Detection and characterisation of carcinogenic erionite fibres in road dust using quantitative SEM-EDX and TEM-SAED analysis

Erionite, a known respiratory carcinogen, is found in altered volcanic ash tuff layers and vesicles within volcanic rocks. In regions with volcaniclastic geology, road cuttings may expose erionite-containing rocks to natural or human disturbances, generating road dust. Airborne erionite fibres in road dust pose significant exposure risks to travellers. The identification of erionite fibres in airborne samples is challenging due to microscopic size, environmental sample contamination, and limitations in analytical techniques and criteria. This study investigated settled dust found on tree twigs along a road near an erionite-containing outcrop in Gawler Downs, New Zealand, where woolly erionite-K was previously reported as a rare occurrence. 

Dust particles were sonicate-washed from twig samples, underwent organic removal processes, and were analysed using SEM-EDX for morphology and chemical composition. Erionite bulk sample from the nearby outcrop were also analysed using SEM-EDS, and quantitative chemical calculations were performed to compared with previous EPMA results for each element under various size and condition scenarios. TEM-SAED analysis was employed to identify thin fibres or single crystal-like fibres.

Respirable-sized fibres were detected in dust from 50% of 20 sampling locations, suggesting possible air dispersion. Detected particles included elongated thin fibres and large bundles, with 65% meeting WHO hazardous fibre size criteria (L > 5 µm, W < 3 µm, AR > 3:1), while 40% were shorter than 5 µm. Quantitative SEM-EDX analysis of erionite bulk samples revealed that fibre width, sample condition and preparation and EDX machine variability significantly influenced the accuracy of elemental detection of fibres. Si and Al detection remained stable in fibres wider than 1 µm, and the Tsi (Si/(Si + Al)) ratio for larger fibrous bundles found matched literature-reported erionite ranges. TEM-SAED analysis confirmed 90% of 30 tested fibres as erionite.

These findings suggest that, despite the rare occurrence of erionite in geological samples from the road cuttings, erionite fibres can be dispersed in road dust and potentially become airborne. Since air-dispersed particles vary in morphology, the study suggests particle size and analytical method are important determinants of the accuracy of SEM-EDX results. The Tsi ratio may therefore only serve as a preliminary indicator that the fibre is erionite, and TEM-SAED continuous data necessary for identification of smaller sized erionite fibres.