Microplastics and human health: Integrating pharmacokinetics

Microplastics are widespread contaminants leading to environmental exposure. While studies on the prevalence in human tissues have multiplied, little is known about their pharmacokinetics. Mechanisms of absorption, distribution, metabolism, and excretion (ADME) must be addressed before effects on human health (i.e. pharmacodynamics) can be understood. Therefore, the objective of this review was to provide an integrated assessment on the fate of microplastics in the human body by gathering information from multiple fields of research (e.g. implants and microspheres). Absorption of microplastics mainly occurs through transcytosis in enterocytes, while larger particles may be internalized through gaps (e.g. persorption) or by uptake by phagocytes. Only microplastics <5 µm may reach the alveolar region, while large particles found in the lungs usually result from entrapment of circulating particles in the narrow pulmonary capillary network. Overall, absorption rates of microplastics are expected to be low. Microplastics are then distributed by the circulatory system, accumulating in the respiratory system, digestive system, liver, spleen, and brain. Metabolism may lead to the biodegradation of microplastics, mainly through enzymes and reactive oxygen species of macrophages, exposure to physiologic fluids, and microbiologic activity in the gut lumen. Finally, most microplastics will be removed by the liver or spleen and excreted in the feces. While this work provides an initial attempt at constructing a pharmacokinetics model for microplastics, further research is required. Ideally, future works should be conducted using histopathology techniques to obtain the precise location in the tissues and radiolabelled particles to allow tracking through time.