Separate and combined associations of heatwaves, air pollution, green spaces, and blue spaces with depressive symptoms: a national cohort study

Depressive symptoms are among the most prevalent mental disorders globally, and identifying their combined risk factors is crucial for promoting population mental health. Extreme heat exposure, air pollution, green spaces, and blue spaces are common environmental exposures, yet their separate effects and interactions on the risk of depressive symptom onset remain unclear. This study aims to investigate the individual and interactive effects of extreme heat exposure and the aforementioned environmental factors on the risk of depressive symptoms. Using five waves of data from the China Health and Retirement Longitudinal Study (CHARLS) from 2011 to 2020. Nine heatwave definitions were employed as indicators of high-temperature exposure in the year preceding the onset of depressive symptoms. PM2.5, NDVI, and the average proportion of open water areas represented exposure levels to air pollution, green space, and blue space, respectively. Using Cox proportional hazards regression models incorporating time-varying variables, we assessed the effects of heatwave combined with air pollution, green space, and blue space on depressive symptoms across three dimensions: separate effects, multiplicative interactions, and additive interactions. Extreme high-temperature exposure, high levels of air pollution, lack of green space and blue space are all significantly associated with increased depressive symptoms risk. Heterogeneity exists across different genders, age groups, and educational levels in terms of individual associations. On both multiplicative and additive interaction scales, synergistic effects were demonstrated between heatwave and high levels of air pollution, as well as low levels of green and blue space, collectively amplifying the risk of depressive symptoms. Extreme high-temperature, elevated air pollution, and insufficient green and blue spaces are all significant environmental risk factors for depressive symptoms among middle-aged and older adults. Furthermore, air pollution and a lack of green and blue spaces amplify the adverse effects of high temperatures on depressive symptoms.