The fate of airborne microfibers in the human respiratory tract in different microenvironments

Αirborne microplastics (MPs) are considered an important exposure hazard to humans, especially in the indoor environment. Deposition and clearance of MPs in the human respiratory tract (HRT) was investigated using the ExDoM2 dosimetry model, modified to incorporate the deposition and clearance of MPs fibers. Fiber deposition was calculated via the fiber equivalent aerodynamic diameter determined using their properties such as size, density and dynamic shape factor. Scenario simulations were performed for elongated particles of cylindrical (base) diameters 1 μm and 10 μm and aspect ratios (ratio of fiber length to base diameter) 3, 10 and 100. Modelling results showed that the highest fiber deposition occurred in the extra-thoracic region due to large particles (fiber cylindrical diameter d > 0.1 μm), whereas particle length (via the aspect ratio) had an influence mainly on smaller base-diameter fibers (d < 0.1 μm) that deposited predominantly in the alveolar region. The ExDoM2 dosimetry model was also used to calculate fiber deposition in the HRT using experimental data for microplastic fiber and fragment concentrations in different microenvironments. The highest deposited number dose (220 fibers) after a 24-hour exposure was calculated in the microenvironment (bus) that had the highest fiber concentration (17.3 ± 2.4 fibers/m). After clearance, the majority (66.4 %) of the average deposited fiber mass was transferred from the respiratory tract to the esophagus via mucociliary clearance, 32.6 % was retained in the respiratory tract, 1 % passed into the blood, and a very small amount (0.0004 %) was transferred to the lymph nodes.