*4.1. Gastric Exposure*

Recent studies into microplastics and nanoplastics exposure and toxicity have indicated that the most significant way humans consume plastic particles is via ingestion [57]. While, there are no studies looking specifically at nanoplastic toxicity in humans, there is research showing that microplastics are being ingested through food and drink [36]. The initial analysis of human stool samples showed that plastic particles were being excreted, which supports the theory that humans are ingesting these particles via food and water. These results, along with research into ingestion uptake in environmental models, clearly

show that humans will be regularly consuming microplastics and nanoplastics [58]. However, no studies have ye<sup>t</sup> investigated what happens to the micro- and nanoplastic particles once they enter the gastrointestinal (GI) tract. It would be pertinent to examine their route through the GI tract and whether particles remain in the gu<sup>t</sup> lumen, or they translocate across the gu<sup>t</sup> epithelia.

It is unlikely that microplastics are able to permeate at a paracellular level as the relevant pores at the tight junction channels have a maximum functional size of approximately 1.5 nm [59]. It is more likely that they enter through lymphatic tissue, and it is particularly possible that they enter via phagocytosis or endocytosis and infiltrate the microfold (M) cells in the Peyer's patches [45]. Following intraperitoneal injections in mice, the peritoneal macrophages were seen to phagocytose 1, 5 and 12 μm polymethacrylate and polystyrene particles [60,61]. Nevertheless, the results indicate that absorption via intestinal tracts in rodent models is low at 0.04–0.3% [61].

The potential of nanoplastics to permeate the gu<sup>t</sup> epithelium, leading to systemic exposure in humans, is a significant issue. Historically, studies have used polystyrene nanoparticles for in vivo and in vitro tests on a variety of animals. The probable oral bioavailability level of 50 nm polystyrene nanoparticles is ten to one hundred times greater than the level of microplastics (2–7%) [62,63]. Similar to the results seen with microplastics, there is no straightforward correlation between the absorption, size and structure of nanoplastics [62]. Previous research has shown that the absorption rates of nanoparticles (50–500 nm) vary greatly across different in vitro intestinal models, with figures of 1.5–10% according to the size and chemical structure of the nanoparticles as well as the type of in vitro model used [62,64,65].

The lumen of the GI tract presents a challenge when researching nanoplastic absorption rates. Once consumed, nanoparticles undergo transformation, and this will impact absorption ability and rates. There are several molecules within the GI tract that nanoparticles may interact with, such as proteins, lipids, carbohydrates, nucleic acids, ions, and water [9]. This then leads to the nanoparticles being encompassed by a collection of proteins known as a 'corona' [66]. Polystyrene nanoparticles may develop into varying forms of complex coronas, according to the conditions they are in [67]. Studies have shown that protein corona changes within an in vitro model representing human digestion, and this leads to greater translocation of nanoparticles [62]. Furthermore, organic matters found in bodies of water will adhere to the surface of nanoparticles; a recent review has examined how dispersed organic materials react with metal (oxide) nanoparticles and determined that these interactions have a significant impact on agglomeration and deposition [68].

It is worth noting that most of the reported studies were based on experiments using polystyrene nanoparticle models, and excluded samples gathered from marine and terrestrial environments. Other plastics such as polypropylene (PP), polyethylene (PE), and polyethylene terephthalate (PET) are, however, the main polymeric materials present in these environments. Thus, it is critical to qualify any extrapolations made from the findings of the research discussed above, which relies solely on polystyrene. Instead, new model studies should include experiments with PP, PE and PET.
