Particle Size, Shape, and Polymer Composition as Determinants of Microplastic and Nanoplastic Toxicity in Freshwater Biota: A Mechanistic and Critical Synthesis

Microplastics (MPs) and nanoplastics (NPs) are pervasive freshwater contaminants of growing ecological concern. This review critically synthesises how particle size, shape, ageing state and polymer composition jointly govern MP and NP toxicity in freshwater biota. Smaller particles frequently elicit stronger responses in laboratory exposures, through enhanced bioavailability, cellular internalisation and surface reactivity, but the relationship is neither universal nor strictly monotonic, and null or threshold responses are common at environmentally realistic concentrations. Because environmental nanoplastic concentrations cannot yet be measured reliably, greater laboratory potency should not be equated with greater environmental risk. Fibres show more prolonged retention and stronger physical effects than spheres under comparable conditions, whereas the relative hazard of fragments is less well resolved and that of films is essentially untested. Adverse effects are reported most frequently for polyvinyl chloride and polystyrene and least frequently for pristine polyethylene and polypropylene; however, no freshwater study has compared several polymers under matched conditions, and the closest designs indicate that additive formulation and particle size, rather than the polymer backbone, drive much of the response. Polymer type is therefore best read as a proxy for formulation and manufacturing history. Most reported effect concentrations exceed measured freshwater concentrations by one to several orders of magnitude, and pristine monodisperse spheres under-represent the weathered, polydisperse and fibre-dominated mixtures organisms encounter. Physical and chemical hazard are accordingly treated as a continuum, since ageing, eco-corona formation and biofilm colonisation act on both simultaneously. We advance the Particle Property–Mechanism–Effect framework, which maps particle properties onto mechanistic tiers and generates falsifiable predictions; it is offered as a structuring hypothesis rather than a validated model. ∙ Toxicity is governed by the interaction of particle size, shape, ageing state and polymer chemistry, not by any single property. ∙ In laboratory exposures, effects generally intensify below approximately 10 µm; null and threshold responses are nonetheless common at environmentally realistic concentrations, and greater laboratory potency does not by itself establish greater environmental risk. ∙ Polymer composition shapes the chemical dimension of hazard, but additive formulation cannot be separated from polymer identity in the designs currently available. ∙ Weathering is treated as a primary determinant of hazard rather than as a modifier of it. ∙ A Particle Property–Mechanism–Effect framework, offered as a structuring hypothesis rather than a validated predictive model, yields testable mechanism-resolved predictions, shown in two worked examples. ∙ Exposure realism and analytical quality assurance are the principal constraints on regulatory translation.