## 3 The Occurrence and Sources of Microplastics and Nanoplastics in the Environment  
MPs and NPs can be classified into primary and secondary sources [19]; the primary are plastics produced at micronized scale, including domestic products, cosmetics, and medicine, while secondary are the result of the degradation of MPs by different processes such as mechanical (erosion, wave action), chemical (photooxidation, temperature, corrosion), and biological activities [20; 21; 22]. Furthermore, plastics are classified on their size into macro- (>25 mm; visible to the naked eye), meso- (<25-5 mm; visible under a light microscope), micro- (5 mm to 0.1 \(\upmu\)m; separated under a microscope and identified using spectrometry), and nano- (<0.1 \(\upmu\)m) plastics [23; 24]. Manufactured plastics contain PP, PVC, PE, and PS as part of their chemical composition [25]. The plastic particles are found in various morphological forms (foam, fibers, pellets, and films), sizes, and colors (Figure 2). The durability of plastic particles makes them highly resistant to environmental degradation as they readily adhere to hydrophobic persistent organic pollutants (POPs), which thus associating them with causing disease and death in numerous aquatic organisms. Remarkably, MPs and NPs are dispersed into the environment by many means such as landfills, sludge [26], discharge of wastewater treatment plants, food waste, terrestrial anthropogenic activities [27], personal care products (e.g., shower gels and shampoos), industrial abrasives (e.g., PS and plastic powder/fluff), residues from plastic processing and recycling plants [28], irrigation (lakes, rivers, reservoirs, and groundwater), street runoff and flooding [29]. Figure 1: Research method of the literature review. The retention and transport of MPs and NPs in groundwater and soil are influenced considerably by their respective parameters (e.g., shape, size, density), soil media (texture, moisture, pH, temperature), and water flow (e.g., velocity, ionic strength, pH) [30]. Bioturbation is defined as interference in the deposition of soil and sediment due to living carriers of MPs and NPs from shallow soil layers to deep ones by ingestion and excretion of microbes [31]. Mites, collembolan, and certain mammals may spread MPs and NPs in soils via chewing, scraping, or even abrasion [32]. Those organisms (collembola, mites and digging mammals) could contribute to the generation of secondary MPs by crushing hard plastic fragments [33]. Many factors affect the quantity of deposition, preservation and transport of MPs and NPs including human activities, properties of particles (shape, size, and density), weather conditions, and environmental geography [34]. MPs and NPs are generated from land-based sources (80%) and sea-based sources (20%) [35]. Terrestrial ecosystems are the main source and transport paths of MPs and NPs into marine environments [36]. In addition, MPs and NPs have been detected in the atmosphere including indoor and outdoor environments [37] and arctic sea ice [38]. The main sources of MPs and NPs in the atmosphere are textiles production, attrition of rubber tires, household, and city dust [39], construction materials and waste incineration [40]. The indoor atmosphere has higher levels of MPs and NPs due to lower removal by dispersal mechanisms [39] which their concentration depends on room partition, ventilation, and airflow [41]. Other possible sources of MPs and NPs that enter the human food chain are the consumption of drinking water [42] bottled water [43], and commercial salt [44]. For instance, a study conducted in 14 countries on tap water and bottled water showed that 80% of the samples contained 4.34 plastic particles/liter, while another examined bottled water and revealed that 90% contained plastic pollutants [45; 46]. Since MPs and NPs commonly escape filtration systems and are found in drinking water, removing plastics from water bodies in the size range of 1-10 um has become a subject of concern [47].