Polystyrene micro- and nanoparticles cellular uptake, toxicological effects, and exocytosis. Short review of in vitro...

Polystyrene micro- and nanoplastics (MNPs) are widely used model particles in in vitro toxicology and represent a relevant fraction of environmental plastic pollution. Due to their small size and modifiable surface properties, MNPs interact with cellular membranes, undergo internalization, and influence intracellular signaling pathways. Reported biological effects range from oxidative stress and inflammation to minimal or absent toxicity. This review summarizes current in vitro evidence on cellular uptake, toxicological outcomes, and exocytosis of MNPs. Biological responses are strongly modulated by particle size, surface functionalization, aggregation state, concentration, and cell type. While smaller nanoparticles generally show higher uptake efficiency, internalization alone does not predict toxicity. Data indicate that the balance between endocytosis and exocytosis is a key determinant of intracellular effects. Efficient clearance may attenuate toxicity, whereas impaired trafficking and lysosomal accumulation promote organelle stress. Overall, MNPs behave as conditional toxicants in vitro, with their biological effects determined by their physicochemical properties and the cellular context. Standardized particle characterization and combined assessment of uptake and clearance are crucial for improving comparability and evaluating toxicological risk.