**2. Sources, transport, and type of microplastics**

In this section, we examine the main sources of microplastics, followed by how they reach the marine environment. Lastly, the types of microplastics predominant in the marine ecosystem are summarized.

#### **2.1 Sources of microplastics**

The sources of microplastics can be categorized into primary and secondary sources, and each category is discussed as follows.

#### *2.1.1 Primary sources*

Primary sources of microplastics include: **plastic pellets**, also known as nibs (diameter: 2–5 mm), which are used to make various types of plastic products [27];

**microbeads,** which are used in the manufacturing of personal care products, face wash, face cleansers, facial scrubs, hair products, nail polish, deodorants, sunscreen, and eye shadows [13–15]; **glitters,** which are shiny substances found in cosmetics and textile products. They are usually made of polyethylene terephthalate (PET) polymer, acrylic, polyvinyl chloride (PVC), and/or polymethyl methacrylate (PMMA) [28]. These primary plastics vary in shape, size, and composition depending upon their applications [15]. For example, certain cosmetic products contain granules of polyethylene and polypropylene (<5 mm), spheres of polystyrene (<2 mm) [29], or irregularly shaped microplastics (<0.5 mm) [15]. Apart from cosmetics, these primary sources of microplastics also find applications in air-blasting technology [14, 29]. This technology uses acrylic, melamine, or polyester as scrubbers at high pressure on machines, engines, and water vessel hulls to scrape off rust buildup or paint [13, 30].

#### *2.1.2 Secondary sources*

### *2.1.2.1 Effluent from water and wastewater treatment plants*

Water and wastewater treatment plants are one of the main sources of releasing microplastics into the marine environment [31]. They are found in the primary stages of water treatment. Because of their small size, they can pass through the filters and enter the secondary units [32]. Microplastics detected in the influents ranged from ~1 to 10,000 particles per liter, and after treatment, microplastics in the effluent ranged from ~0 to 450 particles per liter (as summarized by a number of studies reviewed by Sun et al. [33]). Microfibers, including polyester, acrylic, and polyamide, are detected in the effluent of wastewater treatment plants [34], which implies the limitations of these treatment facilities to remove these microplastics.

#### *2.1.2.2 Wear and tear from normal plastic use*

The most common example of such a source type is the microplastic released as a result of washing clothes and textiles during laundry [35]. As a result, microplastics released from laundry activities eventually reach the marine environment. It is estimated that laundry activities are responsible for 500,000 tons of microplastics in the ocean per year [36, 37]. Apart from textiles/clothes weathering, use of fishing gears, including nets and ropes [38], wear and tear of car tires [39], as well as weathering of household items, including toys, plastics wares, and plastic disposables items [40].

### *2.1.2.3 Airborne dust*

Plastic dust is released from a number of activities including plastic manufacturing facilities, incineration of plastic wastes, traffic emissions, weathering of roads and streets, and urban mining activities [41, 42]. Airborne dust is carried by wind and can settle in indoor settings including schools and houses [43, 44]. In houses, airborne microplastic comes from plastic items used in household items including food packaging, plastic wear, and plastic furnishings [45]. Most recently, during the COVID-19 pandemic, the requirement to wear face masks was made mandatory to prevent the spread of coronavirus. The surgical facemasks were made up of PP, PE, PS, and polyester. Studies showed that wearing these masks exposed the humans directly to inhalation of micro (<1 μm) and nanofibers (<100 nm) [46–48].

## *2.1.2.4 Secondary microplastics*

Primary microplastics may also contribute to the secondary sources of microplastics. Once exposed to the environment, plastic wastes and primary sources of microplastics undergoes weathering and degradation to form secondary microplastics [12]. Details on the degradation process of plastic waste are given in Section 3. Plastic litters including disposable plastic cutlery, plastic cups, food containers, as well as face masks in the era of COVID-19 pandemic (that started in 2019 and is still ongoing in the current year of 2022) end up being dumped on coastal shorelines, where they undergo further degradation and decomposition [48–50].

### **2.2 Transport**

There are four main pathways through which microplastics from different sources reach the marine environment: (a) as surface runoff when the plastic wastes are thrown on the terrestrial lands and eventually travel along with the runoff due to rainfall. Transport via surface runoff is responsible for 44% of the total microplastics being released into the marine ecosystem; (b) via wind, which transports the plastic waste on the terrestrial zone to seas/oceans along with the atmospheric currents. Transport via wind is responsible for 15% of the total microplastics being released into the marine ecosystem; (c) as wastewater discharge in which microplastics can enter the receiving water bodies and is responsible for 37% of the total microplastics released; (d) and lastly, through direct disposal of plastic wastes into the marine environment, which is responsible for 4% of the total microplastics release [7, 13, 30, 51]. Direct disposal of plastic wastes activities includes washing clothes in the rivers, usually in the rural areas [52], coastal tourism activities including fishing and recreational activities resulting in disposable cups and litters [53], and commercial fishing resulting in nets and litters [54].

## **2.3 Types of microplastics**

Microplastics can be categorized into primary and secondary microplastics depending on their sources, as discussed in Sections 1 and 2.1. Depending on their density and chemical compositions, microplastics can be classified into different types including polystyrene (PS), low-density polyethylene (LDPE), highdensity polyethylene (HDPE), polypropylene (PP), polyvinyl chloride (PVC), polyethylene terephthalate (PET), and others (e.g., nylon, polyester) [55, 56]. The different plastic types, properties, and functions where these plastics are commonly used are given in **Table 1**. Microplastics can also be differentiated on the basis of shape: pellets, microbeads, foams, fibers, films, fragments, and microfibers (**Figure 1**) [57].
