Formation and Transformation of Micro(nano)plastics: Mechanisms and Environmental Health Implications

Micro-(nano)-plastics (MNPs; <5 mm plastic particles including nanoscale fractions) are increasingly recognized as pervasive environmental pollutants, with risks arising not only from their original emission sources but also from ongoing transformations. This review systematically integrates current evidence on the formation and transformation of MNPs under mechanical, photochemical, thermal, oxidative, and biological processes, highlighting that environmental MNPs are predominantly aged rather than pristine particles. Aging significantly reshapes particle shape, size, surface chemistry, and reactivity, thereby altering mobility, persistence, and interactions with copollutants, biota, and humans. By adopting a sourcetransformationexposurerisk framework, this review clarifies the critical importance of understanding transformation pathways for tracing pollution origins, revealing exposure routes and doses, and improving health risk assessment. Particular attention is directed toward nanoplastics, whose elevated reactivity and bioavailability remain poorly understood due to analytical limitations. Strategic research priorities are identified, including the development of multifactorial aging simulation models, advanced detection and characterization techniques, and lifecycle-oriented mitigation strategies. This work advances a forward-looking perspective that connects mechanistic understanding with risk governance, providing a scientific foundation for more accurate prediction, monitoring, and control of MNP pollution.