Engemann K, Pedersen CB, Arge L, Tsirogiannis C, Mortensen PB, Svenning JC. Residential green space in childhood is associated with lower risk of psychiatric disorders from adolescence into adulthood. Proc Natl Acad Sci. 2019;116(11):5188-5193.
To assess the impact of childhood residential green space (crGS) exposure on diagnosis of psychiatric illness as an adolescent and/or adult
Design and Participants
This was a retrospective longitudinal study measuring the amount of crGS for individuals (N=943,027) born in Denmark between 1985 and 2003 and living there continuously until the age of 10. Childhood residential green space was determined for the area 210 m x 210 m around their residential address, obtained from the Danish Civil Registration System, for each year from age 0 to 10. Green space was measured using satellite normalized difference vegetation index (NDVI) data, a standardized method for determining land use type. Areas were rated based on amount of green space present and categorized into 10 deciles (eg, most green → least green) for analysis.
Participants’ health data were taken from the Danish Psychiatric Central Research Register (PCRP), which records all inpatient and outpatient psychiatric visits. Presence of psychiatric disorders occurring between the 10th birthday and 2016 were determined by diagnoses in the PCRP based on the International Classification of Diseases, Tenth Revision (ICD-10) codes F01-F99 (Mental, Behavioral, and Neurodevelopmental Disorders), and/or era-associated ICD-8 codes.
The following psychiatric disorders were assessed: substance abuse, schizophrenia, schizoaffective disorder, mood disorders (including bipolar disorders and depressive disorders), neurotic stress-related and somatic disorders (including obsessive-compulsive disorder), eating disorders (including anorexia nervosa), and personality disorders (including borderline type). The cluster of “intellectual disabilities” (F70-79) is in the same ICD-10 category as these psychiatric diagnoses and so was included as a check on methods.
All outcomes were adjusted for urbanization (ie, population density), household socioeconomic status, family history of mental illness, year of birth, and parental age at time of birth.
Levels of crGS were associated with incidence rate ratios (IRRs; a measure of relative risk) for all psychiatric disorders measured. Individuals who grew up in the lowest crGS decile had a 27% increased risk of any psychiatric disorder diagnosis relative to the highest crGS decile. Risk for specific disorder diagnoses ranged from 12% increase (borderline personality disorder) to 37% increase (substance abuse disorder) for the lowest crGS decile relative to the highest crGS decile. All assessed diagnoses showed an inverse linear dose-dependent IRR to crGS, which means risk of each psychiatric diagnosis decreased in a manner that corresponded directly to increasing crGS. Adjusted IRRs for lowest relative to highest crGS for all diagnoses assessed are shown in the table below.
|Psychiatric Disorder||ICD-10 code||IRR||95% Cl|
|Any psychiatric disorders||F00-F99||1.27||1.24-1.30|
|Neurotic, stress-related and somatic disorders||F40-F48||1.33||1.29-1.37|
|Obsessive compulsive disorder||F42||1.21||1.13-1.31|
|Intellectual disability (control)||F70-79||1.01||0.92-1.11*|
*Not statistically significant
Abbreviations: CI, confidence interval; ICD, International Classification of Diseases; IRR, incidence rate ratio
These IRR were of comparable magnitude to other controlled psychiatric risk factors, such as family history of mental illness and parental age at time of birth. Childhood residential green space was a higher risk factor for psychiatric diagnosis than was degree of urbanization. However, crGS was more relevant in determining IRR with increasing urbanization; there was a greater IRR of psychiatric diagnosis in the most urban, population-dense areas compared to the most rural, population-sparse areas for the same crGS decile.
When assessing the impact of all measured risk factors, some psychiatric conditions (eg, mood disorders, especially depressive disorder, and neurotic stress-related and somatic disorders) were more associated with NDVI than any of the other factors measured.
Data also showed that cumulative amount of time spent in higher decile crGS was associated with lower IRR such that living in a high decile crGS from birth until age 10 resulted in lowest risk of psychiatric diagnosis, and living in a low decile crGS from birth until age 10 resulted in the highest risk. Moving housing location and changing to a lower or higher crGS during the study period either increased or decreased IRR, respectively. However, there did not appear to be any specific age-related exposure impacts of crGS on IRR; for example, changing crGS at age 3 vs age 8 did not appear to impact psychiatric IRR.
If these findings can be reproduced or supported, this study could significantly alter the understanding of mental illness etiology and provide exciting new opportunities for prevention.
Two diagnoses (schizoaffective disorder and anorexia nervosa) did not achieve statistical significance. These conditions had relatively few disordered individuals compared to other diagnoses—for example, only 426 were diagnosed with schizoaffective disorder, while 27,814 were diagnosed with depressive disorders—and so may not have been powered enough to achieve statistical significance.
This is a groundbreaking study that uses healthcare records of an entire country to assess the impact of individuals’ childhood exposure to residential green space on mental health and psychiatric diagnoses as adults. The findings show crGS is a significant influential risk factor for all diagnoses measured, associated with up to 37% variability in adult incidence of psychiatric diagnosis. If these findings can be reproduced or supported, this study could significantly alter the understanding of mental illness etiology and provide exciting new opportunities for prevention.
The current study under review is the latest to investigate the impacts of green space on mental illness and mental health. Research in this area began in the 1980s1 and has increased significantly in recent years.2 Specific studies have assessed the impact of adults’ residential green space on rates of depression, and studies reviewed in this journal have explored the mental health effects of living in3 and moving to/from4 areas with greater green space access. There are now systematic reviews and meta-analyses demonstrating the beneficial effects of outdoor natural environments for the mental health of adults and children.5-7 However, this is the first study to show a relationship between exposure to green space as a child and manifestation of psychopathology as an adult.
Multiple mechanisms have been proposed to explain how green space may influence mental health in adults.8 The 2 most commonly cited are the Stress Reduction Theory (SRT) proposed by Roger Ulrich9 and the Attention Restoration Theory (ART) proposed by Stephen and Rachel Kaplan.10
- SRT suggests there is a positive psychophysiological response to natural environments that reduces stress and associated allostatic load, with resultant neurological, hormonal, and immunological effects that impact overall health, including mental health.11
- ART suggests that cognitive processing of natural stimuli requires less “direct attention” than modern industrial stimuli, and is restorative to mental processes, thus reducing effort and strain that can lead to mental illness.12
Both SRT and ART are based on the concept of biophilia, the “inherent human affinity for other natural, living things” that arose in our prehistoric ancestors via millions of years of evolutionary adaptation to natural surroundings.13 Over this extended period our bodies, brains, nervous, and endocrine responses developed in ways that processed natural settings as a restorative baseline to which we are best suited (or, in evolutionary terms, “fit”) to inhabit. For many people, including children, modern living does not provide regular opportunities to experience these environmental benefits, leading to what author Richard Louv has called “nature-deficit disorder.”14 Evidence such as the findings of this current study provide support that these theories are valid.
Other additional mechanisms to explain how green space may be influencing mental health include the following:6
- Increased physical activity around the house and in nearby parks, leading to improved mental health.15 Physical activity is a well-established method for improving mental health and addressing negative moods and behaviors.16
- Mitigation of noise pollution through buffering of absolute and perceived sound levels,17 as well as reduction of total noise levels, which impact mental health.18
- Reduction of ambient temperature and humidity through vegetation buffering,19 which has been associated with decreased prevalence of mental health conditions.20
- Moderation of air quality, especially particulate matter and airborne toxins which adhere to/are absorbed by vegetation and are thus filtered out of the air.21 These toxins have been shown to impact mental health conditions such as schizophrenia and depression.22,23
- Modulation of immune system response with potential mental effects similar to other known cytokine impacts.24 These immunoregulatory mechanisms include
- greater physical contact with flora and fauna, an extension of the “hygiene hypothesis,”25
- inhalation of aromatherapeutic “phytoncides” produced by vegetation,26 and
- observation of natural scenes producing positive psychological states of awe and gratitude.27
- Enhanced personal, emotional, and self-developmental skills as a result of “place attachment” to preferred outdoor green spaces, resulting in greater mental health.28
- Increased social interaction and sense of community, which are both increased with greater amounts of green space and are known to affect mental health.29
These mechanisms have all been studied in adults. It will take a significant amount of research to determine if any of them are relevant for children aged 0 to 10, much less how exposure at this age range could be influencing future psychopathology. Regardless, it is difficult to review the body of evidence and not conclude that environmental factors like green space have some significant impact on mental health.
The results of this study were most pronounced in more densely populated urban regions. This data supports dozens of other studies detecting a large discrepancy in urban vs rural rates of mental illness.30 A study utilizing the same datasets as the current study demonstrated a 40% increased risk of psychiatric disorder for urban adults compared to rural adults, with rates for conditions such as schizophrenia as much as 86% increased.31 Meta-analysis has confirmed this urban association with schizophrenia,32 and measurable neuroanatomical and neurofunctional differences in brain structure predictive of schizophrenia have been detected in children raised in urban environments.33 This type of evidence has prompted some researchers to propose a study of “neurourbanism” to identify ways in which the “new” environment of modern cities may be altering brain development more suited for traditional “biophilic” contexts in which access to green space is more abundant.34
These issues are important to consider as urbanization accelerates in our modern society. In the United States, population is more than 80% urban and rising.35 Since 2008, the global population is more than 50% urban with projections of 65% urban population by the year 2050. Based on the material presented in this review, it is probably not a coincidence that mental health issues are also rising, currently reported by the World Health Organization as the number 1 cause of health-related disability in the world.36 Almost 1 in every 5 people in the United States lives with a diagnosed mental illness, and these numbers are increasing.37
While decline of available green space in urban (and rural) areas is likely not the only cause of these rising rates of mental ill-health, it is worthwhile to consider it as a contributing component. Fortunately, evidence also shows that increasing green space and green space experiences can decrease some psychopathologies such as depression and posttraumatic stress disorder (PTSD). These benefits often occur in a dose-dependent manner, with threshold “doses” of green space being easily within a feasible range of development for those cities willing to make the investment.38 Many municipalities are actively engaging in “urban greening” efforts to improve the health of their residents, both mental and physical, and the health of the local environment.39,40
Given the retrospective design of this study it is difficult to assess the exact mechanisms of action. The measure of crGS only evaluates green space surrounding the residential environment. It does not reflect active or passive engagement; for example, how often the child was outside accessing their environment or even looking out the window. However, other studies exploring these topics have demonstrated a direct relationship between green space access and utilization.41 Similarly, other environmental risk factors such as noise and air pollution were not measured.
As with all observational studies, this research was not able to establish causation. However, this should not be considered a significant limitation in light of the extreme logistical and ethical challenges of randomizing children’s residential location at birth through 10 years of age for the purpose of scientific study.
Exposure to green space in the area surrounding a child’s household may be a protective factor against developing a wide range of mental illnesses as an adolescent or adult. This is shown by the current study and supported by decades of evidence elucidating potential explanatory mechanisms. Conversely, absence of green space in the childhood residential environment may be a significant risk factor for the development of these conditions.
Given the prevalence of mental ill-health in society, it may be beneficial for healthcare systems to investigate the incorporation of environmental approaches for these conditions to address prevention and possible treatment. Clinicians could begin promoting time outside in nature for pediatric patients (and their parents) as much as possible. Individuals wanting to reduce the risk of psychiatric disorders in their own children may want to consider locating their residence in neighborhoods with sufficient green space. Multiple stakeholders, including individuals, communities, public health workers, and municipal administrators, may want to support local efforts to develop, maintain, and/or restore local greenery options for their mental health–promoting benefits.