Impact of climate on mental health among university students: a population-based study across 43 cities in China.

Individuals are often exposed to multiple climate factors simultaneously, but there is limited research on the combined effects of these factors on mental health. This study aims to explore the simultaneous impact of temperature, humidity, and the number of sunny days on mental health, as well as the potential social and behavioral mechanisms underlying these associations. There were 11,954 university students selected through a multistage survey sampling process. The multilevel logistic regression models showed that temperature and sunny days were significantly associated with probable mental disorder. Compared with students living in cities with low annual average temperature below 10 °C, those in cities with moderate temperature of 10-14.9 °C had lower odds of probable mental disorder in the comprehensive model (OR = 0.69, 95% CI: 0.50-0.95). However, students in cities with annual average temperature of 15 °C or above did not differ significantly from the low-temperature group (OR = 1.03, 95% CI: 0.73-1.45). Similarly, compared with students living in cities with fewer than 130 sunny days per year, those in cities with 170 or more sunny days had lower odds of probable mental disorder (OR = 0.69, 95% CI: 0.50-0.95). Formal tests further supported nonlinear associations between city-level annual average temperature and probable mental disorder. In the overall sample, both lower and higher temperatures were associated with higher levels of probable mental disorder, suggesting a U-shaped pattern. Similar patterns were observed in cities with fewer than 130 (F = 604.32, p < 0.001) and 130-169 sunny days (F = 183.35, p < 0.001). In contrast, among cities with 170 or more sunny days (F = 342.42, p < 0.001), the association showed an inverted U-shaped pattern. At the city level, sunny days were generally negatively associated with the prevalence of probable mental disorder. This pattern was consistent in the low- (F = 276.36, p < 0.01) and moderate-temperature (F = 10.08, p < 0.01) groups, but reversed in the high-temperature group (F = 213.46, p < 0.01). The structural equation models analysis showed that while temperature (β = 0.0758, p < 0.01) and the sunny days (β=-0.9963, p < 0.01) all have direct effects on mental health, and they also exert indirect effects through mental stress. This research provides critical insights that can inform both academic understanding and practical approaches to addressing the complex relationship between climate and mental health. The findings may inform climate-sensitive mental health policies, university-based health promotion programs, and broader public health strategies aimed at protecting young adults in the context of increasing climate variability.