Causal Effect of Air Pollution on Outpatient Visits for Chronic Rhinitis

ABSTRACT Objectives Although the association between air pollutants and chronic rhinitis has been extensively investigated, causal inference is lacking. We aimed to investigate the causal relationship between air quality index (AQI) and outpatient visits for chronic rhinitis using instrumental variable (IV) analysis, employing thermal inversion. Methods We used National Health Insurance Service data collected between January 1, 2014, and December 31, 2017, to determine the short‐term causal relationship between air pollution and outpatient visits due to chronic rhinitis. A two‐stage generalized method‐of‐moments Poisson regression model was employed to estimate the causal effects. Stratified analyses by age group and robustness tests for IV with negative controls were conducted. Results A total of 81,210,447 outpatient visits for chronic rhinitis were analyzed. IV analysis demonstrated significant positive associations between AQI and outpatient visits on lag days 0, 3, 5, and 6, with the strongest effect observed at Lag 0 (relative risk [RR] 1.078, 95% confidence interval [CI] 1.045–1.113). Stratified analysis revealed that individuals aged 10–19 years were the most vulnerable, showing statistically significant relative risks across all lag days (RR: 1.039–1.161). Analyses using negative control outcomes and exposures supported the validity of the proposed instrumental variable approach, suggesting a robust causal effect of AQI on outpatient visits for chronic rhinitis. Conclusions We identified a causal relationship between increased air pollution and outpatient visits for chronic rhinitis. Unlike a simple association potentially confounded by various factors, our analysis provides robust evidence of the causal impact of air pollution on disease burden. Level of Evidence 3.