**2. Causes of malignant mesothelioma**

#### **2.1 Asbestos exposure**

The main cause of MM is exposure to air-born asbestos [11, 12]. Asbestos is a silicate mineral classified into two major groups—the amphiboles group that are sharp, needle-like fibers including crocidolite (known as blue asbestos), amosite (brown asbestos), tremolite, actinolite, and anthophyllite, and the serpentines group that are curly fibers, including chrysotile (known as white asbestos) [12–15]. All asbestos fibers are considered as carcinogenic by the WHO and the International Agent for Research on Cancer (IARC) (group 1) [12, 16]. The latency period varies between 20 and 70 years [17, 18].

Asbestos-related mesothelioma cases vary by gender, anatomical region, fiber type, and occupation [19–21]. Most pleural MMs in males are caused by occupational amphibole asbestos exposure. From 2 to 18% of those who were heavily occupationally exposed to amphibole, have developed pleural MM. While the incidence of pleural MM among those occupationally exposed to chrysotile ranges from 0% to 0.47% [19]. Peritoneal MM cases have been reported in those with commercial exposure to amphibole asbestos [22]. However, more recent studies reported that almost 50% of persons with peritoneal MM have fiber load within control values indicating a possible other cause in these tumors [19]. Few studies referred to pericardial or testicular MM and their data did not support the role of asbestos in these sites [23].

The risk of developing MM is related to the type of fiber, the severity, and the duration of exposure [17, 24]. The carcinogenic potency of mineral fibers is determined by the dimensions, durability, dose, and physical properties. Bioavailability after inhalation is affected by fiber dimensions, durability, and dose. Long and thin fibers are associated with higher cytotoxicity and mutagenesis [17, 25]. The WHO distinguished asbestos fibers in short asbestos fibers (SAF) with length < 5 μm and

long asbestos fibers (LAF) with length > 5 μm, diameter < 3 μm, and length/diameter ratio > 3 [11, 26]. The longer asbestos fibers, the more carcinogenic potential [26]. Furthermore, fiber biopersistence influences tumorigenesis. The shorter biopersistence, the lower carcinogenic potential as observed in serpentine chrysotile compared with amphiboles and erionite [17]. However, if the exposure to short biopersistence fibers, such as chrysotile, is prolonged, the mesothelial cells could be transformed [27]. MM from occupational asbestos exposure is mainly caused by crocidolite and amosite fibers [28, 29].

The mass mining of asbestos began in the twentieth century and it was mainly used for insulation against heat, fire, and corrosion, while its previous use was in pottery [12]. Thus, high-risk occupations include engineers who work on brake and clutch lining, builders, dockyard and shipyard workers, plumbers, and electricians [4].

Asbestos exposure can occur mainly occupationally for asbestos workers and nonoccupationally including domestic, neighborhood, or environmental exposure [4, 12, 30]. The risk of developing pleural MM after nonoccupational exposure depends on the types of fibers. The risk is greater to amphiboles exposure than to chrysotile [11]. However, it is complicated to establish accurately asbestos exposure. There is unquestionable asbestos exposure in asbestos miners and shipyards workers. Other categories of workers may not correctly remember events that occurred 30–50 years earlier, as the latency period is too long. Specific questionnaires were developed to identify different levels of exposure within occupational asbestos exposure individuals. Another evidence of asbestos exposure is the measurement of lung tissue fiber content, but this is rarely performed as it is invasive, costly, and for legal implications [11, 12]. The combination of a complete occupational history and radiological evidence of exposure, such as bilateral calcified pleural plaques and/or histological evidence of asbestos fibers in lung tissue, could be used to estimate asbestos exposure. For example, pleural plaques were found in 88% of asbestos exposure patients with mesothelioma [11, 29].

Asbestos use was prohibited in most western countries between 1970 and 2005, except for the USA, where it was only partly banned, and Canada, where the asbestos ban was implemented in 2018 [10, 31]. These countries represent 16% of the world's population [12]. Unfortunately, in developing countries, asbestos use and mining are ongoing with an annual worldwide production of about 2.2 million metric tons [32]. Hence, the incidence of MM will continue to increase worldwide [12].

### *2.1.1 Carcinogenic mechanisms of asbestos*

When asbestos and other fibers enter the pleura and peritoneum via lymphatics, they reside there for months or years, triggering a chronic inflammatory response stimulated by high mobility group protein B1 (HMGB1) secretion and related inflammasome activation in mesothelial cells, which activates the nuclear factor kappalight-chain-enhancer of activated B cells (NF-kB) and phosphatidylinositol 3-kinase (PI3K) [33–36]. This environment promotes the proliferation of mesothelial cells that have spontaneously acquired mutations or are exposed to mutagenic reactive oxygen species generated by inflammatory cells in the area of asbestos deposits [34, 35]. Asbestos-activated macrophages produce reactive oxygen species (ROS), which can cause DNA damage by forming 8-hydroxy-2-deoxyguanosine (8-OHDG) adducts [34]. Ferroptosis, a non-apoptotic, iron-dependent cell death, has recently been linked to asbestos-related carcinogenesis [37]. Hepatocyte growth factor (HGF) may also play

a role in asbestos-induced carcinogenesis by activating the PI3K/MEK5/Fra-1 axis (phosphatidylinositol 3-kinase/mitogen/extracellular signal-regulated kinase kinases 5/(Fos-related antigen 1) [38]. Crocidolite and erionite have a longer biopersistence than chrysotile, which explains their greater pathogenicity [35].

Asbestos fibers are phagocytosed by human mesothelial cells, and once inside the cell, they can mechanically interfere with the cell spindle during mitosis, causing chromosomal mutations responsible for carcinogenesis, but this hypothesis has been ruled out [11, 39].

The carcinogenesis mechanism in pleural MM is complex. Inhaled asbestos fibers move to the pleura. Fibers in the pleural space irritate the tissue, resulting in a cycle of tissue injury and repair. When asbestos fibers are phagocytosed by macrophages, oxygen-free radicals are produced, causing intracellular DNA damage and aberrant repair [40]. Asbestos fibers also enter mesothelial cells, interfering with mitosis, causing DNA mutations, and changing chromosome structure. Inflammatory cytokines are released by asbestos-exposed mesothelial cells, including tumor growth factor, platelet-derived growth factor, and vascular endothelial growth factor (VEGF) [40]. This creates an ideal environment for tumor development. Finally, asbestos increases the expression of proto-oncogenes and promotes aberrant cellular proliferation by phosphorylating different protein kinases (mitogen-activated protein and extracellular signal-regulated kinases 1 and 2) [27]. Asbestos fibers are known to cause DNA damage, which is repaired by homologous recombination (HR) and double-strand break repair, mismatch repair system (MMR), and nonhomologous end-joining (NHE) or nucleotide excision repair (NER), putting people with DNA repair faults at a higher risk of developing MM [1, 41–44].

#### **2.2 Erionite**

Erionite is a fibrous type of zeolite and according to its physical characteristics, it resembles amphiboles amosite or crocidolite [19, 45]. Chemically, it consists of potassium aluminum silicate with various amounts of calcium and sodium [19]. Deposits of erionite have been described in the Cappadocian region of Turkey, in the intermountain west of the United States from Oregon into Mexico and the Sierra Madre region [46, 47]. High amounts of airborne erionite were found in North Dakota, where miles of roads were surfaced with erionite-containing gravel [48]. Also, erionite has been identified in North-Eastern Italy [49].

Studies have shown that erionite is a carcinogenic fiber that causes the MM epidemic in some Cappadocian villages in Turkey [48, 50]. There, erionite was used to build houses and pave roads. Environmental exposure to erionite fibers was documented not only in Cappadocian of Turkey but also in Mexico, North Dakota, Nevada, and California [46–48, 51]. More specifically, in North Dakota erionite has been used to pave roads, in Nevada referred exposure to asbestos, erionite, and other types of fibers, and in California referred exposure to chrysotile and tremolite [48, 51, 52]. These fibers get released into the air due to human activities, such as mining, road construction, and off-road driving. Environmental exposure often begins at birth and occurs randomly among males and females. That is why MM caused by environmental exposure tends to occur at a younger age < 55 years with a 1:1 male: female, contrary to MM caused by occupational asbestos exposure that has a ratio 3:1 [53]. Mineralogical and pedigree analyses in villages in Cappadocian of Turkey have revealed that in addition to environmental exposure there may also be an autosomal dominant genetic susceptibility to MM [50, 54]. Middle-aged patients

*Causes and Pathogenesis of Malignant Mesothelioma DOI: http://dx.doi.org/10.5772/intechopen.103669*

diagnosed with mesothelioma in North America reported living in Mexico at a young age and having emigrated. Their fiber burden analysis in lung tissue demonstrated a high aspect ratio of erionite fibers, which existed in high concentrations in Mexico [46, 55]. Experimental animal studies have confirmed the high carcinogenic potential of erionite [56, 57]. Erionite also could cause other disorders, such as mesothelial hyperplasia, dysplasia, and pleural fibrosis [56]. The way erionite is thought to cause MM is by activation of the NLRP3 (NOD-, LRR- and pyrin domain-containing protein 3) inflammasome, which in turn triggers an autocrine feedback loop in mesothelial cells, modulated by the interleukin-1 receptor [58].

#### **2.3 Fluoro-edenite**

Fluoro-edenite has similar morphology and composition to actinolite and tremolite. This mineral was extracted from an area located at Southeast of Biancavilla in Catania, Eastern Sicily, Italy. It was used as a building material, for road paving, for residential and commercial plaster, and also for mortar construction [19]. A study showed a 10-fold increase in pleural neoplasms among those exposed to fluoroedenite [59]. An experimental animal study revealed that when fluoro-edenite was implanted in peritoneal cavities of rats, MM was induced [60]. Fluoro-edenite was classified as Group 1 carcinogenic to humans [61]. According to studies *in vitro*, fluoro-edenite induces DNA damage and leads to the production of reactive oxygen species (ROS) resulting in decreased cell viability [62].

#### **2.4 Balangeroite**

Balangeroite is a fibrous iron-rich magnesium silicate and it is often intergrown with chrysotile deposits. Deposits have been found in Balangero in Italy, after which it is named. It has a similar morphology but lower bio-durability than commercial amphiboles [63, 64]. The role of these amphibole mineral fibers in the induction of MM in Balangero in Italy is controversial with some authors attributing MMs to balangeroite and others blurring its precise role [64, 65]. The controversy arises from the fact that some Balangero chrysotile miners have commercial amphiboles (crocidolite and amosite) in the mineral analysis of lung tissue. Also, it was known that the Balangero mines occasionally milled imported commercial amphibole from South Africa [63, 65].

#### **2.5 Other minerals**

Other minerals include man-made vitreous fibers, such as rock wool, slag wool, glass fiber and glass filament. All mineral wools are formed by spinning or drawing molten mineral or rock materials, such as slag and ceramics. These are applied to thermal insulation, filtration, soundproofing, and hydroponic medium [66]. The more biopersistent man-made vitreous fibers were classified by IARC (International Agency for Research on Cancer) as "possible carcinogenic to humans" (group 2b) [66]. More biopersistent refractory ceramic fibers have been linked to the induction of MM in Syrian golden hamsters exposed to high-dose chronic inhalation experiments [67]. Some case reports of MM have been related to beryllium, nickel, and crystalline silica, but these data have not been supported by epidemiological studies [19]. There is an increased risk of pleural MM for those exposed to both asbestos and mineral wool or silica according to one study [68].

#### **2.6 Simian Virus 40 (SV40)**

SV40 is a DNA polyomavirus that has been reported as a possible etiologic agent for human MM [69]. Human exposure to SV40 occurred between 1955 and 1963 when inactivated and live anti-polio vaccines were administrated to people in the United States, Canada, Europe, Asia, and Africa [70].

SV40 sequences have been found by the polymerase chain reaction (PCR) method in various human cancers, such as MM, non-Hodgkin lymphoma, osteosarcoma, and thyroid tumors [71]. There are data available on the role of SV40 in the pathogenesis of human MM. For example, this virus activates genes promoting cell progression and proliferation, it induces apoptosis of mesothelial cells transfected with antisense DNA to the SV40, and MM harboring SV40 has a poorer prognosis compared to SV40 negative MM [69]. Mesothelial cells are susceptible to infection and transformation by SV40 [72]. The viral genome encodes oncogenic proteins like large T-antigen (Tag), which inactivate the tumor suppressor activity of p53 and p-retinoblastoma family proteins. However, the presence of SV40 DNA and protein in MM has not led to the definitive causal relationship between the virus and MM development [73]. According to some researchers, SV40 in humans may be a passenger virus in the mesothelial cells without causing pathology or tumorigenesis [69]. Overall, the role of SV40 as an etiologic agent in human MM is still in debate.

#### **2.7 Radiation**

Ionizing radiation is a high-risk factor for malignancy development. Its effect is cumulative, so once received, the effects remain in the body for life. Individuals with increased levels of exposure to ionizing radiation have a greater risk of malignancies later in their life [74]. The carcinogenic risk associated with exposure to ionizing radiation has been evaluated previously in the IARC monographs [66].

The evidence linking radiation to MM in humans comes—(i) from clinical studies involving patients who had previously received radiotherapy for tumors, (ii) from reported cases of MM occurring after the use of the contrast agent thorotrast, and (iii) from studies of workers exposed to prolonged lower levels of radiation [19]. Cases of pleural, peritoneal, and pericardial mesothelioma have been reported after radiotherapy in childhood or adulthood due to lymphoma, genital, renal, and breast neoplasms [75, 76]. The radiation-induced MMs had a latent period from 5 to 50 years and an equal male: female ratio [77]. The intravenous thorotrast administration has caused not only MM but also hepatocellular carcinoma, hemangioendothelioma, and cholangiocarcinoma. The radioactive Thorotrast (232ThO2) is insoluble and after injection, deposits in organs and is associated with slow decay and prolonged alpha-ray emission [19]. Cases of MM have been reported in radiation technologists and among workers in the atomic energy industry [78]. A genetic analysis study has shown that radiation-induced MMs have copy number gains outnumbering deletions, which are more common in asbestos-induced MMs, signifying potential different molecular mechanisms of induction [79]. Overall, radiation is a risk factor to MM in directly irradiated tissues and to a lesser extent in tissues distant from the target site.

#### **2.8 Chronic inflammation and MM**

Chronic serosal membranes inflammations can induce MM of pleura and peritoneum [80]. Therapeutic plombage used as a treatment for pulmonary tuberculosis

*Causes and Pathogenesis of Malignant Mesothelioma DOI: http://dx.doi.org/10.5772/intechopen.103669*

and longstanding chronic empyema could induce pleural MM. Moreover, recurrent peritonitis as a result of relapsing diverticulitis or Crohn's disease or Familial Mediterranean Fever, ventriculoperitoneal shunts for hydrocephaly have been reported as a cause of peritoneal MM [80, 81]. Chronic interleukin-6 production has been linked to MM pathogenesis as a regulatory cytokine in the acute phase response [19].

#### **2.9 BAP1 (BRCA-1-associated protein 1)**

Recently, many researchers are concerned with the role of BAP1 in mesothelioma. BAP1 is a nuclear protein, which is encoded by a tumor suppressor gene (BAP1 gene) located on chromosome 3p21.1 [10, 82]. BAP1 was discovered in 1998 as a novel ubiquitin carboxyl-terminal hydrolase, an enzyme responsible for removing ubiquitin from protein substrates [83]. BAP1 is binding to BRCA1 enhancing its tumorsuppressive activity [7, 83]. Also, BAP1 regulates proteins involved in DNA damage repair, cellular differentiation, chromatin modulation, cell cycle control and cell proliferation, immune regulation, and consequently, it has a tumor-suppressive effect [7]. BAP1 is a nuclear protein that belongs to a family of multiprotein transcriptional regulators that control genes related to metabolism, mitochondrial function, and cell proliferation [33, 37]. The identification of BAP1 as a key regulator of cell death and metabolism aided in the description of the complex set of molecular events mediated by asbestos carcinogenesis [84].

Clinical reports have shown that BAP1 is commonly lost or inactivated in various cancers [85]. An increase in the spontaneous development of breast cancer, lung, ovarian and a few cases of MM that are not related to asbestos in about half of mice with genetically engineered BAP1 mutations that match those found in BAP1 cancer syndrome families supports the idea that BAP1 is a tumor suppressor [86]. BAP1 mutations occur in a wide range of people. According to a study, BAP1 germline mutations were found in 7.7% of spontaneous MM cases [87]. It is suggested that germline mutations in BAP1 are thought to start with just one abnormal allele. Lowlevel asbestos exposure resulted in second allele mutations in genetically vulnerable hosts, resulting in the development of MM linked to BAP1 [88]. Experiments with animal subjects have backed up the aforementioned theory. Experiments in BAP1+/− mice revealed that after intraperitoneal injection of crocidolite asbestos, animals developed MM at twice the rate of wild-type mice, while no MMs were observed in BAP+/− mice not exposed to asbestos [89]. Other researchers discovered that BAP1 knockout mice developed MM without ever having been exposed to asbestos [86]. Another study presented that none of the patients with MM and gene mutations reported occupational asbestos exposure, highlighting that these tumors were either due to low levels of environmental exposure or not due to exposure to carcinogenic fibers [90]. The same study showed that most patients were female and almost half of the tumors were located in the peritoneum, arguing that they were not related to asbestos exposure, as if the cause was both asbestos exposure and genetic predisposition, then the male: female ratio would be maintained and most tumors would be located in the pleura [90].

Taking into consideration the functional role of BAP1 in many cellular pathways implicated in cancer, it is not surprising that the BAP1 gene is mutated in a variety of tumors [85]. BAP1 mutations observed in cancer are primarily inactivating mutations, such as chromosomal deletions of the BAP1 gene, leading to loss of function. BAP1 mutations occur in both germline and somatic forms [7, 85]. Germline

mutations of the BAP1 gene are inherited in an autosomal dominant pattern and constitute a novel tumor predisposition syndrome (BAP1-TPDS) conferring a high risk of hereditary cancers [87, 91]. The cancers associated with this syndrome are MM, uveal or cutaneous melanoma, and renal cell carcinoma [92]. MM is the second most common cancer identified with BAP1-TPDS accounting for 22% of tumors with a median age of onset of 46 years and a seven-fold longer survival rate compared to a patient with sporadic MM [90]. Somatic BAP1 mutations appear in similar types of tumors as in patients with germline mutations. A total of 50% of MM patients were found to have somatic BAP1 mutations and interestingly, they show significantly longer survival than those without mutations on BAP1 [93, 94]. Around 21–63% of MM patients have BAP1 mutations (germline or somatic), and 22% of those who have BAP1 mutations will develop MM. BAP1 genetic mutations are normally present in all cells with one mutant allele, whereas somatic inactivation of a second allele causes cancer [95, 96]. For BAP1 mutation carriers, the gene–environmental interaction is thought to play a key role in cancer susceptibility [95]. In the general population, BAP1 mutations are uncommon, and there are no homozygotes [97]. In distinct cases, their prevalence has been reported to be 1–2% for uveal melanoma, 0.5% for cutaneous melanoma, and 0–7% for MM, rising to 25%, 0.7%, and 20%, respectively, in family cases [43, 44]. In MMs and other malignancies linked to BAP1-TPDS, tumor aggressiveness varies greatly, and the underlying genetic processes are unknown [10].

#### **2.10 Other genes linked to MM pathogenesis**

A handful of the genes implicated in chromatin regulation are mutated in MM patients. Some genes [CDKN2A (cyclin-dependent kinase inhibitor 2A gene), TMEM127 (transmembrane protein 127 gene)] encode tumor suppressor proteins involved in cell growth, proliferation, and survival. Other genes, such as NF2 (neurofibromin 2), encode proteins that modulate signaling pathways for modulating cell shape, cell growth, and cell adhesion [10, 98]. Other genes, such as KDR (kinase insert domain receptor), encode vascular endothelial growth factor (VEGF) receptors that increase endothelial cell proliferation, survival, migration, and differentiation [99]. The pathophysiological mechanisms underlying the development of MM as a result of these genetic mutations are still unknown.

Many studies highlight different clinical parameters that can predict the presence of an inherited mutation in MM, such as minimal asbestos exposure, peritoneal disease, young age, and a second cancer diagnosis [42–44]. This finding is significant because it could lead to the development of clinical panel-based genetic testing and the adoption of clinical genetic testing recommendations. For MM patients, genetic testing would be extremely beneficial because it would allow for the early detection and prevention of malignancies in high-risk individuals. The earlier cancers are detected and treated, the better the chances of survival [10].

According to comprehensive Genome-Wide Association Studies (GWAS) on MM, the most significant SNP (single nuclear polymorphisms) were found in genes involved in cell adhesion, migration, and apoptosis, and may promote carcinogenesis via mechanisms triggered by the human immune system's response to asbestos fibers [100]. Reactive oxygen species (ROS) and free radicals, which arise as a result of inhaled asbestos fiber, are thought to have a role in asbestos toxicity and carcinogenicity. Genetic polymorphisms in detoxification genes encode proteins that are involved in the detoxification and clearance of ROS or change enzyme function, which may increase cancer risk. Furthermore, genetic polymorphisms in DNA repair genes result *Causes and Pathogenesis of Malignant Mesothelioma DOI: http://dx.doi.org/10.5772/intechopen.103669*

in a deficiency in DNA repair pathways, which fail to defend against the oxidative stress generated by asbestos fibers, ultimately leading to an increased risk of carcinogenesis [101]. Individuals who were homozygotes or heterozygotes in one of four DNA repair genes were more likely to develop pleural MM than controls [10, 99, 101].

Reduced expression of critical molecules in the p53 tumor-suppressor gene pathway, such as p14, p16, and NF2-MERLIN (Moesin-ezrin-radixin-like protein), has been discovered as a result of genetic profiling of pleural MM [27, 102]. Other genes, such as BRCA-associated protein 1 (BAP1), set domain containing 2 (SETD2), unc-like autophagy activating kinase (ULAK2), ryanodine receptor 2 (RR2), cilia and flagella associated protein 45 (CFAP45), and set domain bifurcated 1 (SETDB1), have been shown to have deletions or loss mutations in pleural MM [17].

More research is needed to provide a full picture of the genes that predispose to mesothelioma and their role in the molecular pathways of asbestos carcinogenesis that have been revealed, such as chronic inflammation and altered metabolism.

#### **2.11 The role of genes and environment**

Carcinogenesis is frequently linked to somatic gene changes that disrupt DNA repair systems, resulting in an accumulation of DNA damage and an increase in the proportion of cells with damaged DNA. Cancer could arise if these cells kept their survival mechanisms. Inherited mutations affecting DNA repair and other genes may exacerbate carcinogenesis by increasing vulnerability to environmental carcinogens [11, 33]. In the subject of carcinogens, the current method is to explore genes and environment interactions by combining genetic and environmental investigations.

#### **2.12 Carbon nanotubes and mesothelioma**

Carbon nanotubes are one-dimensional fibrous nanomaterials that resemble asbestos fibers in their physical properties. In 2014, WHO and IARC (International Agency for Research on Cancer) classified long, rigid multiwall carbon nanotubes as group 2B possibly carcinogenic for humans [61, 103]. Only one type of carbon nanotube was categorized in group 2B-possibly carcinogenic to humans in 2014 and this was a commercial product called "MWCNT-7." (multiwall carbon nanotubes-7) [61, 104, 105]. MWCNT-7 are multiwall carbon nanotubes with a structure comparable to asbestos and are biopersistent [106]. Toxicological investigations in animals showed that some forms of carbon nanotubes can cause MM. In animal experimental models, long, large-diameter, rigid multiwall carbon nanotubes supplied by intraperitoneal or intrascrotal injection or trans-tracheal intrapulmonary spraying were shown to develop MM [106–108]. Longer, rigid multiwall carbon nanotubes translocated to the parietal pleura, causing more inflammation, fibrosis, and localized mesothelial cell proliferation than shorter, thinner agglomerates [89]. Many animal experimental studies have shown similar findings [106, 109]. Asbestos fibers and multiwall carbon nanotubes have physicochemical similarities and differences [103]. The "fiber pathogenicity paradigm" identifies width, length, biopersistence, and mechanical bending stiffness as predictors of pathogenicity and carcinogenicity in carbon nanotubes, or other fibrous nanomaterials, and metallic nanowires [110].

Near the end of the twentieth century, carbon nanotubes were found and manufactured. Composite materials, thin coatings and films, microelectronics, energy storage, environmental remediation, and nanomedicine are all areas where they are used [111]. The Royal Society and Royal Academy of Engineering acknowledged

the physical similarities between carbon nanotubes and asbestos fibers in 2004, as well as the potential for human health hazards [112]. Chemical vapor deposition (CVD) is the most common industrial method for producing carbon nanotubes, with transition metals catalyzing the breakdown of a carbon-containing organic vapor. Carbon nanotubes can be discharged as dry powders during the manufacturing and processing phases [110]. The National Institute of Occupational Safety and Health (NIOSH) in the United States has set a recommended exposure limit of 1 μg/m3 [113]. To prevent repeating the history of asbestos-related disorders, the ultimate goal is to commercialize nanomaterials while simultaneously considering potential human health risks [103].

### *2.12.1 Pathogenicity of carbon nanotubes*

Some forms of carbon nanotubes resemble asbestos fibers in length and rigidity. Long, thin, biopersistent fibers are thought to enter the pleural space, obstruct clearance through lymphatic stomata on the parietal pleura, and cause frustrated phagocytosis, oxidant generation, and persistent inflammation, ultimately leading to MM [110]. Furthermore, the hydrophobicity of raw carbon nanotubes hampered their dispersion in biological conditions, causing them to clump together in rope-like formations or tangled clumps. These clusters clump together to create discrete multifocal granulomas, which comprise macrophage and fibroblast aggregates [114, 115]. Carbon nanotubes were also found in lymph nodes in the mediastinum. Individual carbon nanotubes may be gradually released from pulmonary agglomerates over time and translocated to the lung interstitium, pleura, and regional lymph nodes. Also, many researchers discovered that inhaling well-aerosolized single or multi-walled carbon nanotubes caused persistent inflammation and fibrosis [106]. Very thin carbon nanotubes are more prone to form tangled agglomerates than thicker, rigid multiwall nanotubes. DNA damage can be caused by short multiwall carbon nanotubes with a length of 1 μm [103, 116].
