Systemic translocation of micro- and nanoplastics in humans: Evidence, mechanisms and future perspectives

Background Microplastics and nanoplastics (MNPs) are now widespread environmental contaminants, and polymeric material has been reported in a growing number of human tissues. Ingestion and inhalation are considered the main routes of exposure, yet the processes that govern particle uptake, tissue translocation, systemic distribution, retention, and clearance remain poorly understood. Objective To critically examine the available evidence on MNP translocation and organ biodistribution and to assess the analytical methods currently used to detect and characterize these particles in human tissues. Methods We conducted a critical narrative review using PubMed and Web of Science, complemented by manual screening of relevant reference lists. Particular attention was given to studies published between 2023 and 2026, while earlier landmark and methodological studies were included when necessary. Human observational studies, experimental MNP models, analytical investigations, and non-plastic nanoparticle research were evaluated according to their relevance, directness, analytical reliability, and methodological limitations. Results Polymeric material has been reported in the human placenta, lungs, liver, kidneys, brain, and other biological compartments. These findings indicate the presence and broad distribution of MNP-associated material in the human body and suggest possible systemic translocation, though the exact physiological pathways remain unconfirmed. Evidence from experimental models suggests that translocation may involve M-cell-mediated transport, endocytic uptake, vesicular trafficking, and changes in barrier integrity. These processes are strongly influenced by particle size, surface characteristics, exposure conditions, and the biological model studied. Vibrational spectroscopy, thermal analysis, and high-resolution imaging provide complementary information on polymer identity, mass burden, morphology, and tissue localization. However, contamination, matrix interference, incomplete recovery of nanoscale particles, and the absence of standardized analytical protocols continue to limit comparison across studies. Conclusion Current human studies support the occurrence of MNP-associated polymeric material in multiple tissues, whereas most mechanistic evidence for barrier crossing and cellular uptake still comes from experimental models. The routes, toxicokinetics, and clinical implications of MNP distribution in humans remain uncertain. Progress in this field will depend on standardized analytical workflows, rigorous contamination control, orthogonal particle confirmation, and larger, well-characterized human studies.