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There are a large number of microplastic (MPs) in the sea or on land, most of which undergo physical, chemical or biological processes leading to the release of dissolved organic matter (DOM). In this study, we analyzed the change of Polyurethane microplastic (PU-MPs) valence bond under different aging conditions thanks to Fourier transform infrared spectroscopy (FTIR) and its surface characteristics using scanning electron microscopy (SEM) and also described the characteristics of DOM dissolved from PU (PU-DOM) under UV aging process in two different medium (water and air), based on Dissolved organic carbon (DOC) measurements, UV-visible spectrometer and Three-dimensional excitation emission matrices (3D-EEMs). The DOC data both showed that Under UV aging of different systems, PU-DOM concentration increases with the extension of aging time, and correspondingly, its toxicity to human colon adenocarcinoma cells also increases, but the release amount of PU-DOM under air aging is higher than that of PU-DOM in water. We speculate that it may be the refraction and scattering of water, which leads to the reduction of the intensity of UV radiation. 3D-EEMs identified tryptophan-like fluorescent component and tyrosine-like component, meanwhile, the liquid chromatography-mass spectrometer (LC-MS) data further confirmed the formation of acid substances. The results further confirmed that the composition of PU-DOM in different systems is the same, but the release amount is different. The contents of the produced conjugated carbonyls and Reactive oxygen species (ROS) because of light irradiation increased likewise. The cytotoxicity of PU-DOM was consistent with the changing trend of ROS level in PU-MPs, suggesting that the produced ROS induced the in vitro toxicities. The results not only highlight the adverse health effects of photoaged PU-MPs, but also provide new perspectives for the environmental risks of MPs., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2022 Elsevier B.V. All rights reserved.)