In a prospective cohort study published in the journal Nature Mental Health, researchers in China investigated the potential association between exposure to residential greenness and the incidence of anxiety and depression and explored the underlying pathways. They found prolonged residence in green environments was linked to a lower risk of depression and anxiety, potentially via air pollution.
Study: Long-term exposure to residential greenness and decreased risk of depression and anxiety. Image Credit: p-jitti / Shutterstock
Background
Mental disorders, including depression and anxiety, are among the most debilitating conditions, with their impact on global health burden rising steadily. These disorders affect millions worldwide and are influenced by genetic, behavioral, and environmental factors. Recognizing modifiable factors associated with mental health issues could offer valuable targets for interventions and inform potential treatment strategies.
A growing body of evidence now recognizes residential greenness as a significant environmental factor in reducing stress and improving health, particularly mental well-being. While some longitudinal studies suggest a negative association between green spaces and depression, inconsistent findings highlight the need for larger, well-designed prospective cohort studies to better understand this relationship. Although long-term exposure to greenness is hypothesized to provide cumulative mental health benefits through various pathways, there is a dearth of population-based evidence, and the predominant mechanism remains uncertain. In the present cohort study, researchers explored the link between long-term exposure to residential greenness, depression, and anxiety while examining the potential pathways and factors.
About the study
Data from 409,556 participants were obtained from the United Kingdom (UK) Biobank, a large prospective cohort. The median age of participants was 58 years, and 52.4% were female. About 90.8% of the participants were white, and 86.2% resided in urban areas. Participants provided detailed information through questionnaires, physical measurements, and biological samples. The analysis focused on associations between residential greenness and incident depression and anxiety, excluding those with pre-existing mental health issues. Subsets of participants were analyzed to explore the potential pathways. The mean follow-up duration was 11.9 years.
Greenness around residential areas was evaluated using NDVI (short for Normalized Difference Vegetation Index), a measure based on land surface reflection of infrared wavelengths within buffer regions of sizes 300 m, 500 m, 1,000 m, and 1,500 m. Data were obtained from moderate-resolution imaging spectroradiometer remote sensing. Preprocessing was performed to remove cloudy and snow-covered areas, and water body impacts were addressed. Anxiety or depression diagnoses at baseline and follow-up were confirmed using hospital admissions, death registry, primary care records, and self-reports verified by healthcare professionals. Cases were identified using the International Statistical Classification of Diseases and Related Health Problems (ICD-10) codes.
Various potential confounders and mediators were considered in the analysis, including sociodemographic factors (age, gender, ethnicity, income, education, residence location), lifestyle factors (body mass index (BMI), smoking status, drinking status), and environmental factors (air pollutants, noise, water percentage, urban morphometric measures). Statistical analyses involved time-varying Cox proportional hazard models, hazard ratios (HR), restricted cubic splines, mediation analysis, subgroup analysis, and sensitivity analysis.
Results and discussion
In the study period, about 4.1% and 3.5% of the total participants were diagnosed with anxiety (HR = HR = 0.86) and depression (HR = 0.84), respectively. Residential greenness exposure consistently showed a protective effect on depression and anxiety, with significant reductions in risk observed across different buffer sizes. Mediation analyses indicated that air pollution, mainly particulate matter of diameter 2.5 microns (PM2.5), NO2, NOx, SO2, and O3, significantly mediated the associations between NDVI and both depression and anxiety. Further, IMD (short for index of multiple deprivation) was found to mediate the association between NDVI and depression. Additionally, factors such as distance to coast, factory, and healthcare places, as well as lifestyle factors like sleep duration and social engagement, showed small but significant mediation effects. The protective effects of green surroundings were found to be more pronounced against depression and anxiety in older adults and males.
Sensitivity analyses confirmed the robustness of the main findings. The large sample size, long follow-up duration, detailed adjustment for potential confounders, and exploration of potential mediators strengthen the findings. However, the study is limited by the lack of details on green space quality, the potential influence of extraneous factors on greenness exposure, healthy volunteer bias, and potential bias introduced by using diagnosis rather than symptom onset. Further research is warranted to confirm these findings.
Conclusion
In conclusion, this is the first prospective cohort study to provide comprehensive insights into the association between exposure to greenness and mental health. The findings reveal that long-term exposure to residential greenness is linked to lower risks of depression and anxiety. Higher levels of green surroundings showed more substantial positive effects on mental health, with reduced air pollution identified as a significant mediator. These findings call for strategic urban planning interventions by local governments to promote mental well-being by enhancing green spaces.
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