weighed and validated for sufficient mass gain. Samples exceeding the minimum mass threshold are analyzed for RCS. In MNM mines, analysis is conducted using XRD techniques, following protocols such as MSHA P-2, NIOSH 7500, and OSHA ID-142. In contrast, samples from coal mines are analyzed using Fourier transform infrared spectroscopy (FTIR). Silica content in MNM mine samples is measured by comparing the mass of quartz or cristobalite obtained through XRD to the total respirable dust mass. Exposure calculations for MNM and coal miners are based on an 8-hour time-weighted average (TWA) for standard shifts. However, for shifts exceeding eight hours, the calculation methods differ: in coal mines, exposures are assessed based on the actual shift length, whereas in MNM mines, the assessment remains anchored to an 8-hour TWA regardless of shift duration. This procedural distinction reflects the regulatory frameworks established for each sector.. Table 2 compares MSHA procedures for RCS analysis in MNM mines and coal mines [15].

3. Respirable Crystalline Silica Concentrations in U.S. MNM Mines from 2000 to 2023

The data for this study were obtained from the MSHA Mine Data Retrieval system, specifically the databases containing personal health samples and mine information. The personal health samples were filtered in SQL Server Management Studio using a contaminant code 523, which specifically identifies respirable quartz dust with a quartz content of over one percent. After merging datasets based on mine ID and filtering for metal and nonmetal (MNM) mines, a total of 65,621 records were imported into Excel for further analysis.

Permissible Exposure Limits (PELs) were computed based on the 1973 ACGIH TLV® formula[16], which defined a variable PEL ranging from 0.1 to 3.33 mg/m³ depending on quartz content. Out of the 65,624 dust samples examined using the MSHA method, 6,054 samples fell outside the specified lower and upper limits. Additionally, three samples with respirable dust concentrations of 0 mg/m³ were removed due to measurement errors. Seventeen samples were also excluded because their back-calculated RCS concentrations were below the MSHA P-2 method detection limit of 5 µg/m³. Finally, three incorrectly coded samples labeled "Quartz >1% respirable fraction" but confirmed to contain less than 1% quartz were discarded. After these exclusions, a total of 59,547 valid samples collected between 2000 and September 2023 were included for analysis. The data were analyzed based on annual distribution of RCS concentrations, mine type, geographical location, mine size, and canvas code (metal, non-metal, sand & gravel, and stone).

Annual Distribution of RCS Concentrations: Figure 1 shows the annual distribution of RCS sample concentrations using a box-and-whisker plot. Outliers above 500 µg/m³ were excluded from visualization for better clarity. The majority of the samples fell within the permissible exposure limit (PEL) of 50 µg/m³. However, more than half of the samples collected each year exceeded the action level of 25 µg/m³, highlighting the necessity for preventive dust control strategies. A comparison between the annual mean and median concentrations underscores the influence of outliers on the mean values, reinforcing the need to focus on the number of samples exceeding regulatory thresholds for a more accurate assessment.

Mine Type: Samples were categorized into surface mines, underground mines, and facilities within mines. Among the 57,547 entries, 51,642 were associated with surface mines, 3,477 with underground mines, and 4,428 with mine facilities. Figure 2 and Figure 3 illustrate the average RCS concentration per year and the number of samples exceeding the PEL of 50 µg/m³ per year respectively in different MNM mine types.

As evident in Figure 2 and Figure 3, the RCS concentration and the number of samples exceeding PEL in surface mines and facilities are higher than in underground mines. This may result from inadequate dust control measures, extensive material handling activities, the proximity of workers to crushers and loading equipment, and the challenges associated with dust management in open-air environments. Underground mines, by contrast, often employ targeted ventilation systems that more effectively capture and redirect dust away from workers. Nevertheless, the number of samples exceeding the PEL under the new regulations is significant in all three cases, requiring the immediate implementation of effective control programs in mines.

Mine Size: Based on MSHA classification, mines employing fewer than 20 workers are categorized as small mines, while those with 20 or more workers are considered large mines. Of the 59,547 samples, 43,624 was collected from small mines and 15,923 from large mines. Figure 4 and Figure 5 illustrate the average RCS concentration per year and the number of samples exceeding the PEL of 50 µg/m³ per year, respectively, in different MNM mine sizes.

The results indicate that large mines tend to have higher RCS concentrations and a greater number of samples exceeding the PEL. This can be attributed to the larger scale of operations, more intensive material processing, and higher dust generation rates associated with the use of heavy machinery in large mines.

Canvas code: MNM mines were further categorized based on their primary or secondary canvass codes into metal, nonmetal, sand and gravel, and stone mines. After accounting for mines classified under multiple categories, the final dataset contained 60,881 records. Figure 6 illustrates the annual average concentration of RCS, while Figure 7 shows the yearly count of samples that exceed the acceptable PEL of 50 µg/m³ for various canvas codes in MNM mines.

Metal and nonmetal mines exhibited slightly higher RCS concentrations and a greater number of exceedances compared to sand and gravel or stone mines. This trend likely reflects the higher silica content of ores in metal and nonmetal mines, the more aggressive miner equipment and drilling and blasting methods employed, and the extensive ore processing, as well as the larger volume and scale typical in these operations.

Geographical location: Mines were grouped into three geographical regions: Western, Central, and Eastern United States. The dataset included 15,841 samples from the Western region, 22,940 from the Central region, and 20,766 from the Eastern region. Figure 8 exhibits the yearly mean levels of RCS for different mine locations, whereas Figure 9 depicts the proportion of samples beyond the PEL of 50 µg/m³ each year for distinct mining locations.

Western region mines exhibited slightly higher RCS concentrations and a greater number of samples exceeding the PEL compared to the Central and Eastern regions. This may be attributed to the higher concentration of large-scale MNM mining operations in the Western United States.

Statistical Analysis: Because the datasets were non-normally distributed with unequal variances across groups, non-parametric tests were applied. Mann–Whitney U and Kruskal–Wallis H tests were conducted using Minitab software to evaluate differences in RCS concentrations among groups based on mine type, mine size, canvas code, and geographical location.

The Mann–Whitney U test was employed for comparisons between two groups (e.g., small vs. large mines), while the Kruskal–Wallis H test was used for comparisons across more than two groups (e.g., mine types, canvas codes, regions). These tests are non-parametric alternatives to the independent t-test and One-way ANOVA, respectively, and are suitable for analyzing ordinal or non-normally distributed data [29].

For each comparison, the null hypothesis (H₀) stated that all group medians are equal, while the alternative hypothesis (H₁) stated that at least one group median differs. Tables S1–S4 in supplemental information summarize the statistical results. Given that all P-values were below the conventional significance threshold (p < 0.05), the null hypothesis was rejected in each case. This indicates that significant differences in RCS concentrations exist among different mine types, mine sizes, canvas codes, and geographical locations in U.S. MNM mining operations.

4. Characterization Techniques in Effect in MNM Mines

Understanding the methods of dust analysis in MNM mines recognized by regulatory bodies for control and monitoring of dust levels is essential in determining the effectiveness of risk management. One practical and regulated approach is the use of predictive environmental indicators, which are the parameters or values derived from these parameters that offer quantitative insights into environmental risks associated with mining operations. A deeper understanding of these risks and potential liabilities is crucial for the sustainable development and effective management of mine sites [30].

Mineral dust generated during mining activities poses significant threats to both human health and the environment. Understanding the size distribution of dust particles is essential for evaluating their impact on the environment, occupational health, and human physiology. Dust particles, typically ranging from 0.001 to 60 μm in diameter, are typically categorized into nuisance dust, fugitive dust, inhalable dust, thoracic dust, and respirable dust. Noble et al. examined several key aspects of mineral dust associated with mining metalliferous ores, including dust sources, control measures, monitoring techniques, and methods for characterizing dust in terms of particle concentration, size, morphology, and chemical composition. Predicting the physical and mineralogical properties of dust is critical for effective dust management. However, current testing procedures fall short in accurately predicting the chemical and mineralogical characteristics of dust generated during mining operations. Further research is needed to better understand how minerals are distributed across different particle size fractions and to develop a rapid testing method for predicting dust composition [30].

The primary techniques used for the characterization of respirable mineral dust in MNM mines include the following:

• Infrared (IR) Spectrophotometric and X-ray Diffraction (XRD): These methods are standard techniques for monitoring quartz in dust samples [31]. These methods are widely used in regulatory protocols such as NIOSH Method 7602 (IR) and Method 7500 (XRD) for crystalline silica measurement [30,32].
• Particle Size Distribution Analysis: Particle size distribution analysis provides insights into the depth and site of particle deposition within the human respiratory tract. Measurements are typically conducted by analyzing particle suspensions and measuring impedance changes as particles pass through an aperture, which generates a signal proportional to the particle volume [30,32,33].
• Scanning Electron Microscopy (SEM): SEM offers detailed information on the physical and chemical properties of dust particles, including morphology, size, surface characteristics [34,35,36,37]. Although SEM provides extensive characterization capabilities, it is important to remember that SEM provides less accurate characterization in elements with low atomic masses [30,38].
• Transmission Electron Microscopy (TEM): TEM enables high-resolution analysis of individual particles, providing detailed information about particle structure and composition. Although phase identification through diffraction patterns is time-consuming, TEM is particularly valuable for selective analysis of submicron particles [30,39].
• Raman Spectroscopy: This technique is utilized for the structural and chemical characterization of atmospheric aerosol and dust. It is especially useful for investigating the chemical interactions between dust and organic materials present in atmospheric aerosols [30,40].
• Geochemical Assessment: The assessment involves analyzing the elemental composition of dust to identify the presence of metals and metalloids associated with health risks. Such analyses are crucial for understanding environmental contamination and occupational exposure in MNM mining environments [30,39,41].

MSHA employs a standardized approach for the collection and analysis of respirable dust samples at MNM mines, focusing primarily on the quartz and, occasionally, cristobalite content. A review of global studies that characterize respirable dust and silica exposure in mining environments is summarized in Table 3.

5. Toxicity of Dust in MNM Mines

Dust toxicity studies on MNM mines are limited compared to those on coal mines. However, existing literature highlights numerous proven health issues associated with MNM dust exposure. The release of RCS dust into the environment during mining or milling processes creates an airborne hazard, which is extremely toxic and causes severe health hazards. A significant proportion of the particles have the potential to cause or intensify disease processes because they are insoluble and resistant to being converted into less harmful compounds through metabolism. The size and surface features of respirable crystalline silica significantly impact its toxicity. Exposure to some compounds activates inflammation pathways, leading to various negative outcomes, such as lung fibrosis. This process involves a repetitive cycle of cell destruction, the production of oxidants, inflammation, scarring, and fibrosis. Exposure to a pathogen can trigger an immune response that may cause consequences outside of the lungs, including an increase in the levels of antibodies in the blood (hypergammaglobulinemia) and damage to the kidneys due to immune reactions. In addition, the suggested mechanisms for carcinogenesis include direct DNA damage, suppression of tumor suppressor genes, disruption of cell cycle regulation, activation of growth factors, and production of oncogenes. Gaining a comprehensive understanding of these intricate systems is essential for effectively reducing the harmful health effects linked to exposure to respirable crystalline silica [15,80,81,82,83].

Expanding on the mechanisms of toxicity, the angularity of RCS dust causes lung tissue damage and inflammatory responses via various pathways. First, the sharp-edged, angular structure of RCS particles, as shown in Figure 10 allows them to penetrate deep into the alveoli, producing micro-injuries to the epithelial cells that line these air sacs, resulting in cellular damage [84,85]. Second, this physical damage triggers an immunological response in which macrophages attempt to absorb and digest silica particles via phagocytosis. However, the insoluble and long-lasting nature of silica frequently causes macrophage rupture, resulting in the release of inflammatory cytokines and chemokines, which exacerbate inflammation by attracting more immune cells [86,87,88,89]. Over time, this continuous inflammatory response causes fibrosis, as fibroblasts create excess collagen to heal the damage, resulting in silicotic nodules. These hardened, fibrotic regions diminish lung elasticity and respiratory efficiency, emphasizing the harmful effect of angular RCS particles on lung function [87,90,91].

A comprehensive review of global studies on respirable dust and silica exposure in MNM mining environments reveals significant health risks and the varied effectiveness of exposure control measures [43,48,65,69,72]. All process areas, particularly at primary-secondary crushers, consistently exhibit high levels of inhalable dust, highlighting areas of significant respiratory risk [45]. Furthermore, ultrafine particles pose systemic risks that could lead to severe lung pathologies [44,45]. Effective ventilation management is crucial for reducing dust exposure, especially in challenging environments where worker rotation and wet drilling have been shown to be beneficial [46,48,54,69]. Tailings dust has also shown in vitro toxicity to human airway cells, stressing the need for stringent dust management practices [47]. Additionally, the mean exposure levels to respirable silica often exceed the safety limits set by regulatory bodies, underscoring the challenge in managing silica exposure [65,66,68,71]. The studies also reveal a stark variance in dust and quartz concentrations across different mining locations and processes, emphasizing the need for tailored interventions [54,61]. Effective dust control measures, including enhanced ventilation and regulated work rotations, are critical in mitigating these health risks [48,54,69]. In some mining sectors, the geometric mean of RCS exposure has significantly exceeded recommended levels. This means that there needs to be immediate implementation of comprehensive monitoring and control strategies to protect miners from the long-term health effects of silica dust exposure.

Chen et al., in 2006, studied silica exposure in Chinese tin mines from 1972 to 1974. Their analysis showed that mortality rates from all causes closely matched the national averages. However, a significant percentage of death causes were attributed to diseases such as malignant neoplasms, cerebrovascular disease, and cardiovascular diseases (68.6%). This relationship has been found to be quite significant in their discovery: cumulative dust exposure related to mortality in lung cancer, and some arsenic particles and crystalline silica were identified as the cause of lung cancer death [92].

NIOSH, through the centers for disease control and prevention (CDC), has conducted research to prevent workplace illnesses and injuries, including identifying priority areas through surveillance of occupational diseases and injuries. However, there is limited data on the current health status of MNM miners in the U.S., primarily due to the absence of targeted health surveillance programs for this population. Existing studies have linked longer underground work and greater exposure to radon and diesel exhaust with higher rates of lung cancer among MNM miners. Additional research suggests that exposure to radon, silica, or diesel emissions may increase lung cancer risk. One major challenge in studying these associations is the difficulty of accurately measuring past exposures. Nonetheless, findings highlight the need to investigate the combined effects of radon and silica exposure, along with specific mined commodities, on the development of lung cancer and other respiratory diseases [93].

6. Results and Discussion

This review article consolidated multiple studies to provide an overview of the various effects of exposure to RCS in MNM mines. Information shows that RCS is a common risk in different mining settings and is often connected to serious health issues like silicosis and lung cancer. These studies show that the link between RCS exposure and lung problems varies by location, which affects how common these diseases are. Evaluations conducted by scientific, technological, and legislative authorities indicate that there is evidence of progress in surveillance and reducing exposure. However, it is important to note that the existing protections in place are not enough to eliminate all related dangers. This review highlights the practicality and significance of implementing surveillance and protective laws to prevent exposure. However, it emphasizes that these restrictions should be strictly enforced and regularly revised to adapt to the advancements in mining techniques and health standards.

Many intriguing findings emerged from the analysis of data on RCS concentration monitoring in U.S. MNM mines between 2000 and 2023. There is a noticeable upward trend in the average RCS concentrations and the number of samples beyond the PEL, particularly in the past five years, across various types of mines (underground, surface, and facilities). When comparing, it mostly refers to the alarming increase in the number of samples that exceed the PEL and indicates a significantly higher level of concern over safety and workers' exposure.

An analysis of the data on mine size reveals that larger mines tend to have slightly higher RCS compared to smaller ones. This finding is consistent with more samples from large mines exceeding the PEL. Consequently, the majority of the acquired samples indicate that workers in large operations are more likely to be exposed to RCS. This might be ascribed to the operational scale, the equipment utilized, and the sophisticated ventilation systems implemented in huge mines. In terms of mine type, surface mines and facilities have greater RCS concentrations and more samples that exceed PEL than underground mines. We blame this discrepancy on the greater scope of surface operations, workers' close proximity to dust-producing activities, and insufficient ventilation. Surface mines function in open spaces where dust spreads easily, making control difficult, in contrast to underground mines with specialized ventilation systems. Workers frequently work close to loaders and crushers, and the widespread use of large machinery exacerbates the creation of dust.

When classified according to the Canvas Code (metal, non-metal, sand & gravel, and stone), it was found that metal mines and non-metal mines had higher concentrations of RCS compared to sand & gravel mines and stone mines. Therefore, the type of material being mined may be a contributing factor to the degree of respirable silica dust. The differences may result from the greater silica content in material composition, stronger rock fragmentation and drilling techniques, significant ore processing, and the large size and scale of operations typical in metal mines and non-metal mines. This analysis, geographically, places the mines of the western part of the US with marginally higher instances of RCS samples exceeding the PEL than those of the central and eastern. This disparity may result from the concentration of huge MNM mines in the western region of the country.

A comprehensive summary of global studies on the characterization of respirable dust and silica exposure in mining environments shows that high dust levels are a common concern in different mining operations. Crushers and tailings often have high levels of inhalable and respirable dust, which may greatly raise the risk of workers being exposed to excessive amounts of dust and suffering from respiratory problems. Furthermore, the concentration of silica in fine dust particles differs significantly among various types of mines, and in some cases, the levels of quartz in mines exceed the PEL. The presence of a significant amount of silica is associated with severe health hazards, such as silicosis and other respiratory illnesses, highlighting the immediate requirement for more efficient methods of dust control.

Health assessments, such as epidemiological and toxicological studies, often prioritize the investigation of the effects of dust exposure on respiratory health. These evaluations frequently result in suggestions for more stringent restrictions, periodic health examinations for employees, and the use of personal protective equipment. Implementing dust control strategies, such as wet drilling, improved ventilation, and rotating workers in high-exposure zones, has shown success in certain cases by reducing dust levels and lowering the incidence of respiratory illnesses. Advanced sampling and analysis techniques, such as SEM, EDS, FTIR spectrometry, and XRD, offer important information about the size, shape, mineral composition, and chemical properties of dust particles. These details are essential for evaluating the health hazards linked to dust exposure in mining environments.

7. Conclusions

This review paper offers a thorough examination of the properties, toxicity, and regulatory frameworks of RCS exposure in MNM mines. The findings emphasize the considerable health hazards linked to RCS exposure, especially the rise of silicosis and lung cancer, and reinforce the necessity for ongoing initiatives to alleviate these risks by enhanced monitoring, characterization methods, and regulatory compliance.

1) Contemporary Methods for Characterizing Silica Dust in MNM Mines

The study of silica dust in MNM mines uses several advanced methods, including XRD, FTIR, SEM, and EDS. These approaches are crucial for assessing dust particles' dimensions, morphology, mineralogy, and chemical characteristics. XRD and FTIR are particularly adept at estimating crystalline silica concentration, whilst SEM and EDS offer comprehensive insights into particle form and elemental composition. Moreover, studies on particle size distributionand geochemical evaluations are employed to analyze the potential health effects of dust exposure. The integration of these methodologies with ongoing monitoring and sampling is essential for pinpointing high-risk regions and executing tailored dust control strategies.

2) Toxicity of Silica Dust and Its Association with Pulmonary Diseases

The toxic effects of silica dust are extensively documented, as its angular and intractable characteristics inflict considerable harm on pulmonary tissue. Upon inhalation, RCS particles get into the alveoli, resulting in micro-injuries, inflammation, and fibrosis. Silica exposure induces an immunological response that frequently leads to macrophage lysis, resulting in the release of inflammatory cytokines and chemokines that worsen lung injury. This results in the development of silicotic nodules, diminished lung elasticity, and compromised respiratory performance. Epidemiological studies have consistently shown a strong connection between RCS exposure and the occurrence of lung diseases, especially silicosis and lung cancer, in MNM mine workers. The severity of these health consequences is affected by factors including the duration and intensity of exposure, particle size, and the presence of additional hazardous compounds in the dust.

3) Efficacy of Regulatory Measures and the New MSHA Regulations

Previous rules, like the PEL set by OSHA and MSHA, have been somewhat effective in reducing dust exposure in MNM mines. However, the data reviewed in this analysis show that RCS levels often exceed safety guidelines, particularly in large mines and those with high silica content in their materials. The newly enacted MSHA regulations, effective April 2024, establish a PEL for RCS at 50 µg/m³ and adopt an action level of 25 µg/m³, reflecting a proactive strategy to reduce occupational health hazards. The action level functions as a preliminary alert, urging businesses to adopt supplementary dust management strategies prior to exposure reaching hazardous levels. Although these restrictions represent progress, their efficacy hinges on rigorous enforcement, consistent monitoring, and the use of sophisticated dust control devices.

Progressing Ahead

A comprehensive strategy is essential to tackle the persistent issues of RCS exposure in MNM mines. This encompasses:

• Enhanced Monitoring and Surveillance: Ongoing assessment of dust concentrations, along with systematic health surveillance initiatives, will facilitate the identification of high-risk zones and personnel, allowing for prompt responses.
• Enhanced Dust Control Strategies: The implementation of engineering controls, including ventilation systems, wet drilling, and dust suppression technologies, can markedly decrease dust levels in mining settings.
• Worker Education and Training: Instructing miners on the hazards of respirable crystalline silica (RCS) exposure and the significance of utilizing personal protective equipment (PPE) can mitigate exposure and enhance overall safety.
• Research and Development: Additional investigation is required to enhance comprehension of the health risks associated with RCS exposure, especially the synergistic implications of silica in conjunction with other deleterious compounds, including radon and diesel exhaust. Establishing swift and precise testing methodologies for dust characterization will be essential for efficient risk management.

In conclusion, although considerable advancements have been achieved in comprehending and alleviating the health hazards linked to RCS exposure in MNM mines, substantial efforts are still required. The new MSHA standards establish a robust framework for minimizing exposure; nonetheless, their efficacy will rely on stringent enforcement, the ongoing enhancement of dust management strategies, and a dedication to safeguarding the health and safety of miners. Employing a multidisciplinary strategy that integrates advanced characterization methods, robust regulatory frameworks, and proactive health monitoring can establish a safer and healthier workplace for MNM mine workers.