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Is 5 mm still a good upper size boundary for microplastics in aquatic environments? Perspectives on size distribution and toxicological effects
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This review questions whether the widely used 5-millimeter upper size limit for defining microplastics is still scientifically appropriate for aquatic research. After analyzing over 80 studies, researchers found significant inconsistencies in how the boundary is applied, which makes comparing results across studies difficult. The evidence indicates that smaller particles tend to cause greater biological harm, suggesting that refining size categories could improve both research consistency and risk assessment.
Plastic is everywhere as an essential component of industries' products, but accumulation and degradation of plastics into microplastics occurs continuously in aquatic environments. Despite numerous studies investigating the influence of microplastics, challenges remain when comparing comprehensive results due to the lack of agreement regarding microplastics sizes. Over 80 studies and reports were reviewed, revealing the inconsistencies in defining the upper size limit for microplastics, and are the basis of this exploration of the need to redefine the latter by focusing on pragmatic factors such as size distribution and toxicity endpoints in aquatic environments. Reviewed articles indicate a gap between recommendations for microplastics definitions and the current status of microplastics. We suggest initiating a discussion regarding downscaling the broadly accepted 5 mm upper limit to 1000 μm, considering environmentally realistic conditions and SI nomenclature. We encourage continued international discussion of redefining the upper size limit defining microplastics from this pragmatic view.
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Researchers argued that the current less-than-5-mm size definition of microplastics, adopted from marine research, is insufficient for soil ecosystem studies and proposed a more precise, differentiated size classification to better target the distinct challenges that plastics of varying sizes pose to soil functions.
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This methods paper critically examines how choices about the lower size limit and counting method for microplastics affect reported concentrations, arguing that different research questions require different approaches. The authors call for transparency in reporting methodological choices so that studies using different size thresholds can still be meaningfully compared.
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Microplastics in aquatic systems: A review of occurrence, monitoring and potential environmental risks
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Researchers review the presence of microplastics — tiny plastic fragments less than 5 mm — across freshwater and marine environments worldwide, finding that polyethylene, polypropylene, and polystyrene are the most commonly detected types. Exposure disrupts feeding, movement, and reproduction in aquatic wildlife, and the authors call for standardized measurement methods and legal limits to protect ecosystems.
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