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

Abstract Halloysite nanotubes (HNTs) are abundant naturally-occurring hollow aluminosilicate clay mineral fibers with a typical diameter  −1 , which increased concentrations from the background level up to 2900 cm −3 and 6.4 μm 2  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 × 10 4 images when the filter loading is good. Thus, the fiber sampling and quantification procedures needs to be improved significantly if nanofibers