What Are Microplastics?

Microplastics are small plastic particles typically defined as being less than 5 millimetres (mm) in size, with dimensions ranging from approximately 100 nanometres (nm) to 5 mm. Particles smaller than 100 nm are often classified separately as nanoplastics, which are even more difficult to detect and study due to their extremely small size (Ocean Conservancy, 2023).

Rather than representing a single substance, microplastics constitute a diverse group of contaminants that vary widely in size, shape, colour, polymer type, and chemical composition. Treating microplastics as a uniform pollutant has contributed to confusion in both scientific research and policymaking, particularly when developing effective mitigation strategies (Rochman et al., 2019).

Types of Microplastics: Primary and Secondary

Microplastics are commonly classified into two main categories based on their origin:

• Primary microplasticsThese are intentionally manufactured at small sizes. Examples include microbeads—tiny polyethylene spheres once widely used as exfoliants in cosmetics and personal care products—as well as industrial abrasives and plastic pellets used as raw materials in plastic production (NOAA, 2023).

• Secondary microplasticsThese form through the fragmentation of larger plastic items such as packaging, bottles, textiles, tyres, and fishing gear. Degradation occurs as a result of sunlight exposure, heat, mechanical abrasion, and environmental weathering processes (Rochman et al., 2019).

Both primary and secondary microplastics ultimately enter the environment and contribute to widespread contamination.

Shapes and Physical Characteristics

Microplastics occur in a wide range of shapes and forms, which can often provide insights into their sources. Researchers commonly classify them into categories such as fibres, fragments, spheres (beads), pellets, films, and foams (Rochman et al., 2019).

Common examples include:

• Fibres – commonly shed from synthetic clothing, carpets, and upholstery

• Spheres – often linked to personal care products or industrial scrubbers

• Pellets – associated with plastic manufacturing feedstock

• Films and foams – typically originating from packaging and insulation materials

Microplastics also vary in colour and density, influencing how they move through air, water, and soil and affecting their environmental distribution (Rochman et al., 2019).

Sources and Pathways into the Environment

Microplastics enter the environment through multiple pathways, making them extremely difficult to trace back to a single source. Key pathways include:

• Direct littering and fragmentation of plastic waste

• Wastewater effluent

• Surface runoff from roads and agricultural land

• Wind transport

• Maritime activities such as fishing and shipping

• Use of personal care products containing microplastics

(Jolaosho et al., 2025)

Due to their small size, microplastics can be transported over long distances. Research shows they can move through the atmosphere across thousands of kilometres, be deposited via rainfall, and accumulate far from their original sources. Rivers often contain microplastic concentrations orders of magnitude higher than marine environments, acting as major transport routes from land to sea (Santos, Machovsky-Capuska and Andrades, 2021).

As a result, microplastics have been detected in environments ranging from Arctic mountain tops to the depths of the Mariana Trench, highlighting their global reach.

Environmental Distribution and Ubiquity

Microplastics are now present across all environmental compartments, including oceans, rivers, soils, air, and living organisms. Numerous studies have documented plastic ingestion across entire food webs, from microscopic zooplankton to apex predators such as fish, birds, and marine mammals (Santos, Machovsky-Capuska and Andrades, 2021).

Globally, at least 1,565 species across marine, freshwater, and terrestrial ecosystems have been reported to ingest plastics (Santos, Machovsky-Capuska and Andrades, 2021). Their widespread presence is largely attributable to their durability, as most conventional plastics are not biodegradable and can persist in the environment for centuries, continually breaking down into smaller particles rather than fully degrading.

Reference List

Jolaosho, T.L., Rasaq, M.F., Omotoye, E.V., Araomo, O.V., Adekoya, O.S., Abolaji, O.Y. and Hungbo, J.J. (2025). Microplastics in freshwater and marine ecosystems: Occurrence, characterization, sources, distribution dynamics, fate, transport processes, potential mitigation strategies, and policy interventions. Ecotoxicology and Environmental Safety, 294, p.118036. https://doi.org/10.1016/j.ecoenv.2025.118036

National Oceanic and Atmospheric Administration (2024). What are microplastics? [online] Available at: https://oceanservice.noaa.gov/facts/microplastics.html [Accessed 18 Dec. 2025].

Ocean Conservancy (2023). Microplastics Facts & Figures: What are microplastics? [online] Available at: https://oceanconservancy.org/wp-content/uploads/2023/02/Microplastics-Fact-Sheet-FINAL-2.3.23.pdf [Accessed 18 Dec. 2025].

Rochman, C.M. et al. (2019). Rethinking microplastics as a diverse contaminant suite. Environmental Toxicology and Chemistry, 38(4), pp.703–711. https://doi.org/10.1002/etc.4371

Santos, R.G., Machovsky-Capuska, G.E. and Andrades, R. (2021). Plastic ingestion as an evolutionary trap: Toward a holistic understanding. Science, 373(6550), pp.56–60. https://doi.org/10.1126/science.abh0945