Occupational exposure during handling and loading of halloysite nanotubes – A case study of counting nanofibers

Halloysite nanotubes (HNTs) are abundant naturally-occurring hollow aluminosilicate clay mineral fibers with a typical diameter < 100 nm and an aspect ratio of up to 200. Here we assessed the potential inhalation exposure to HNTs in an industrial research laboratory. Inside a fume hood, ten times 100 g of HNTs were poured at rate of 0.5 kg min⁻¹, which increased concentrations from the background level up to 2900 cm⁻³ and 6.4 μm² cm⁻³. Inside the fume hood, the respirable mass concentration was 143 μg m⁻³ including background particles. Outside the fume hood we did not measure elevated concentrations. We classified 1895 particles according to their length and aspect ratio. Five particles were in aspect ratio > 3 and in length > 2 μm. These particles were agglomerated and/or aggregated particles where the longest individual fiber was 2 μm in length. The occupational exposure limits for refractory mineral fibers vary from 0.1 to 2 fibers cm⁻³. Following standard protocols for fiber analysis, detection of 0.1 fibers cm⁻³ would require analysis on 4 × 10⁴ images when the filter loading is good. Thus, the fiber sampling and quantification procedures needs to be improved significantly if nanofibers <100 nm in diameter are included in regulatory exposure assessment. Due to very limited toxicological information of HNTs we recommend avoiding inhalation exposure.