Depression gives rise to a large proportion of the health burden from mental disorder and substantially diminishes quality of life with an increased prevalence across the globe (Moreno-Agostino et al., 2021; Vos et al., 2020). Given the social and health care burden of this disorder, it is imperative to identify modifiable risk factors for prevention of depression. In addition to the identified social and behavioral factors (e.g., low socioeconomic status and smoking) (Li et al., 2021; Ribeiro et al., 2017), ambient air pollution has increasingly been recognized as an emerging risk factor for depression (Chen et al., 2018; Pun et al., 2017; Xue et al., 2021). Fine particulate matter (PM2.5) has been widely identified as an important environmental determinant of depression onset and aggravation in several recent meta-analysis (Borroni et al., 2022; Braithwaite et al., 2019), while other major air pollutants such as ozone (O3) has not been adequately investigated.

Associations of O3 exposure with mental health deserve more epidemiological investigations. Biological mechanism study indicated that exposure to O3 may be a potential risk factor for depression due to the high neurotoxicity and powerful oxidizing properties (Zhang et al., 2019). Emerging epidemiological studies had reported a positive association between short-term O3 exposure and depression across the globe (Lu et al., 2020; Nguyen et al., 2021; Tsai et al., 2020), while limited researches investigated the long-term impact of O3 exposure on depression (Borroni et al., 2022). Available longitudinal O3-depression evidence was mainly reported in North America (Kioumourtzoglou et al., 2017) and Europe (Bakolis et al., 2021; Pelgrims et al., 2021), wherein great heterogeneities still existed between studies. For instance, a large American cohort study reported increased risk of depression associated with long-term O3 exposure (Kioumourtzoglou et al., 2017), while non-significant association was observed in a recent regional study in Britain (Bakolis et al., 2021). Besides, the effect of O3 exposure on depression may vary among subpopulations, which might be due to differential susceptibility of the subgroups to health effects of air pollution (Simoni et al., 2015) and confounding effect of comorbidities (Loop et al., 2013). Related evidence was largely sparse in developing countries such as China, where most locations have been experiencing serious O3 air pollution (Lu et al., 2018) and rapid increase in depression prevalence during recent decades (Ferrari et al., 2022).

To fill this research gap, we designed a repeated measurement study based on a Chinese nationwide cohort of middle-aged and older men and women during 2011–2018. We primarily aimed to quantify the long-term association between O3 exposure and depression in Chinese adults, and to depict the concentration-response (C-R) relationship across a wide range of exposure levels.