Photoaging of Microplastics and Nanoplastics: Physicochemical Transformations, Cellular Responses, and Toxicological Implications.

Microplastics (MPs) and nanoplastics (NPs) are emerging contaminants that are continuously altered by weathering processes, including light exposure, after their release into the environment. This review provides an overview of the physicochemical transformations and associated cytotoxic effects of photoaged MPs/NPs across various biological models. Current evidence indicates that photoaging substantially alters the physicochemical properties of MPs/NPs. These transformations may influence particle dispersion, cellular uptake, and particle-cell interactions. Photoaged MPs/NPs have been associated with reduced cell viability and other cytotoxicity-related changes in various biological models. Although photoaged MPs/NPs generally exhibited greater toxicity than their pristine counterparts, the magnitude and nature of these effects varied considerably across studies, likely reflecting differences in polymer type, photoaging conditions, particle size, exposure scenarios, and biological models. Overall, existing evidence indicates that photoaging can modify, and often enhance, the cytotoxicity of MPs/NPs. Further studies are needed to improve the comparability and environmental relevance of photoaging protocols, implement more comprehensive particle characterization, and elucidate the mechanisms linking photoaging-induced transformations to biological effects.