Finding needles in a haystack: measuring ambient concentrations of naturally occurring elongated mineral particles in an urban environment

Naturally occurring elongated mineral particles (EMPs) are microscopic particles derived from rocks and soils and are defined by their high aspect ratios. They include both asbestiform minerals, some of which are recognised carcinogens, and non-asbestiform minerals. While occupational exposure to commercially used EMPs such as asbestos has been extensively studied, population-scale exposure to naturally occurring EMPs in environmental settings remains poorly understood. Direct measurements of ambient EMP concentrations are rare due to their low abundance and the analytical difficulty of isolating fibres from complex ambient particulate matter. To address these challenges, a cost-effective, filter-based air-sampling network was established in Riverhead, Auckland, New Zealand, where erionite and other elongated mineral particles have been reported in local geology. A multi-tiered analytical workflow was applied, combining automated scanning electron microscopy with energy dispersive spectroscopy (SEM-EDS) for high-throughput screening with transmission electron microscopy and selected-area electron diffraction (TEM-SAED) for mineralogical confirmation. Automated SEM-EDS screening identified 449 EMPs across the sampled locations, of which 29 exhibited Si:Al ratios within the range characteristic of erionite. However, subsequent TEM-SAED analysis confirmed that none of these particles were erionite. These results indicate that airborne erionite was not detected in the analysed filters within the spatial and temporal scope and analytical detection limits of the monitoring campaign. The study demonstrates the utility of combining dense monitoring networks with automated SEM-based screening as a scalable framework for environmental EMP surveillance in regions where fibrous minerals occur in local geology.