Higher levels of vegetation are associated with a lower risk of hospital admissions for mental disorders, according to an analysis of data from seven countries over two decades, published in the climate issue of The BMJ.
The results suggest that this protective effect increases with greater exposure to green spaces, with no clear threshold. According to the researchers, these data could guide urban planning and health policies to better protect mental health.
Mental health remains a global challenge. It is estimated that, in 2021, 1.1 billion people had mental disorders, representing 14% of the global burden of disease, with associated economic and social costs.
There is growing evidence that exposure to vegetation may reduce the risk of mental health problems, but most previous studies are limited to a single country, short time periods, or specific mental health outcomes.
To fill these gaps, the researchers analyzed 11.4 million hospital admissions for mental disorders in 6,842 localities across seven countries (Australia, Brazil, Canada, Chile, New Zealand, South Korea and Thailand) between 2000 and 2019.
All types of mental disorders were included, as well as six specific categories: psychotic disorders, substance use disorders, mood disorders, behavioral disorders, dementia, and anxiety.
Vegetation was measured using the Normalized Difference Vegetation Index (NDVI), a reliable satellite-derived indicator widely used to assess vegetation levels in an area.
Factors such as population, meteorological conditions, air pollution, socioeconomic indicators, and seasonality were taken into account, and models were stratified by sex, age, urbanicity, and the season of the year.
The results show that local vegetation is associated with a 7% reduction in hospital admissions for all types of mental disorders, with stronger associations for substance-related disorders (9%), psychotic disorders (7%), and dementia (6%).
However, associations varied between countries and disorder types. For example, Brazil, Chile, and Thailand showed consistent protective associations for most disorders, while in Australia and Canada vegetation was associated with a slight increase in the risk of hospitalizations for overall mental disorders and for some specific disorders.
Overall, the protective effects were stronger in urban areas, where it is estimated that 7,712 hospitalizations per year could potentially be avoided with greater exposure to green spaces.
Seasonal patterns in cities were also identified, suggesting that climate and meteorological conditions play an important role in the use and perception of green spaces.
An additional analysis indicated that a 10% increase in vegetation was associated with fewer hospitalizations, ranging from about 1 per 100,000 inhabitants in South Korea to approximately 1,000 per 100,000 inhabitants in New Zealand.
The researchers emphasize that this is an observational study, so it is not possible to establish cause-and-effect relationships. They also acknowledge uncertainties associated with using hospital admission data from multiple countries and note that the results only capture severe disorders that require hospitalization, thus underestimating the true impact on mental health.
Nevertheless, the authors state that the study suggests that “a considerable proportion of hospitalizations for mental disorders may be associated with exposure to vegetation and could be reduced through greening interventions in realistic scenarios.”
“The mental health benefits may also bring broader economic and social advantages, including reduced health costs, less burden on hospital systems, increased productivity at work, and improved community well-being,” they add.
Researchers also argue that future studies should explore the differentiated effects of various types of green spaces, such as parks or forests, and assess the quality and accessibility of these spaces for the population.
