# A Systematic Study of the Cryogenic Milling of Chrysotile Asbestos
## 1 Introduction  
The term asbestos refers to a group of six mineral fibres of significant industrial and economic importance that have been widely used since antiquity [1]. Members of the asbestos group are chrysotile (serendine group), and five species of amphiboles: crocidolite (fibrous ribeleckite), amosite (fibrous cummingonite-grurnerite) and the fibrous variety of anthophylllite, actinolite and tremolite [2, 3]. Today, there is enough scientific evidence to state that inhalation of asbestos may induce fatal lung diseases, such as lung cancer, malignant mesothelioma (MM), and asbestos [3, 4]. Accordingly, asbestos is included by the International Agency for Research on Cancer (IARC) in Group 1 as a carcinogen for human [3]. The most insidious asbestos-related disease is undoubtedly MM [5, 6, 7]. This aggressive type of cancer originates from the pleural cavity (i.e., the serous membrane that separate the lungs and the wall of the thoracic cavity) to which asbestos fibres can reach due to their favourable aerodynamic diameter [8, 9]. The carcinogenesis of MM is initiated by the inflammatory response to those fibres that are retained in the pleural cavity, being too long to exit through the stomatal openings in the parietal pleural surface (diameterin the range 3-10 \(\upmu\)m) and migrate to the lymphatic capillary system [5; 7; 8]. Hence, the toxicity potential of long fibres is higher. It should be pointed out that this is only one piece of the puzzle. The complex toxicity mechanisms, being multifactorial and related to different cell-fibre interactions, is not yet fully understood [5; 10]. In addition, there is still debate as to whether or not chrysotile is less potent than amphibole asbestos in causing mesothelial carcinogen in humans. The main reason for which chrysotile is considered less hazardous than the other asbestos minerals is its lower biodurability [11]. The chrysotile fibres are, in fact, rather quickly dissolved in the intracellular macrophage environment in contrast to the amphiboles that remain in the lungs, even for a lifetime [12; 13]. The prochrysotile community therefore sustains that asbestos-induced diseases apparently linked to chrysotile exposure are actually due to contamination by amphibole asbestos [14]. This so-called \"amphibole hypothesis\" is now rejected by most of the scientific community and recently published occupational epidemiology studies assert a clear correlation between chrysotile exposure and increased mortality from MM [15; 16]. Nevertheless, this issue cannot be considered univocally resolved, as only a relatively few studies were aimed at evidencing the dose-response relationship for cancer following exposure to different fibre types in the same exposed population [3; 17]. In this context, the technological advances in the field of _in vitro_ models can provide new and effective approaches for studying asbestos toxicity. However, a confident and unequivocal protocol for making asbestos fibre samples with defined and proper textural characteristics suitable for _in vitro_ testing is still lacking [18]. Following the continuous awareness of health hazards related to asbestos exposure, the exploitation of amphibole asbestos is now globally banned. However, for the reasons discussed above, controlled use of chrysotile is still allowed in ca. 66% of all countries in the world [19]. The 2019 trade data show that the extractions in the Russian Federation (750,000 t/year), Kazakhstan (200,000 t/year), China (125,000 t/year), Brazil (15,000 t/year) and Zimbabwe (2500 t/year) stand for over 99% of the total world production of chrysotile (1,100,000 t/year) (USGS, 2020). The by far most important producer is the Russian Federation that alone stands for ca. 68% of the total [20]. Extraction sites are located in the Urals Mountains.