Ultrafine Particles and Mortality: A Scoping Review of Epidemiologic Evidence and Exposure Assessment Limitations.

Ultrafine particles (UFPs; <100 nm) can penetrate biological barriers and trigger inflammation, oxidative stress, and endothelial dysfunction, yet they remain largely unregulated and understudied relative to PM2.5. This scoping review, conducted following PRISMA-ScR guidelines, systematically searched PubMed and Scopus for epidemiological studies examining ambient UFP exposure and mortality. Of 704 articles screened, 21 studies published between 2007 and 2025 met inclusion criteria. Fourteen assessed short-term exposure (≤1 month) using time-series or case-crossover designs with central-site particle number concentration monitors, and seven assessed long-term exposure using land-use regression, chemical transport, or machine learning models. A majority of both short-term (10 of 14) and long-term (6 of 7) studies reported positive associations between UFP exposure and mortality, particularly for respiratory outcomes. The strongest associations were observed in studies using higher-resolution or source-specific exposure methods, a pattern consistent with reduced misclassification. However, exposure assessment approaches varied widely in spatial resolution, instrument type, and particle size definitions, introducing substantial heterogeneity that limits comparability across studies. These findings underscore the need for standardized UFP measurement protocols, high-resolution exposure assessment, and further investigation of disparities in exposure and health outcomes to inform regulatory consideration.