Global climate change leads to changes in the frequency and/or magnitude of extreme weather and climate events, resulting in significant human morbidity and mortality, and adverse effects on mental health (Hrabok et al. 2020; Ebi et al. 2021). This study is the first to comprehensively examine the relationship between short-term exposure to extreme meteorological factors (including sunshine duration, wind speed, and precipitation) and anxiety outpatient visits in Suzhou, a city with a temperate climate in China. The results showed that extremely low sunshine duration, low wind speed, and high wind speed increase the risk of outpatient visits for anxiety. The associations were robust after adjustment by adding air pollutants (PM2.5, NO2) into the model. Furthermore, the effects were modified by gender, age and season. In the context of climate change, the findings may contribute to the development of weather-based early warning systems to minimize the impact of extreme meteorological factors on anxiety outpatient visits.
While the adverse effects of extreme climatic factors on anxiety disorders have been studied previously, most studies have focused on unfavorable temperatures (Trang et al. 2016; Zhang et al. 2020; Nori-Sarma et al. 2022; Li et al. 2022); studies on the effects of extreme sunshine duration, extreme wind speed and precipitation are limited. A study in Ningbo, China showed a significant and non-linear association between sunshine duration and hospital admissions for schizophrenia, and lack of sunlight increased the risk of hospital admissions for schizophrenia (Gu et al. 2019). However, the association between sunshine duration and outpatient visits for anxiety is largely unclear. Accoding to our study, exposure to extremely low sunshine duration increased the risk of anxiety outpatient visits. This means that people with anxiety disorders should spend appropriate amounts of time outdoors when the sun is full, rather than avoiding the sun.
From a biological perspective, vitamin D metabolism or circadian rhythms regulation could be involved in the relationship link sun exposure and anxiety disorder. A study showed that anxiety groups have an average lower vitamin D levels than the healthy group (Liu et al. 2022). Several studies have suggested that an imbalance in serotonin (5-hydroxytryptamine; 5-HT) neurotransmission may contribute to the development and persistence of anxiety disorders. (Eison 1990; Griebel 1995). Oxidative stress may also play a role in the neurobiology of anxiety disorders (Gautam et al. 2012; Boldrini et al. 2018). Vitamin D supplementation have seen in some studies to improve the severity of anxiety disorders (Eid et al. 2019; Borges-Vieira and Cardoso 2022). This may be due to the increased conversion of serotonin promoted by vitamin D, or the anti-inflammatory effect of vitamin D by reducing oxidative stress (Patrick and Ames 2014; Eid et al. 2019). In addition, natural sunlight may affect the suprachiasmatic nucleus of the hypothalamus, which regulates the body's circadian rhythm. One of its main regulatory functions is to inhibit the conversion of serotonin to melatonin by the pineal gland in sunlight (Kent et al. 2009).
Males and middle-aged and older adults were more sensitive to changes in natural sunlight than other groups, according to subgroup analyses. Previous research has shown that the skin's ability to produce vitamin D is significantly reduced in older adults (Wacker and Holick 2013; Heiskanen et al. 2020), who may be more prone to vitamin D deficiency and symptoms of anxiety disorders. Suzhou, known as the Cloud Capital, is home to East China's largest cloud computing data center. It is one of China's five largest communication node cities and a CG animation cluster rendering base. These industries have historically been male-dominated, which may lead men to doing more indoor computer work and putting them under more mental stress than women.
Our study found that both extremely low and high wind speed increased the risk of outpatient visits for anxiety. However, empirical research on the relationship between wind speed and mental health is scarce. Wind speed is associated with mood, violence, suicide and agitation in previous research (Schory et al. 2003; Denissen et al. 2008; McWilliams et al. 2014; Lickiewicz et al. 2020). Consistent with research showing that weather fluctuations have a particularly strong effect in spring, we found that extreme wind speed had a more adverse effect on outpatient visits for anxiety during winter and spring. Warm temperate semi-humid monsoon climate makes Suzhou hot and humid in summer and cold and dry in winter. This may be due to the calming effect of the moist, fresh, mild sea breeze (Yackerson et al. 2012), so experiencing cold and dry winds represents more of a hassle. Although previous research has shown no association between anxiety and wind speed (Bulbena et al. 2005), the association between anxiety and wind speed needs to be backed up by more research.
Our study has several advantages. This may be the first study to use a time-series approach to comprehensively explore the short-term effects of meteorological factors (including sunshine duration, wind speed and precipitation) on anxiety outpatient visits in a temperate climate city in China. We also looked for susceptible groups based on gender, age and season. Males as well as middle-aged and older adults appeare to be more susceptible to the cumulative effects of extremely low sunshine duration. The adverse effects of extreme wind speed were more pronounced in the cold season. Our study adds to the epidemiological evidence for the effect of extreme meteorological factors on anxiety outpatient visits. At the same time, it provides a reference for the government and medical authorities to formulate targeted intervention measures to protect vulnerable groups.
The study also has some limitations. First, we only studied a typical hospital in Suzhou, and the findings may be limited in generalizability. Second, anxious subjects who did not seek treatment may not have been captured, which may underestimate the effects of air pollution. Third, the use of ecological research may cause ecological bias. Where individual exposures were limited, we used monitoring data from weather stations rather than individual-level exposures. Fourth, we did not further investigate the relationship between extreme weather and various anxiety disorder subtypes due to limited data. Finally, individual confounding factors such as chronic diseases and smoking were not included in this study, and the impact of these factors needs to be addressed in the future.