4.6. Inhalation of microplastics

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Micro/nanoplastics can enter the human body not only orally and through the skin, but also through inhalation. The microplastic content of the air depends on the size of the surrounding population, human activities, and environmental factors. The main source of airborne microplastic content is microfibres from synthetic textiles, but after decomposition, larger plastic particles from other sources are also released into the air by the force of the wind. In large cities, tire wear from traffic also contributes to microplastic pollution. Currently, there are limited data on concentrations of microplastic particles measured in the air and comparing them is difficult due to differences in sampling and analysis methods. On average, the concentration of microplastics in air varies between 0.3 and 1.5 particles/m3 outdoors and between 0.4 and 59.5 particles/m3 indoors [16]. Indoor air microplastic levels are often much higher than outdoor air levels. In homes, microplastics are generated by the wear and tear of upholstered furniture, cleaning, and everyday activities, which are less diluted in the air by ventilation than outdoors. In terms of the shape of microplastic particles found in the air, fibers and fragments are the most common. Children, since more particles are found near the ground, and workers in the polymer industry are more exposed to inhalation of microplastics [145]. Size and concentration are critical in assessing the health effects of inhaled particles, while deposition in the lungs is proportional to the aerodynamic equivalent diameter, so shape can also be an important factor [81], [108]. In general, the smaller the inhaled micro/nanoplastic particles and the higher their concentration, the more likely the occurrence of a toxic effect. In addition, the duration of exposure also determines the severity and type of the induced biological response [81]. As for the shape, thinner fibers are more likely to enter the lungs, and it has been observed that particles with sharper surfaces are less deposited [74]. After inhalation, some of the microplastic particles may be removed by the clearance mechanisms of the respiratory system (sneezing, mucociliary clearance, phagocytosis by macrophages, and lymphatic transport) [145]. Although most particles cannot reach the alveoli of the lungs due to the mucociliary clearance mechanism, particles with a lower density and size of less than 10 µm more easily reach the lower airways, where they are deposited [110]. However, the particles that do not deposit in this manner and are removed from the respiratory tract enter the gastrointestinal tract via the pharynx. After inhalation, particles that bypass mucociliary clearance may reach lung epithelial and endothelial cells by diffusion, direct cellular penetration, or active cellular uptake. Fibers enter cells by endocytosis, and their removal by clearance is more difficult [81]. Micro/nanoplastic particles accumulated in the alveoli cause oxidative stress, cytotoxicity, activate the immune system, and cause local chronic inflammation, which in the long term can lead to the development of malignant processes [16], [81], [105]. Due to increased endothelial and epithelial permeability during inflammation, microplastic particles are more readily absorbed into the systemic circulation (typically particles smaller than 1 µm) [81].

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Two-thirds of workers in the polymer industry have already experienced respiratory symptoms [145]. Throat irritation, cough, shortness of breath, decreased lung capacity, allergic reactions, asthma, and increased mucus production have been observed in some workers in synthetic textile mills as a result of long-term exposure from inhalation of synthetic particles. Histopathological changes such as interstitial fibrosis and the presence of granulomas containing foreign bodies have been described in workers in the synthetic industry chronically exposed to inhalation of microplastics [74], [81]. Among workers in the synthetic textile industry, the inflammatory lung disease caused by inhalation of small fibers and associated with cough and dyspnea also became known as “flock worker’s disease”. The disease was first identified as an occupational risk in the 1990s, but its exact cause was recognized later [146]. Since then, workers in such factories have had to comply with much stricter regulations regarding protective equipment. Among workers in the synthetic textile industry, the incidence of lung cancer is also higher [108]. In terms of adverse health effects, not only the local immune reaction caused by the particle or the consequences of particle absorption, but also the toxic effect of organic pollutants (e.g. polycyclic aromatic hydrocarbons) or heavy metals adhering to the inhaled microplastic particles, as well as respiratory infections caused by pathogenic microorganisms bound to the surface of microplastics, may be a problem [145].
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