Is 5 mm still a good upper size boundary for microplastics in aquatic environments? Perspectives on size distribution and toxicological effects

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.