Microplastics Categorised into Primary and Secondary Types
“According to their origin, microplastics can be categorised into two main categories: primary and secondary microplastics. Although the two types are both responsible contributors to the world's plastic burden, their routes to the environment and physical characteristics vary significantly. Primary Microplastics Primary microplastics are intentionally produced in a microscopic size to fit particular industrial or household use. They are plastic granules, pellets, and microbeads used in personal care products such as exfoliating cleansers, cosmetics, and toothpastes (Razeghi et al., 2021). In addition to consumer products, which are the main sources of microplastics, primary microplastics are also commonly used as sandblasting scrubbers in industrial processes or as the granulated raw resin (nurdles) used in mass plastic production. These particles enter the environment directly and, in most cases, bypass wastewater plants due to their small size. In addition, synthetic fabrics contribute a large amount of primary microplastics in microfiber form; these are lost during washing and released into water through household graywater (Razeghi et al., 2021). The more recent research focuses on the fact that the particle of the tyre wears, which occurs via friction between the road and vehicle tyres, represents another substantial and often neglected source of primary microplastic emission in urban runoff (Chen et al., 2025). Secondary Microplastics Secondary microplastics, on the other hand, are not produced in small size form, but rather occur as the products of the natural breakdown of larger plastic objects or macroplastics, in the environment. When plastic bottles, fishing gear, or packaging are dumped, they are subjected to mechanical, chemical, and biological stressors. In particular, the mechanical energy of wave action, the chemical effects of ultraviolet (UV) radiation, and the metabolic activity of some microbes will work in harmony to weaken the plastic's structural integrity (Lambert & Wagner, 2018; Klein et al., 2018). This is referred to as fragmentation, and it breaks large debris into smaller pieces that ultimately become invisible to the naked eye. It has been observed that the percentage of such fragmentation is increasing worldwide, driven by rising plastic volumes in some hotspots, such as the Great Pacific Garbage Patch (Zhu et al., 2025). Morphological Variations A microplastic particle's physical morphology is a key determinant of its origin and potential effects on marine and terrestrial ecosystems. These shapes are mostly classified into the following: Fragments: These are hard, sharp and three-dimensional fragments that are formed as a result of the disintegration of rigid plastic objects. They are characterised by an extreme colour variety and unequal edges (Anderson et al., 2017). Fibres: Fibres are thread-like strands that form the most common type of acquisition in environmental samples; these are products of synthetic clothing (e.g., polyester, nylon, and polypropylene). They are also particularly dangerous to aquatic organisms due to the high surface-area-to-volume ratio, which allows the toxin to be adsorbed (Geyer et al., 2024). Films: Thin and flexible, films are two-dimensional particles made from plastic bags, agricultural mulch, or packaging foils. They are usually inconspicuous and, due to their transparency and buoyancy, easily blend with other natural prey in aquatic ecosystems. Pellets: Brightly coloured, small, sphere-shaped or ovoid granules. Brightly coloured, small, sphere-shaped or ovoid granules (usually). Although some microplastics are the main components of soaps, others are the building blocks of all plastic products and are likely to be lost during maritime transportation (Rezania et al., 2018). Foam: Foams consist of lightweight porous materials (expanded polystyrene- Styrofoam ), which can be used in the packing of food products and insulation. These particles have a significant impact on the environment due to the chemical additives that leach into adjacent water (Anderson et al., 2017).”
Summary
Microplastics are correctly divided into primary (intentionally produced as micro‑particles for products such as cosmetics, detergents, industrial pellets, and synthetic textiles) and secondary (generated by fragmentation of larger plastic debris). The described morphological categories—fragments, fibres, films, pellets, and foam—match the standard classification used in current scientific literature. Recent studies also identify tyre‑wear particles as an emerging primary source, which aligns with the statement.
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The origin of microplastics in the aquatic environment is divided into two major sources: the first source of microplastics (primary microplastics) corresponds to plastics produced directly as microparticles, and the second source (secondary microplastics) corresponds to the fragmentation of ...
- Microplastics in the aquatic and terrestrial environment: sources (with a specific focus on personal care products), fate and effects - PMC
Consequently, quantitative information on the contribution of different sources to the overall amount of macro- and, especially microplastics in the environment is generally lacking [8, 104, 105]. There are, e.g. no reliable estimates of the percentage of plastic packaging material reaching the marine environment [25]. Given the large amount of macroplastics entering the environment, it is generally assumed that most microplastics in the environment are secondary microplastics, i.e. a result of weathering of larger plastic debris [11, 25]. However, fragmentation rates of macroplastics are largely unknown [57, 59, 99]. As a result, no quantitative information is available on the relative contribution of primary and secondary microplastics to the overall amount of microplastics in the environment [50].
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Microplastics in the ocean are mostly irretrievable and persistent1,9. Our synthesis of subsurface microplastic observations over a 10-year period generates a global benchmark (Table 1). Despite observational uncertainties, the substantial presence of subsurface microplastics in both nearshore and offshore waters underscores the ocean water column as a critical yet uncharacterized reservoir of plastics. Small microplastics show a gradual decrease in abundance with depth, suggesting a more uniform distribution and longer lifespan in the water column, whereas large microplastics are more effectively trapped by stratification. Offshore surface accumulation zones37 extend into subsurface waters but are primarily composed of large microplastics.
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These primary microplastics can ... care products, such as toothpaste, face washes, and cosmetics. Secondary microplastics come from larger pieces of plastics, such as beverage bottles, bags, and toys....
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Microplastics in the ocean are mostly irretrievable and persistent1,9. Our synthesis of subsurface microplastic observations over a 10-year period generates a global benchmark (Table 1). Despite observational uncertainties, the substantial presence of subsurface microplastics in both nearshore and offshore waters underscores the ocean water column as a critical yet uncharacterized reservoir of plastics. Small microplastics show a gradual decrease in abundance with depth, suggesting a more uniform distribution and longer lifespan in the water column, whereas large microplastics are more effectively trapped by stratification. Offshore surface accumulation zones37 extend into subsurface waters but are primarily composed of large microplastics.
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Both types are recognized to persist in the environment at high levels, particularly in aquatic and marine ecosystems, where they cause water pollution. >35 % of all ocean MPs come from textiles/clothing, primarily due to the erosion of polyester, ...
- Twenty years of microplastic pollution research—what have we learned? | Science
This eLetter addresses a serious mistake in a recent review by R. Thompson et al., “Twenty years of microplastic pollution research—what have we learned?” (1). In this paper, in Fig. 1A, the authors proposed a classification of microplastics and included “pigments in paint” as an example of the much-discussed and already banned in some countries intentionally added primary microplastics (2) (3rd type of primary microplastics).
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Primary microplastics are intentionally manufactured in small sizes for their use in consumer products, such as cosmetics or biomedical products. Secondary microplastics are plastic particles that break down from larger plastic materials, such ...
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