Mapping internal exposure and biokinetics of micro-nanoplastics in humans: a scoping review of knowledge and gaps

Abstract

Humans are inevitably exposed to micro-nanoplastics (MNPs) via inhalation and ingestion. While there is an increasing amount of evidence underlying the risk of MNPs on human health, a robust risk assessment of MNPs is currently not available. Biokinetic knowledge of MNPs is limited in availability and needs a comprehensive summary of what is known and unknown. Therefore, this scoping review provides an overview of the reported state of knowledge and gaps in biokinetics – absorption, distribution, metabolism, and excretion (ADME) of MNPs in humans. The literature review was combined with expert interviews to gain deeper insights into the latest research findings and kinetic hypotheses. The combined data was disproportionate between four aspects of ADME. The absorption of MNPs was found dependent on plastic particle size and dosage. Endocytosis and phagocytosis by macrophages were introduced as probable absorption mechanisms. After respiratory or intestinal absorption, MNPs can be distributed to various organs and tissues in the body via the blood circulatory system. The literature and experts considered the metabolism of MNPs in humans unlikely, but there is a limited amount of data to conclude. In vitro assessments reported a potential metabolism of plastic particles. Currently, biokinetic data still lack quantitative absorption and excretion rate, and plastic polymers other than polystyrene are left unexplored. Most importantly, realistic biokinetic data are scarce. Therefore, future MNPs research work should consider conducting experiments to assess a realistic uptake and excretion rate. Researchers should also involve various polymers such as polypropylene, and work to discover probable distribution mechanisms of MNPs.