# Mineral Fibres and Asbestos Bodies in Human Lung Tissue: A Case Study
## 1 Introduction  
Minerals with fibrous crystal habit are widely distributed in the Earth's crust. Among them, the most relevant are commercial fibres that belong to the asbestos family: chrysotile and five amphiboles, i.e., tremolite asbestos, actinolite asbestos, anthophyllite asbestos, crocidolite (fibrous-asbestiform variety of riebeckite), and amosite (fibrous-asbestiform variety of cummingtonite-grunerite) [1; 2; 3]. Amphiboles are composed of double chains of corner-sharing SiO\({}_{4}\) tetrahedra with a Si(Al)O ratio of 4:11. The simplified general formula for amphiboles is: A\({}_{0-1}\)B\({}_{2}\)C\({}_{5}\)T\({}_{8}\)O\({}_{22}\)W\({}_{2}\), where A are cations (Na\({}^{+}\), K\({}^{+}\), Ca\({}^{2+}\), Li\({}^{+}\)) having coordination in the range 6-12; B (i.e., M(4) site) are irregular octahedral or 8-fold coordinated cation sites (Na\({}^{+}\), Li\({}^{+}\), Ca\({}^{2+}\), Mn\({}^{2+}\), Fe\({}^{2+}\), Mg\({}^{2+}\)); C are cations (Mg\({}^{2+}\), Fe\({}^{2+}\), Fe\({}^{3+}\), Mn\({}^{2+}\), Mn\({}^{3+}\), Al\({}^{3+}\), Ti\({}^{3+}\), Ti\({}^{4+}\), Li\({}^{+}\)) that occur in the regular octahedrally coordinated cavities \(M(1)\), \(M(2)\) and \(M(3)\); T are the tetrahedrally coordinated sites within the silicate chain (Si\({}^{4+}\), Al\({}^{3+}\)); and W are anions (OH\({}^{-}\), F\({}^{-}\), Cl\({}^{-}\), O\({}^{2-}\)) [3; 4]. Chrysotile is a trioctahedral hydrous phyllosilicate based on a 1:1-layer structure with a Si-centred tetrahedral sheet and a Mg-centred octahedral (brucite) sheet, with the ideal chemical formula Mg\({}_{3}\)Si\({}_{2}\)O\({}_{5}\)(OH) [5]. Archaeological findings suggest that asbestos fibres were used to create artefacts from a very long time ago [6]. However, the industrial age of asbestos started in the 17 th century, when asbestos mining and industrial use began taking advantage of its outstanding technological properties (e.g., flexibility, abrasion and fire resistance, high tensile strength, large surface area, weak sound-transmission coefficient, low thermal conductivity) [2; 6]. Thanks to these properties, asbestos fibres were used in more than 3000 industrial applications, including various types of composite products (e.g., asbestos cement), known as asbestos-containing materials (ACMs) [2]. The first studies that suggested a correlation between lung cancer and asbestos exposure date back to the beginning of the last century [7; 8]. Subsequently, animal carcinogenicity tests and long-term epidemiological studies provided evidence that inhalation of asbestos fibres, after a latency period of decades, may induce fatal lung diseases, namely malignant mesothelioma (MM) [8]. Following this evidence, the International Agency for Research on Cancer (IARC) classified the six types of commercial asbestos fibres as carcinogenic to humans [8]. Toxicity and carcinogenicity of asbestos are related to their biopersistence, i.e., the ability to persist in the human body despite the physiological clearance mechanisms [9; 10; 11; 12]. Biopersistence of mineral fibres is linked to their physical, chemical, structural properties, and biodurability, i.e., the resistance of fibres to chemical/biochemical alteration. The peculiar morphology and length: width ratio (generally >3:1) of mineral fibres make it possible for them to be inhaled and to reach the alveolar region of the lower respiratory tract [13; 14]. In this environment, fibres can be engulfed or dissolved by alveolar macrophages [15; 16; 17]. Unfortunately, long fibres (length >10 \(\upmu\)m) cannot be completely internalised by macrophages and cannot be cleared [12; 13; 14; 15; 16]. When the clearance mechanism fails, fibres can be coated with an iron-protein-mucopolysaccharide material, termed \"asbestos body\" (AB) [9; 18; 19; 20; 21; 22; 23; 24; 25]. Various models have been postulated to explain the growth mechanism of ABs [9]; however, most of them are based on the physiology of rats [9; 18], and therefore, both the actual coating process and the material involved in the development of human ABs are poorly known.