Respirable mineral fibres in leaf surface dust: Micro-characterisation and evidence of a geogenic origin

Naturally occurring fibrous minerals, including erionite and amphiboles, are classified as Group 1 carcinogens, and pose an environmental health risk when inhaled in respirable sizes. In volcaniclastic regions, road cuttings that disturb and expose fibre-bearing formations can release airborne fibres through weathering and human activities. To investigate this process, this study collected surface dust and rock samples from roadside environments in a New Zealand volcaniclastic setting and analysed fibre abundance, characteristics, and the connection between rock sources and airborne fibres. Fibrous particles were detected in surface leaf dust on both sides of the road at all 11 sampling sites, with fibre abundance ranging from 462 to 61,595 fibres/cm². Of the 338 fibres measured, 93 % were within the respirable size range (geometric diameter <3 μm), and 15 % met the WHO criteria for hazardous fibres (length ≥5 μm, width <3 μm, aspect ratio >3:1). Chemical analysis indicated that most fibres had Si/(Si + Al) ratios consistent with mordenite, while crystallographic data confirmed 84 % mordenite, 13 % erionite, and 3 % amphibole. Mordenite was also the dominant crystalline phase in the rock samples, and fibre abundance in surface dust correlated moderately with mordenite concentrations in the corresponding bulk samples (Kendall's tau = 0.49, p = 0.04). These findings reveal an under-recognised environmental exposure pathway of respirable mineral fibres from roadside exposed volcanic outcrops, and highlight the need for further monitoring and health risk evaluation in volcanic regions where fibrous zeolite and amphibole minerals are naturally occurring and disturbed.