In a recent study published in the journal Planetary Health, research conducted a massive, long-term, population-scale study comprising 2.3 million individuals from Wales to elucidate whether greater exposure to green and blue spaces (GBS) could be linked with mental health outcomes. Results from this 10-year-long longitudinal dynamic panel study revealed that greater exposure was associated with reduced common mental disorders (CMD), with these results more significant for individuals from more socio-economically deprived quintiles. These findings can help inform government policy on GBS, especially those planning GBS set-up to improve the mental health of their communities.
Study: Ambient greenness, access to local green spaces, and subsequent mental health: a 10-year longitudinal dynamic panel study of 2·3 million adults in Wales. Image Credit: Dmitrijs Bindemanis / Shutterstock
Mental health and GBS
Common mental disorders (CMD) remain a primary contributor to the worldwide disease burden, resulting in an estimated 4.9% reduction in disability-adjusted life-years. Recent research has hypothesized that increased exposure to greener and blue spaces (GBS) or living close to green spaces may be responsible for improved mental health outcomes. Unfortunately, there is a dearth of longitudinal scientific evidence to support this hypothesis.
GBS comprises open-air natural settings, including parks, gardens, forests, beaches, lakes, and ponds, and has long been categorized as cultural ecosystem services. High-quality GBS accessibility and distribution, however, is unequal, with individuals living in deprived areas, older adults, the sick and disabled, and minority ethnic communities being far more deprived than their more affluent, younger, and more mobile counterparts.
Small-scale cross-sectional and longitudinal studies have suggested that increased GBS exposure may result in better mental health and well-being. However, they have predominantly included area- rather than household-level census data and been restricted to urban settings. Furthermore, the results from these pilots have been inconclusive, differing in estimated lag times between GBS exposure and beneficial mental health outcomes.
About the study
In the present study, researchers investigated the adult (>16 years) outcomes of accrued household GBS availability, with explicit consideration of health inequalities. They further delinked GBS exposure and living in greener areas, allowing them to analyze the benefits of each separately. Since households may change location over time, the research methodology accounted for these shifts and periodically tracked household movement (anonymously) via a routine updation of household residence, as opposed to previous studies, which focused on the region rather than families or households.
The study cohort comprised adult individuals from Wales, United Kingdom (UK), tracked electronically between 2008 and 2019. The dataset was derived from the Welsh Demographic Service Dataset and consisted of anonymized individual-level demographic characteristics of citizens registered with National Health Service (NHS) general practitioners (GPs). This dataset comprises 80% of the total Welsh population. Geographical location was participant-provided and updated when a participant or GP reported a change in household residency. The study design nested all individuals above 16 years within a household.
The demographic data thus obtained was linked to GP records, including CMD reports. Data from the Annual District Death Extract from the Office for National Statistics mortality register was used to periodically add individuals who had attained the age of adulthood and remove those who had died from the dataset. The Welsh Index of Multiple Deprivation and rural-urban Office for National Statistics classifications for Lower Layer Super Output Areas were used for the socioeconomic and demographic classification of study cohort participants.
This study defined ‘household ambient greenness’ as living within 300 m of a GBS and ‘local GBS access’ as 1. living within 1600 m of a GBS easily accessible via foot or 2. living in proximity to a GBS with a road-accessible network buffer. Greenness was measured using the Enhanced Vegetation Index (EVI), which improves over the hitherto prevalent Normalized Difference Vegetation Index (NDVI) by correcting for atmospheric conditions (e.g., cloud cover) and canopy background noise. EBI and NDVI are obtained from geospatial satellite imagery, with data for this study from Landsat 2008-2019.
“The substantial topographical variation in our study area (Wales, UK) made EVI the most appropriate measure as it is less susceptible to the effects of topographic seasonal factors. We calculated this within 300 m of each home in Wales.:
Building on earlier research, this study posited that certain advantages of green-blue space (GBS) exposure, such as serene natural views and lower noise levels compared to industrial environments, could be experienced without direct physical interaction with the GBS. To minimize noise from cloud cover, images were extracted during the springtime between May and June of each year.
“We used the cloud masks to set pixels covered by cloud in the satellite imagery to NULL to prevent these values from influencing the final greenness density. A larger EVI score does not necessarily equate to more greenness by area but instead represents a larger volume (increased biomass) of green. A small forest, for example, could produce the same EVI score as a large area covered in grass.”
Obtained GBS imagery in tandem with local authority technical advice notes (TAN 16) was used to classify GBS into those that can be 1. seen and 2. physically accessed. The CMD (anxiety or depression) data was categorized into 1. no CMD, 2. CMD diagnosed one year prior to study commencement, and 3. CMD was diagnosed within the eight years preceding study initiation, for which medication was ongoing.
Statical covariates included sex, age group, household births and deaths, area-level socioeconomic deprivation, household movement, and region category (urban or rural). Statistical analyses comprised the application of multivariate logistic regression to elucidate correlations between normalized time-aggregated EVI and access measures.
Results from this study highlight that both increased ambient greenness and GBS access were independently associated with a reduced likelihood of future CMD. Stratified statistical analyses showed that this association was strongest for people living in deprived areas (10% reduction per 0·1-unit increase) compared to those living in areas with low deprivation (6% reduction).
These results also discovered that individuals’ CMD history alters the benefits of ambient greenness and GBS access – individuals with a previous history of CMD were found to benefit more than those without prior CMD diagnoses. However, both showed positive associations, indicating that GBS has both restorative and preventive functions in combatting CMD.
“Although the study period for this research preceded the COVID-19 pandemic, the need for GBS to access or view was brought to the fore during the early stages of the pandemic, particularly for those living in urban areas with poor or no access to private or communal garden spaces. Our results suggest that investing in improved ambient greenness, as well as making public GBS accessible, might lead to future mental health benefits for adults with and without a history of CMD.”
In combination with the above, GBS, especially in urban settings, might provide additional co-benefits, including food and job creation, flood prevention, carbon sequestration, and biodiversity promotion. GBSs can, therefore, be thought of as both public health and social investments. This study thus forms the scientific repository that policymakers and urban planners can use in their decision-making processes, thereby helping their respective communities from both ecological and mental health standpoints.
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