Personal Exposure Monitoring of Respirable Dust at Waste Processing Operations of a South African Gold Mine Using Direct-Reading Instruments

Occupational exposure to respirable dust (RD) and respirable crystalline silica (RCS) remains a significant health risk in mining environments, and direct-reading instruments (DRIs) for particulate matter (PM) have the potential to improve exposure assessment. The aim of this study was to evaluate personal RD exposure patterns during waste processing operations with side-by-side sampling following a standard gravimetric sampling protocol alongside DRIs. Bulk tailings dam material was analyzed for mineralogical composition and particle size using X-ray diffraction (XRD) and automated static imaging analysis, respectively. Forty paired side-by-side personal RD samples were collected at three gold mine waste processing operations (one tailings dam and two remining sites) using the gravimetric MDHS 14/4 method and DRIs (Nanozen DustCount 9000i or TSI SidePak AM520i). Additionally, filters from gravimetric sampling were analyzed for quartz by an external laboratory following the NIOSH 7500 analysis method. Bulk analysis of material revealed that samples contained 84 to 86% of quartz, and approximately 90% of the particles were in the respirable size fraction (<10 μm) and >10% were submicron in size. Gravimetric RD 8-h time-weighted average (8h-TWA) concentrations ranged from 0.044 to 0.585 mg/m3, while RCS 8h-TWA exposure concentrations ranged from <0.001 to 0.170 mg/m3, of which 5 samples exceeded the RCS occupational exposure limit (OEL) of 0.05 mg/m3. According to Bland–Altman analysis, the Nanozen DustCount 9000i and the TSI SidePak AM520i underestimated respirable dust 8h-TWA exposure concentrations compared with the gravimetric method (mean bias = −0.107 mg/m3 and −0.089 mg/m3, respectively). DRIs identified task-related and environment-related short-duration peak exposures not identified by gravimetric sampling. The DRIs underestimated RD mass concentrations compared to gravimetric methods but identified exposure patterns, making them a practical complementary tool in occupational hygiene monitoring programs.