Impact of traffic-related air pollution exposure on fractional exhaled nitric oxide: A systematic review and Meta-analysis

Traffic-Related Air Pollution (TRAP) is a complicated mix of gases and particulates associated with respiratory oxidative stress and immunological dysfunction. FeNO (Fractional exhaled Nitric Oxide) is emerging as an essential biomarker for inflammation. The duration, specific exposure patterns, and magnitude of associations of FeNO with TRAP remain unclear. This research synthesizes and analyzes the evidence linking specific TRAP to FeNO alteration in human population. A systematic review across 3 databases: PubMed, Scopus and Embase, until 3 February 2026, yields a qualitative synthesis of 48 papers, with a majority reporting increase in FeNO levels with various TRAP exposure (33 out of 48, 71.1%). A network meta-analysis of 9 studies ( n = 5761) across a network of 10 pollutants found black carbon (BC, 10.8, 95% CI (confidence interval): 2.1, 20.2) and polycyclic aromatic hydrocarbons (PAH, 9.5, 95% CI: 0.8, 18.9) as the pollutant associated with the highest 24-h percent FeNO difference per interquartile range (IQR) of pollutants. Heterogenous results may related to confounders and effect modifiers affecting FeNO, such as demographic differences, follow-up time inconsistencies, varying baseline FeNO levels and IQR of pollutants. FeNO may serve as a biomarker for adverse health effects from TRAP exposure for BC and PAH. Consistent approach for defining air pollutants components and measurement units is advocated for future research to be cohesive. Though a short term 24-h for FeNO level is the most common, future research should standardize protocol of follow-up time and confounder adjustments to ensure comparability of the statistical estimates across other time points. This study focuses primarily on the association between exposure to Traffic Related Air Pollution (TRAP) and the levels of Fractional Exhaled Nitric Oxide (FeNO). The PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) guideline and the Newcastle-Ottawa Scale (NOS) were utilized to evaluate relevant and acceptable studies. Studies on occupational exposure and in vivo/in vitro studies have been excluded, as this study concentrates on the general situation of exposure to TRAP. A total of 48 studies were included in the systematic review. Nine papers were selected for meta-analysis based on a 24-h exposure criterion. The network geometry diagram indicates that the most often studied TRAP are PM 10 , ozone, and NOx. Black carbon (BC) and polycyclic aromatic hydrocarbons (PAH) exhibited a statistically significant correlation with FeNO levels. • Majority of literature shows FeNO associated with ambient TRAP exposure • Different pollutants, TRAP levels, and timing of FeNO measurement influence results • Network meta-analysis identifies BC and PAH with the highest 24-h FeNO changes • Future research: unified follow-up protocol and accurate TRAP measurement