# Stabilized Sewage Sludge as Fertilizer: Risks Related to the Presence of Microplastics
## 1 Introduction  
Microplastics are plastic particles no larger than 5 mm. Scientists classify them based on various criteria, such as size (nanoplastics, sub-microplastics, small microplastics, large microplastics), color (transparent, white, black, colored), shape (fragments, spheres, fibers, pellets, films, foams), and source--meaning the type of plastic from which they are made [1]. The primary sources of microplastics include hygiene products, tire and road marking abrasion, synthetic washing materials, and improper waste management [2]. Scientists have warned about the presence of microplastics in every ecosystem--and even in the human body [3]. Studies in rodents have demonstrated that inhalation of polypropylene dust, even at low concentrations, induces inflammation and respiratory disorders [4]. Research on the health effects of micro- and nanoplastics (MNPs) is challenged by the lack of standardization in exposure doses and forms. Many studies employ varying concentrations and measurement units, which complicates risk assessment. Most research focuses on spherical polystyrene particles, which represent only a small fraction of environmental microplastics [5]. In reality, micro- and nanoplastics come in diverse shapes (e.g., fibers, sheets) and have different physicochemical properties that may influence their toxicity. Additionally,particles used in studies are often surface-modified or contain solvents and dyes that can have toxic effects, potentially skewing results [5]. While there is evidence of cytotoxicity and genotoxicity from microplastics, including oxidative stress and DNA damage, further studies using more representative samples are necessary to accurately determine their impact on human health [5]. Moreover, experiments on the effects of polyethylene present in agricultural mulch on _Lumbricus terrestris_ (Oligochaeta, Lumbricidae) showed increased mortality in these organisms and a significant reduction in their growth rate [6]. Studies have also examined the effects of polypropylene micro- and nanoplastics on soil microbiota [7]. The results showed that polypropylene particles decreased bacterial diversity, with particle size exerting a more decisive influence than concentration. Nanoplastics reduced the complexity and connectivity of both bacterial and fungal networks, with bacteria exhibiting greater sensitivity to plastic-induced stress than fungi. A shift was observed in the dominant ecological processes governing bacterial communities, from stochastic drift to deterministic selection. Nanoplastics disrupted interactions within the microbial community, as well as those between soil, microorganisms, and plants. These findings suggest that polypropylene nanoplastics represent a significant ecological threat to soil microbiota and their interactions with vegetation [7]. Fluorescent polystyrene microbeads (0.2 \(\upmu\)m) were observed to accumulate in the root cap mucus of lettuce, appearing as a visible dark green tip. Confocal imaging revealed that the beads traversed the apoplastic pathway into the vascular system, ultimately reaching the stems and leaves via transpiration flow. In root and stem tissues, they formed grape-like clusters, while in leaves, they were more dispersed. This study provides clear evidence that submicrometer microplastics can be taken up, transported, and accumulated in edible plant tissues, highlighting a potential route of human exposure through contaminated crops [8]. Physical, chemical, and biological processes drive the degradation of polymers in soil. Mechanical forces, such as soil movement and organism activity, cause the fragmentation of larger particles. Chemical reactions, including oxidation and exposure to UV light, break down polymer chains. Additionally, soil microorganisms can biologically degrade certain plastics through enzymatic activity. Together, these processes fragment polymers into smaller micro- and nanoplastics, increasing their environmental impact [9].