4. Health effects of micro/nanoplastics

Plastics are considered inert materials, however there are various routes through which micro/nanoplastics can cause harm to living organisms. It was already known that aquatic animals can become entangled in the plastic debris present in water bodies, especially in fibers and rings. This entrapment can hinder their movement and feeding, leading to exhaustion, suffocation, or injury, including the development of ulcerated, necrotizing wounds, or arterial blockages [97]. However, the impacts of microplastics in water on the aquatic ecosystem have only been recognized in recent decades. From studies investigating the problems caused by microplastics in aquatic organisms, it can be concluded that the harmful effects partly stem from the physical damage caused by the microplastic particles. In addition, ecotoxicological studies also suggest biological responses caused by the particle itself or by additives, monomers, other pollutants presumably bound to its surface, and pathogenic microorganisms [98], [99], [100], [101]. Toxicity associated with micro/nanoplastics is generally dose-dependent, and the probability and severity of toxic effects are inversely proportional to particle size [102]. However, the effect of microplastics entering the body depends not only on their size, but also on their shape, polymer type, chemical composition, adsorption and desorption properties, and the degree of degradation. Regarding shape, fibers induce more severe inflammation and higher superoxide dismutase enzyme activity in the digestive system of fish than fragments or beads [44]. In addition, fibers are more prone to embedding in tissues and have a longer transit time in the digestive tract of fish [44]. Polymers that make up microplastics show different degrees of toxicity. In a freshwater mussel species (Corbicula fulminea), for example, PVC and PS induced greater histological abnormalities compared to PET or PE [103]. When evaluating the impact of microplastics on living organisms, it is crucial to consider the experimental conditions. Studies investigating the effects of micro/nanoplastics on living organisms usually use the original structure, regular spherical particles that have not yet degraded and do not contain additives. They mostly study PS, PE and PVC micro/nanoparticles and use concentrations of micro/nanoplastics higher than those found in the natural environment. Therefore, the effects experienced under real conditions may differ from those observed inlaboratory settings [81]. When designing experiments, it is also important to consider that plastic particles, especially nanoparticles, are likely to occur in nature as homo- or heteroaggregates formed with other substances. This alters both their physicochemical properties (e.g. sedimentation) and biological activity [104]. Micro/nanoplastics can cause problems not only at the individual level. They can also enter and accumulate in organisms at higher trophic level via the food chain.