Divergent responses of human erythrocytes to nano-, micro-, and size-mixed polystyrene particles reveal distinct cellular effects of environmental plastics

Plastic particles have been detected in almost all areas of the human body. Consequently, the impact of nano- and microplastics has been investigated in various cell and animal models, mainly using polystyrene (PS) spheres. The objective of this study was to examine the effects of 100 nm and 1 µm PS spheres on human erythrocytes. Erythrocytes were isolated from human blood and incubated in protein-free medium with 0-100 µg/mL PS particles for up to 20 h. Cell counts and erythrocyte morphology were subsequently assessed by light microscopy. Compared with controls, 100 µg/mL of either 100 nm or 1 µm PS spheres did not alter cell number. In contrast, 100 nm spheres induced echinocyte formation, consistent with hydrophobic membrane interaction. At 1-100 µg/mL, 1 µm spheres showed minimal interaction and no morphological changes. Notably, combined exposure to both sizes induced erythrocyte aggregation at 100 µg/mL, accompanied by additional echinocyte formation. These findings confirm the effects of 100 nm PS nanospheres on erythrocytes but contradict previous reports on 1 µm microspheres, which here exhibited little membrane interaction. Aggregation as a cellular response to size-mixed particles illustrates distinct size-dependent effects under identical conditions. Moreover, these results underline the importance of standardised assay conditions to ensure comparability across studies. Although the applied concentrations exceed those expected from typical environmental exposure, inhaled and ingested particles in real-world settings also vary in size. When differences in shape and polymer composition are additionally considered, the variability of potential cellular responses in vivo may substantially increase.