Beyond surface functionalization: Reassessing aging claims in micro- and nanoplastics for hazard interpretation

Surface-functionalized micro- and nanoplastics continue to be labeled as "aged" or environmentally relevant in experimental studies, despite limited empirical validation of this designation. In practice, engineered functionalization-most commonly inferred through surface charge similarity-is frequently treated as equivalent to environmentally weathered states, implicitly substituting descriptor-based resemblance for transformation-driven aging. Here, we synthesize evidence demonstrating that this equivalence lacks mechanistic support. Direct comparisons across particle states show that particles with comparable ζ-potentials, whether engineered, laboratory-weathered, or environmentally aged, exhibit systematically divergent aggregation behavior, contaminant interactions, and biological responses, indicating that charge similarity does not confer functional equivalence. The persistence of this assumption, compounded by the scarcity of coordinated comparative datasets, introduces structural bias in studies of fate, exposure, and toxicity and can distort environmental inference and hazard interpretation. To address this gap, we advance a transformation-based framework that distinguishes three mechanistic regimes-engineered, laboratory-weathered, and environmentally aged particles-and establishes evidentiary criteria for substantiating aging claims. By decoupling particle descriptors from transformation history, this framework redefines "aging" as a process-dependent state, clarifies the limits of proxy substitution, and improves the interpretability of experimental outcomes for environmental assessment and regulatory evaluation.