In vitro testing of hazardous mineral fibres: The issue of the concentration metric

This communication addresses the matter of the appropriate concentration metrics for the in vitro testing of mineral fibres, a specific technical issue affecting the correct determination of their toxic/carcinogenic potential. The exposure to certain mineral fibres (e.g., asbestos and erionite) is well-known for its detrimental effects on human health, with caution exposure limits set to 0.01 ff/cm³ by the European Council in 2023. In this regard, in vitro tests have a crucial role in the preliminary determination of the hazardous potential of mineral fibres, although selecting the appropriate concentration metrics and doses is currently controversial. Here, we address the complex technical issues of the current normalisation methods (i.e., mass normalization and fibre number normalization) with their advantages and disadvantages, ultimately concluding that mass normalisation should be recommended. In fact, considering two fibrous species with the same chemical composition, mass normalisation guarantees that the concentration of atomic species and ROS-inducing metals remains equal, while this parameter becomes an additional variable with fibre number normalisation.