The probability of an organism ingesting microplastics is determined by factors such as the size, density, shape, and color of the microplastic, the degree of erosion, and the type, size, and feeding habits of the organism. [44]. Animals tend to ingest particles similar in size and color to their natural food. Lower density PE and PP particles float on the surface and are ingested by pelagic animals, while higher density PVC and PET accumulate in the sediment on the seafloor, making them more likely to be ingested by deep-sea animals [56], [81], [100]. It has been observed that zebrafish (Danio rerio) are more likely to ingest microplastics when a significant amount of other organic matter is present. If only microplastics or few other pollutants are present, zebrafish can distinguish them and refrain from ingesting them [102]. Heteroaggregation of micro/nanoplastic particles with natural nanoparticles is more likely with charged particles. Such aggregates are more easily ingested by organisms because they mistake them for their natural food [44]. Aging of microplastic particles also promotes biofilm formation. Some marine animals ingest biofilm-coated microplastic particles more readily compared to non-biofilm-coated ones. However, smaller animals are less likely to ingest them due to their larger size. During aging, the color of the microplastic particle may also change (e.g. it may become yellow), which could further facilitate ingestion in certain instances [106]. Microplastics can be ingested either directly by organisms or indirectly through the food chain [44], [107]. Typically, nanoparticles can enter higher trophic level organisms through the food chain. Plastic particles were detected in the brains of fish fed with water fleas that had ingested nanoplastic particles previously [104]. The study demonstrated that nanoplastic particles were able to cross the blood-brain barrier of fish. Moreover, the presence of these particles as foreign bodies in the brains of fish caused behavioral disorders in the animals [104]. Through this indirect route, micro/nanoplastics can enter even the human body via seafood. With respect to seafood, it is believed that organisms not consumed whole, such as fish, do not pose such a risk, as microplastics mostly accumulate in their digestive tracts, which are generally not consumed. However, mussels, crabs, oysters, and other mollusks are consumed whole [103]. At the same time, smaller microplastics can be absorbed and distributed to various tissues through the bloodstream of the organism, potentially being consumed by humans [100]. Although to a lesser extent, microplastics adhering to the skin of fish can also cause problems when consumed [74].

After ingestion, larger particles can cause not only physical damage, such as ulcers and ruptures, but also blockages in the digestive tract, which can lead to starvation, decreased digestive efficiency, reduced development, impaired movement and reproduction, or even death [100], [101]. Some of the microplastic particles that enter the living organism, especially the larger ones, are usually excreted into the feces, while particles smaller than 10-20 µm can be absorbed by the gastrointestinal tract. Due to their size, nanoparticles can penetrate the intestinal epithelium more easily. After translocation across the gut-blood barrier, nanoparticles can reach the circulation [98]. Interestingly, oral exposure to PS nanoparticles, resulted in increased iron absorption in chicken, suggesting that the presence of nanoplastics may affect the barrier function of intestinal epithelial cells [98]. In the long term, alterations in the properties of the gut epithelium may also lead to problems in the absorption of other nutrients and vitamins. Chickens exposed to nanoplastics chronically were found to have an increased blood clotting time, attributed to a vitamin K deficiency [107]. After absorption, particles smaller than 1 µm can accumulate in certain organs via the bloodstream [103]. The accumulation of plastic particles smaller than 10 µm has been observed in the liver, gills, and lymphatic system of fish. Microplastics have also been detected in earthworms, as well as in the intestines, stomachs, and feces of seabirds and chickens [105], [107]. In dogs, PVC particles of 5-110 µm have been found in the portal vein, indicating their passage to the liver [98]. Based on animal study data, after absorption, microplastics are excreted in bile, urine, cerebrospinal fluid, and breast milk. Moreover, PVC nanoparticles can pass through the placenta. [74], [107], [108]. Deposition of PVC granules resulted in embolization of small vessels in animals after chronic oral administration [74]. Microplastic particles ranging from 50–500 µm in size were detected even in human feces, in a small pilot study involving eight participants [109].

Absorption of microplastic particles from the intestine may occur through endocytosis by the Peyer’s patches of the ileum or by persorption through the intestinal wall. Paracellular transport of nanoparticles is also possible through the tight junction structures of the intestinal epithelium (close-fitting cell junction structures that serve as diffusion barriers) [107]. In the digestive tract the so-called protein corona can attach to the plastic particles, which can either impair absorption, or in some cases even facilitate it, and increase toxicity [74], [104], [107]. The smaller the particles are, the easier they enter the systemic circulation and into the organs. The chemical properties and surface charge of the particle also play a pivotal role in absorption; hydrophobic particles are more readily absorbed [44], [74], [97]. In addition, the nature of the surface functional groups also affects absorption [110]. However, the type of absorption depends not only on the size and surface chemistry of the plastic particle, but also on the type of cell. A positively charged particle may be more readily taken up by cells or cause greater toxicity due to binding to negatively charged sugar derivatives on the cell surface, while a negatively charged surface may inhibit endocytosis due to electrostatic repulsion [104].

Humans most commonly meet micro/nanoplastics through drinking water, food (mainly seafood, e.g. in the form of edible mussels), table salt, as well as inhalation and skin contact. Absorption through intact skin is very unlikely due to the barrier function of the stratum corneum. However, certain excipients used in cosmetics, such as urea and glycerin, which soften the skin and enhance the permeability of the stratum corneum (etc.) may facilitate the penetration of nanoparticles through the skin [104]. While we consume relatively small amounts of salt, the resulting microplastic load may not be as significant. However, a notable amount of microplastics can enter the human body through seafood, air, and drinking water, especially bottled water [108]. The presence of PET and PE microparticles in bottled water probably results from the degradation of the packaging material (bottle/ cup), but microplastics can also enter bottled water during cleaning, production, and filling. The more often a single-use plastic bottle (PET, HDPE) is opened and closed, the more microplastic particles enter the water from the bottle mouth and cap. More microplastics have been found in water in reusable bottles (older bottles) than in single-use bottles [35], [79]. Despite the significant microplastic pollution present in our immediate environment, little is known about the effects of micro/nanoplastics on human cells. In contrast to the numerous animal models, there are limited in vitro studies conducted on human cell lines and tissues [99].