Characterization of particle exposure during tunnel excavation by tunnel boring machines

Tunnel boring machines (TBMs) are used to excavate tunnels in a manner where the rock is constantly penetrated with rotating cutter heads. Fine particles of the rock minerals are thereby generated. Workers on and in the vicinity of the TBM are exposed to particulate matter (PM) consisting of bedrock minerals including alpha-quartz. Exposure to respirable alpha-quartz remains a concern because of the respiratory diseases associated with this exposure. The particle size distribution of PM and alpha-quartz is of special importance because of its influence on adverse health effects, monitoring and control strategies as well as accurate quantification of alpha-quartz concentrations. The major aim of our study was therefore to investigate the particle size distribution of airborne PM and alpha-quartz generated during tunnel excavation using TBMs in an area dominated by gneiss, a metamorphic type of rock. Sioutas cascade impactors were used to collect personal samples on 3 separate days. The impactor fractionates the dust in 5 size fractions, from 10 mu m down to below 0.25 mu m. The filters were weighted, and the alpha-quartz concentrations were quantified using X-ray diffraction (XRD) analysis and the NIOSH 7500 method on the 5 size fractions. Other minerals were determined using Rietveld refinement XRD analysis. The size and elemental composition of individual particles were investigated by scanning electron microscopy. The majority of PM mass was collected on the first 3 stages (aerodynamic diameter = 10 to 0.5 mu m) of the Sioutas cascade impactor. No observable differences were found for the size distribution of the collected PM and alpha-quartz for the 3 sampling days nor the various work tasks. However, the alpha-quartz proportion varied for the 3 sampling days demonstrating a dependence on geology. The collected alpha-quartz consisted of more particles with sizes below 1 mu m than the calibration material, which most likely affected the accuracy of the measured respirable alpha-quartz concentrations. This potential systematic error is important to keep in mind when analyzing alpha-quartz from occupational samples. Knowledge of the particle size distribution is also important for control measures, which should target particle sizes that efficiently capture the respirable alpha-quartz concentration.