Associations between ambient NO2 exposure and multi-system diseases in four subtropical humid monsoon cities in China

Evidence remains limited regarding the cumulative multi-system health effects of nitrogen dioxide (NO2) exposure and its lag–response patterns. This study aimed to evaluate the associations between short-term NO₂ exposure and multi-system healthcare utilization, with particular emphasis on lagged effects. Daily data on ambient air pollution, meteorological factors, and healthcare utilization across multiple disease systems were collected from 2023 to 2024 in four subtropical humid monsoon cities in China. Using a time-series design, distributed lag non-linear models (DLNMs) were applied to examine lagged and exposure–response associations between short-term NO2 exposure (lag 0–7 days) and daily healthcare visits. Stratified analyses by sex, age, healthcare utilization type, and season, as well as sensitivity analyses, were conducted. Significant positive associations between NO2 exposure and healthcare utilization were consistently observed across all four cities, with relative risks (RRs) ranging from 1.016 (95% confidence interval [CI] 1.008–1.084) to 1.290 (1.185–1.404), 1.072 (1.040–1.105) to 1.168 (1.128–1.210), and 1.072 (1.032–1.114) to 1.145 (1.099–1.193), respectively, for mental/behavioral, cardiovascular, and respiratory diseases, associated with a 10 μg/m3 increase in NO2 concentration during lag01–lag07. Statistically significant cumulative risks were also observed for neurological and metabolic/endocrine diseases in certain cities. Exposure–response indicated a predominantly non-linear, upward association between NO₂ concentrations and healthcare utilization, with relative risks increasing more sharply at higher concentrations. Further subtype analyses indicated that the cumulative lag effects were mainly concentrated in schizophrenia and delusional disorders, affective disorders, hypertensive diseases, and acute upper respiratory infections. Short-term NO₂ exposure was associated with increased healthcare utilization across multiple disease systems, underscoring the need for strengthened NO₂ control to mitigate its broader health impacts.