Associations between short-term absolute increases in ambient air pollution and hospital admissions and outpatient visits for depression: A nationwide case-crossover study

Short-term absolute changes in ambient air pollution levels may result in disease events such as depression, whereas evidence from large-scale epidemiological studies has been insufficient. A time-stratified case-crossover study was performed to investigate the associations between air pollution increases between neighboring days (APINs, generated as the differences in absolute air pollution concentrations on the current days minus those on the previous days) and morbidity risks of depression based on hospital admission and outpatient visit data from 239 Chinese cities of prefecture-level or above during 2013–2017. Daily absolute concentrations of particulate matter with an aerodynamic diameter ≤ 2.5 μm (PM 2.5 ), particulate matter with an aerodynamic diameter ≤ 10 μm (PM 10 ), nitrogen dioxide (NO 2 ), sulfur dioxide (SO 2 ), carbon monoxide (CO) and ozone (8 h O 3 ) were obtained, and conditional logistic regression models were used to evaluate the associations between absolute concentrations and APINs of major air pollutants with morbidity risks of depression. A total of 330,610 hospital admissions and 3,726,390 outpatient visits for depression were included. Per interquartile range (IQR) increase in APINs of major air pollutants at lag 0–3 were associated with increases ranging from 0.59% (0.06%, 1.13%) (8 h O 3 ) to 1.66% (1.09%, 2.22%) (NO 2 ) in hospital admissions for depression; and with increases ranging from 0.42% (0.15%, 0.68%) (8 h O 3 ) to 0.85% (0.56%, 1.14%) (NO 2 ) in outpatient visits for depression, which were robust to adjustments for absolute concentrations of major air pollutants (with the exception of 8 h O 3 ). This study provides novel evidence that short-term absolute increases in air pollution levels are associated with elevated risks of depression, which is worth consideration in future disease prevention practices.