Evaluation of the potential toxicity of respirable halloysite elongate mineral particles: Insights from the Fibre Potential Toxicity Index (FPTI) model

Halloysite is a member of the kaolin group, with an ideal chemical composition of Al2(OH)4Si2O5.2H2O, displaying wide and diverse industrial application spanning from traditional ceramics to drug delivery. Halloysite often occurs as elongate mineral particles (EMPs) with nanometer-size tubular, cylindrical or fibrous shape. Despite these features and its extensive use, the International Agency for Research on Cancer (IARC) has not yet evaluated halloysite for its potential carcinogenicity. Existing literature only focused on its toxicity following oral administration mostly in a pharmaceutical contexts, but findings remain inconclusive. In this study, a commercially available halloysite from Matauri Bay (North Island, New Zealand) was investigated to provide further insight into potential occupational exposure to airborne nano-particles in the ceramic industry. A comprehensive characterization of the sample was performed using XRPD, TGA-DTA, XRF-WDS, BET, particle size distribution, SEM, TEM, Mossbauer and UV-Vis spectroscopies. This multi-technique approach permitted to determine its toxicity potential through the Fibre Potential Toxicity Index (FPTI) model, which had already proven robust in evaluating the hazardousness of respirable mineral fibres. Results indicated that the sample contains approximately 90 wt% halloysite, with minor quartz and cristobalite. Halloysite nanotubes are respirable but not "regulated fibres" with a FPTI value of 1.92(0.10), comparable to that of non-carcinogenic mineral fibres suggesting, with caution, that halloysite is a nanomaterial with low toxicity. Although the study focused on samples from a specific locality, the results should be considered applicable to halloysite samples from any quarry; nevertheless, it should be noted that halloysite from this deposit contains respirable quartz and cristobalite which are a IARC Group 1 known human carcinogens.