Two Decades of Nanotoxicology: Evolution, Impact, and Future Challenges.

The rapid expansion of nanotechnology, with over 11,100 products and 100,000 patents, has raised safety concerns for organisms and ecosystems. Nanotoxicology emerged from ultrafine particle studies in the 1990s and was formally established in 2004. This review synthesizes approximately 160 publications and regulatory documents, selected from 220 compiled, to analyze the field's evolution from 2004 to 2025. Early studies focused on nanostructures in medicine and their effects on organs and tissues, driving research aimed at mitigating risks. Current studies increasingly address the molecular mechanisms underlying nanostructure toxicity, which arise from dynamic interactions among physicochemical properties, exposure conditions, biological transformations, and environmental fate. Research extends beyond human health to assess long-term environmental impacts. Artificial intelligence and machine learning have enabled predictive models linking physicochemical properties to toxicological outcomes, although reliability remains limited by data set heterogeneity, restricted external validation, and overfitting. Bibliometric analysis revealed a publication peak around 2014-2016 (>380 articles/year), followed by a decline through 2023-2024 and partial recovery in 2025, reflecting diversification into nanosafety, safety-by-design, nanoecotoxicology, and AI-based prediction. Standardized protocols, harmonized data sets, and globally accepted safe exposure limits remain unavailable. Future progress depends on integrating biologically and environmentally relevant models, robust computational approaches, and harmonized regulatory frameworks to advance sustainable nanotechnology.