## 5 Toxicity of MPs  
The toxic effect of MPs on living organisms results from the characteristics of the MP particles (type of plastic material, shape and size, chemical structure of the surface), additives that are added in the production process (PACs) and later released into the environment and biota and the vector capability of MPs to the transfer of pollutants in the environment into living organisms [14, 36]. One of the primary environmental risks related to MPs is their availability to living organisms in the environment [48]. Animals often exchange MPs for food and swallow them or ingest other organisms that are already polluted with MPs. If the organism is exposed to MPs for a long time, poisoning may occur due to the presence of the toxic chemicals [5]. Digestive fluids and low pH conditions potentiate the release of the pollutant ingested together with MPs, which can increase their assimilation in the organism [47, 48]. The hydrophobic properties of POPs favor their accumulation in the fatty tissue of living organisms [47]. MPs are detected in marine and freshwater animals (birds, fish, turtles, crustaceans, bivalves, echinoderms, anemones, arthropods, polychaetes, mammals), soil invertebrates (colemolas, cridarians and isopods), plants, algae and microorganisms (bacteria, fungi and protozoa) [14, 25, 41]. A large number of contaminated living organisms at different trophic levels indicates the transfer of MPs through food chains [41] and consequently into the human body. Figure 4 shows some examples of food chains in aquatic and terrestrial ecosystems. Figure 3: Factors that determine interactions of MPs with pollutants in the environment (adapted from [58]). The mechanisms of MP impact on living organisms are not completely clarified [41; 63], but numerous studies point to harmful effects at the level of populations (the number of species and biomass), organisms (survival, reproduction, growth, feeding, embryonic development, motility and photosynthetic activity) [63], organs and tissues (the damage of digestive and respiratory organs, inflammatory processes, metabolic disorders, hepatotoxicity, neurotoxicity and immunotoxicity) [47; 54] and cells and molecules (reduced lysosomal stability of digestive glands, oxidative damage, damage DNA, reduced antioxidant capacity, changes in gene expression, enzymatic activity and ion exchange) [63]. The studies on the effects of MPs on living organisms and their modes of action report the four most important mechanisms of harmful effects that stand out: reduced food intake (i.e., reduced nutritional value of food), internal physical injuries due to sharp edges, external physical injuries and oxidative stress [63]. In addition, MPs can have harmful effects by transferring pathogenic microbes to living organisms, which are often colonized on the surface biofilm (e.g., _Vibrio_ spp.) [36; 61]. MP pollution is spread worldwide, but the potential consequences and risks for human health and the environment are greater in specific areas, where the abundance of MPs is greater due to specific industries, wastewater discharges [64], wastewater treatment plant effluents [65] and inefficient waste management (inappropriate landfills, burning, dumping in soil and water, etc.) [66]. Various enzymatic tests, histological analyses and fluorescence monitoring are used to examine the effect of POPs introduced by MPs on aquatic organisms [47]. However, according to current guidelines, toxicological testing of chemicals is conducted using high concentrations of individual substances, and the results are then extrapolated to low doses. It is not possible to precisely determine the risk of exposure to low doses with such procedures nor to obtain data on the risks of simultaneous exposure to a combination of different pollutants [49]. Laboratory studies have proven that the simultaneous exposure of organisms to MPs and other chemicals can induce and/or change their toxic effects [48; 49].