BACKGROUND AND OBJECTIVE When multiple complex air pollutants are combined in real-world settings, the reliability of estimating the effect of a single pollutant is questionable. This study aimed to investigate the combined effects of changes in air pollutants on small airway dysfunction (SAD).

METHODS We analysed Korea National Health and Nutrition Examination Survey (KNHANES) V-VIII database from 2010 to 2018 to elucidate the associations between annual changes in air pollutants over a previous 5-year period and small airway function. We estimated the annual concentrations of five air pollutants: NO₂, O₃, PM_2.5, SO₂ and CO. Forced expiratory flow between 25% and 75% of vital capacity (FEF_25%-75%) <65% was defined as SAD. Using the quantile generalized-Computation (g-Computation) model, the combined effect of the annual changes in different air pollutants was estimated.

RESULTS A total of 29,115 individuals were included. We found significant associations between SAD and the quartiles of annual changes in NO₂ (OR = 1.10, 95% CI = 1.08-1.12), O₃ (OR = 1.03, 95% CI = 1.00-1.05), PM_2.5 (OR = 1.03, 95% CI = 1.00-1.05), SO₂ (OR = 1.04, 95% CI = 1.02-1.08) and CO (OR = 1.16, 95% CI = 1.12-1.19). The combined effect of the air pollutant changes was significantly associated with SAD independent of smoking (OR = 1.31, 95% CI = 1.26-1.35, p-value <0.001), and this trend was consistently observed across the entire study population and various subgroup populations. As the estimated risk of SAD, determined by individual-specific combined effect models, increased and the log odds for SAD increased linearly.

CONCLUSION The combined effect of annual changes in multiple air pollutant concentrations were associated with an increased risk of SAD.