Higher levels of greenness were associated with lower risks of hospital admissions for mental disorders in an analysis of data from seven countries over two decades. The results suggest that this protective effect increases with greater exposure to greenness, with no clear threshold - evidence that can inform urban design and health policy to better protect mental health, say the researchers.

Researchers analyzed 11.4 million hospital admissions for mental disorders of various types from 6,842 locations in seven countries (Australia, Brazil, Canada, Chile, New Zealand, South Korea, and Thailand) from 2000 to 2019. Greenness was measured by the normalized difference vegetation index (NDVI), a widely used and satellite-derived metric for assessing vegetation levels in a given area.

Factors including population levels, weather conditions, air pollutants, socioeconomic indicators and seasonality were taken into account and models were stratified by sex, age, urbanization, and season.

Local greenness was associated with a 7% reduction in hospital admissions for all- cause mental disorders, with stronger associations for substance use disorders (9%), psychotic disorders (7%), and dementia (6%), although results varied across countries and disorders.

Overall, protective associations were strongest in urban areas, where an estimated 7,712 hospital admissions for mental disorders annually were potentially preventable through greater exposure to greenness. Seasonal patterns were also found in urban areas, suggesting that climate and weather conditions play a crucial role in how green spaces are used and perceived, say the authors.

Further analysis in urban areas suggested that a 10% increase in greenness was associated with fewer hospital admissions for mental disorders ranging from around 1 per 100,000 in South Korea to approximately 1,000 per 100,000 in New Zealand.

Notably, age and sex showed no meaningful effect on the findings.

This is an observational study so no firm conclusions can be drawn about cause and effect. Nevertheless, the authors say this study suggests “a considerable proportion or rate of hospital admissions for mental disorders may be associated with exposure to greenness and could potentially be reduced through greening interventions under realistic scenarios.”

To download the full study, published in The BMJ’s climate issue, click here