Health impacts of micro- and nanoplastics in humans: systematic review of in vivo evidence

BACKGROUND: Microplastics and nanoplastics (MNPs) are pervasive environmental contaminants with potential human health implications. Although laboratory models implicate MNPs in oxidative stress, inflammation, and endocrine disruption, a comprehensive synthesis of direct in vivo human evidence is lacking. We aimed to systematically review studies measuring MNPs in living human subjects and summarise associated health findings. METHODS: We systematically searched PubMed, Web of Science, Scopus, Cochrane and Embase through 26 December 2024. Two investigators independently screened and selected original research articles that quantified MNPs in biological samples from living humans. We excluded animal, in vitro, cell-line, and injection-based studies, as well as reports on non-plastic micro- and nanoparticles. Data extraction, performed in duplicate, included study design, participant characteristics, detection methods, polymer types, and reported health outcomes. Methodological quality was appraised using Risk Of Bias in Non-Randomized Studies-of Exposures (ROBINS-E). The primary outcome was the presence and burden of MNPs; secondary outcomes were clinical or biomarker associations. No metaanalysis was performed due to heterogeneity. RESULTS: From 5 522 records, 25 studies met inclusion. Studies employed pyrolysis-gas chromatography/mass spectrometry (n = 9), Raman spectroscopy (n = 8), infrared spectroscopy (n = 7), and Fourier-transform infrared spectroscopy (n = 3), often combined with microscopy for MNP detection. Predominant polymers were polyethylene, polypropylene, polyvinyl chloride, polyethylene terephthalate, and polystyrene. In cardiovascular research (5 studies; n = 454), higher thrombus and plasma MNP burdens correlated with inflammatory markers and adverse cardiac events. Reproductive research (seven studies; n = 327) linked semen and tissue MNPs to reduced sperm quality and accumulation in tumor and placental samples. Gastrointestinal research (9 studies; n = 537) associated fecal MNPs with liver enzyme elevations and gut dysbiosis. Respiratory (3 studies; n = 171) and ocular (1 study; n = 49) research detected MNPs in airway fluids and vitreous humor respectively, with links to airway inflammation and increased intraocular pressure. ROBINS-E assessments indicated moderate to high risk of confounding and exposure-measurement bias; consistency across detection modalities was limited. CONCLUSION: Human in vivo evidence confirms that MNPs accumulate in multiple organ systems and are associated with inflammation and functional impairment. Methodological heterogeneity and bias constrain causal inference. Prospective cohort studies with rigorous exposure assessment and confounder control are needed to advance understanding and guide policy.