A review of respirable crystalline silica dust concentration, characteristics, toxicity, and regulation in US metal and nonmetal mines

Occupational exposure to respirable crystalline silica (RCS) remains a significant health concern in metal and nonmetal (MNM) mining operations, contributing to the development of silicosis, lung cancer, and other chronic respiratory conditions. This review examines the prevalence and effects of RCS exposure in MNM mining environments, the toxicity of silica dust, and the effectiveness of regulatory interventions aimed at controlling exposure and mitigating health hazards. Key factors influencing RCS concentrations, including mine type, size, and geographic location, are analyzed, with particular focus on the impact of recent regulatory updates from the Mine Safety and Health Administration (MSHA). Advanced characterization techniques, including X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), and energy-dispersive spectroscopy (EDS), are assessed for their application in dust monitoring and composition analysis. Our analysis of nearly 65,000 samples collected between 2000 and 2023 revealed that RCS concentrations frequently exceed permissible exposure limits, particularly in large surface mines and facilities located in the western United States. Surface mines exhibited higher dust concentrations than underground mines, and metal and nonmetal mines showed greater silica hazards compared to sand and gravel and stone operations. While regulatory initiatives, including MSHA's new action-level plan, represent a step forward, their success hinges on rigorous enforcement and the adoption of comprehensive dust control strategies. This review highlights the urgent need for an integrated approach, combining advanced monitoring technologies, effective dust suppression methods, and targeted worker training programs, to mitigate RCS exposure and improve occupational health outcomes in the mining industry.