FIGURE 2.

Potential internalization and clearance mechanisms of microplastics in the lungs. The lung lining fluid (surfactant and mucus) reduces the chance of microplastic displacement. Particles <10 μm are cleared by mucociliary, while particles <1 μm are uptaken through the epithelium and also may penetrate the thin lung lining fluid and contact the epithelium, and circulate and metastasize through diffusion or active cellular uptake. The surface charge and surface-molecular interactions of different MPs affect the immune cell clearance. For macrophage, positive charged MPs and coated-MPs (combined with protein or other substances) are more likely to interact with the cell membrane. Other immune cells are also involved in the defenses against MPs, such as antigen presentation by dendritic cells and the trapping and phagocytosis of MPs through the release of NETs by neutrophils, causing immune activation.