Dermal exposure, review of current knowledge on the uptake of micro-and nano-plastics

Micro- and Nano-plastics (MNPs) represent an emerging concern for human health. While ingestion and inhalation pathways have been extensively studied, limited knowledge exists on how MNPs interact with the skin and their capacity to penetrate deeper skin layers. Although the skin serves as a highly effective barrier, there are concerns that smaller particles may be able to circumvent it under certain conditions. This review evaluated current literature on dermal uptake of MNPs. Methodological approaches for assessing dermal uptake are critically reviewed, including test systems, skin source, and exposure conditions. This is followed by an evaluation of empirical evidence on MNP penetration, considering the influence of particle size, shape, surface chemistry, and polymer composition, with findings from nanomaterial studies used to address key data gaps. Forty-six relevant studies were identified spanning ex vivo human and animal tissues, in vitro skin models, and in vivo animal studies. Most studies focused on fluorescently labelled polystyrene particles, limiting the applicability of findings to environmentally relevant MNPs. Most studies reported retention of MNPs in the outer skin layers (stratum corneum and hair follicles), with limited evidence of penetration into deeper tissues. Isolated studies indicated that particles < 2 μm may penetrate deeper layers; however, methodological inconsistencies prevent establishing definitive size-based thresholds for regulatory purposes. Multiple factors, including particle shape, surface charge, skin condition, and co-exposure with cosmetic or occupational products, appear to influence uptake but remain inconsistently studied. Major gaps persist, including the absence of long-term exposure studies, limited data on vulnerable populations, minimal research on environmentally weathered particles, and a lack of standardised test methods specific to MNPs. While existing guidance provides established frameworks for dermal absorption of chemicals and cosmetic ingredients, they were not developed for particulate MNPs and are infrequently applied or validated for this particle class in the current evidence base. Future research should adopt harmonised, standardised approaches to improve hazard identification, risk assessment, and regulatory decision-making for dermal exposure to MNPs.