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Cities, Green Space, and Mental Well-Being

Summary and Keywords

Mental and behavioral disorders account for approximately 7.4% of the global burden of disease, with depression now the world’s leading cause of disability. One in four people in the world will suffer from a mental health problem at some point in their life. City planning and design holds much promise for reducing this burden of disease, and for offering solutions that are affordable, accessible and equitable. Increasingly urban green space is recognized as an important social determinant of health, with the potential to protect mental health – for example, by buffering against life stressors - as well as relieving the symptom severity of specific psychiatric disorders. Pathways linking urban green space with mental wellbeing include the ability of natural stimuli – trees, water, light patterns – to promote ‘involuntary attention’ allowing the brain to disengage and recover from cognitive fatigue. This article brings together evidence of the positive effects of urban green space on common mental health problems (i.e. stress, anxiety, depression) together with evidence of its role in the symptom relief of specific psychiatric disorders, including schizophrenia and psychosis, post-traumatic stress disorder (PTSD), dementia, attention deficit/hyperactivity Disorder (ADHD) and autism. Urban green space is a potential force for building mental health: city planners, urban designers, policy makers and public health professionals need to maximize the opportunities in applying green space strategies for both health prevention and in supporting treatment of mental ill health.

Keywords: city green space, mental health, depression, stress, psychiatric disorder, lifestyle, rehabilitation, health resilience

Introduction

The world is facing a health crisis of growing proportions as mental health problems increase, both within economically advanced countries and low- and middle-income countries, who struggle with competing health priorities such as HIV/Aids, Ebola, malaria and the growing rise in noncommunicable disease, such as heart attack and stroke. Mental and behavioral disorders now account for approximately 7.4% of the global burden of disease, with depression the world’s leading cause of disability (Ferrari et al., 2013). Advancing mental health is now a global public health priority sanctioned by the 66th World Health Assembly (WHA) in their adoption of the World Health Organization’s (WHO) Comprehensive Mental Health Action Plan (CMHAP), recommending a comprehensive and coordinated approach to reduce the global burden of mental disorders (WHO, May 2013). The increasing trend in mental health disorders may be related to increased urbanization (Peen, Schoevers,Beekman, & Dekker, 2010); with 77% of people in the world’s more developed regions living in cities (UN, 2015)—with consequent reduction in access to natural settings—this presents an increasing challenge for mental health services. Building on recent evidence showing the stress regulation benefits of green space among the urban poor (Roe et al., 2013; Ward Thompson et al., 2012; Ward Thompson, Aspinall, Roe, Robertson, & Miller, 2016), this article explores how city green space might relieve the burden of mental global health problems and facilitate rehabilitation and recovery in specific mental health problems.

Urbanization is happening at rapid speed, projected to reach almost 70% by 2050 (UN, 2015). Owing to the combined effect of increased birth rates and migration, Africa, India, and China are set to experience exponential growth in the coming decades. This growth, in turn, means city dwellers face increasing socio-economic and environmental stressors such as rising rental costs and poorer-quality housing, busier streets and roads, resulting in increased air and noise pollution, and having less access to “escape” places, such as parks, for stress relief. Mental health problems are more prevalent in socio-economically disadvantaged people (Patel, 2015). In addition, the rising numbers of migrants moving to cities potentially adds to the burden of mental health care, since immigrant populations have an increased risk of psychiatric disorders (Veling & Susser, 2011; Selten, Cantor-Graae, & Kahn, 2007). Abbott (2012) has posited that the rising stress in our cities is triggering higher levels of psychosis, citing the Camberwell Study in South East London, where the incidence of schizophrenia almost doubled between 1965 and 1997 during a period in which there was no comparative rise in the general population (Boydell et al., 2003).

This has led to calls for a “lifestyle medicine” approach that advocates recognizing the importance of both individual health behavior change, and the environment. Lifestyle factors such as being physically active and taking part in community voluntary activities reduce the risk of depression. As part of an integrated approach to health promotion, “shinrin-yoku” (translated as “forest bathing”) is a practice promoted by the Japanese Forest Agency for healthy lifestyle; in addition, “green prescriptions,” such as taking a woodland walk, have been advocated by the former Chief Medical Officer for Scotland, Harry Burns. Both of these national health policy initiatives draw on a growing body of evidence showing the salutogenic properties (i.e., health-improving) of natural environments. These range from national-level epidemiological studies exploring self-reported mental health (Alcock, White, Wheeler, Fleming, & Depledge, 2014) to localized case studies exploring physiological and psychological outcomes in populations suffering from life stressors such as job loss (Roe et al., 2013; Ward Thompson, Roe, & Aspinall, 2013) and specific mental health problems such as schizophrenia and psychosis (Roe & Aspinall, 2011a). However, there are few studies that relate city green space attributes (e.g., quantity, quality, access, and distance to) to measures of specific mental health problems widely experienced by the general population, such as depression and anxiety, and much less evidence relating outcomes to rehabilitation and recovery in specific mental health disorders.

Two questions have prompted this targeted and thematic review. For people living in cities (a) What evidence is there to indicate natural environments benefit depression, anxiety, and stress in the general population? (b) What evidence is there to indicate natural environments can alleviate the symptoms of people with diagnosed psychiatric problems? Question (a) pertains to green space as health-protective mechanism; question (b) is concerned with green space as a potential rehabilitation intervention to relieve the severity of specific conditions.

The evidence was gathered based upon current systematic reviews and meta analyses in the field (Bowler, Buyung-Ali, Knight, & Pullin, 2010; Barton & Pretty 2010), recent reviews (i.e., WHO/CBD, 2015; Allen & Balfour, 2014; Kuo, 2015; Natural Environments Initiative, 2014), along with searches in online literature databases in relation to specific mental health problems (i.e., Science Direct, Google Scholar), and the author’s own knowledge and expertise in the field. Prior to setting out the evidence thematically by mental health condition, this article presents the theoretical framework for green space as a health resource in the mental health field, identifying the key pathways posited to link green space to mental health outcomes (see Figure 1).

Firstly, to clarify terminology, what is a mental health problem? From a clinical standpoint, this term designates a disorder in various forms of mental functioning, that is, in thought, speech, emotion, and behavior. These disorders are myriad and mixed, and range from Autism Spectrum Disorders to Attention Deficit Hyperactivity Disorder (ADHD), through to Obsessive-Compulsive Disorder, anxiety disorders, postpartum depression, recurrent depressive illness, dementia, and chronic health problems like Parkinson’s disease, which has high rates of anxiety and depression. Psychiatric professionals stress the importance of the premise that a person is not defined by a diagnosis, and that an individual with a diagnosis may, in fact, be coping much better on medication than someone without a diagnosis. For the purpose of this review, we refer to “people with mental health problems,” which acknowledges that many people experience mental distress and this may be a “problem” but not necessarily an “illness” (Mental Health Foundation, n.d.). In relation to question (b) of this review (i.e., concerning evidence of the benefits of a natural environment), we refer to people with specific psychiatric diagnoses.

Green Space as a Concept in the Mental Health Field

The Restorative Health Model

The mental health benefits of natural settings have been explored to date under the rubric of “psychological restoration,” that is, the ability of nature to produce physiological, affective, or cognitive recovery in the short term, termed “discrete” or micro-restorative experiences (Hartig, 2007). Restoration itself has been defined as “the process of recovering physiological, psychological and social resources that have been diminished in efforts to meet the demands of everyday life” (Hartig, 2007). Resources are defined as the “capabilities that come into play when people try to meet demands” (Hartig, 2007). There is now considerable evidence showing that contact with nature can promote psychological restoration in the form of improved mood (Hartig, Evans, Jamner, Davis, & Gärling, 2003; Roe & Aspinall, 2011a), improved cognitive attention (Hartig et al., 2003; Berman, Jonides, & Kaplan, 2008), and reduced stress and anxiety (Ulrich et al., 1991; Hartig et al., 2003; Grahn & Stigsdotter, 2003). Within deprived social housing communities in Chicago, research has consistently shown the benefit of green space both to cognitive restoration (Kuo, 2001; Faber-Taylor et al., 2002), self-discipline (Faber-Taylor et al., 2002), reduced aggression (Kuo & Sullivan, 2001a), and reduced crime (Kuo & Sullivan, 2001b).

In addition, there is evidence supporting discrete physiological restoration from contact with nature, impacting on the parasympathetic nervous system (e.g., Gladwell et al., 2012; Brown, Barton, & Gladwell, 2013) blood pressure (e.g., Hartig et al., 2003), cortisol (e.g., Park, Tsunetsugu, Kasetani, Kagawa, & Miyazaki, 2010; Roe et al., 2013; Ward Thompson et al., 2012), neurophysiology (Roe, Aspinall, Coyne, & Mavros, 2014; Aspinall, Mavros, Coyne, & Roe, 2013; Ulrich, 1981; Kim & Kaplan, 2004), and heart rate (Laumann, Garling, & Stormark, 2003). It is posited that these short-term psychological and physiological effects—if experienced regularly—could account for longer-term health effects (Kuo, 2015).

The discrete model is the dominant theoretical model in the field to date, although Hartig proposed a longer-term restorative process called instoration, defined as the strengthening of an individual’s capability to meet everyday demands, such as improved self-identity or self-esteem (Hartig, Böök, Garvill, Olsson, & Gärling, 1996). Instoration is a deeper and more profound type of recovery that facilitates acquisition of coping resources, a process I suggest is more akin to concepts of health resilience described in the following discussion.

The restorative affordances of green space are thought to stem from nature’s ability to promote “soft fascination”—a form of automatic, effortless attention as opposed to the cognitive demands required to negotiate a busy urban street (Kaplan & Kaplan, 1989). The framework draws on psychologist William James’s distinction (1892) between two kinds of attention: directed attention, which is characterized by an exertion of effort, and involuntary attention, which is effortless and responsive to interesting stimuli. Natural settings, inherently rich in fascinating stimuli, invoke involuntary attention, which in turn, allows room for contemplation and recovery of depleted cognitive resources. In addition to fascination, natural settings are believed to promote restoration owing to their ability to promote a sense of being away (i.e., psychological and/or geographical distance from routes that impose demands on directed attention); extent (a setting “rich enough and coherent enough so that it constitutes a whole other world” [Kaplan, 1995, p. 173]) and with compatibility (i.e., a good fit between an individual’s goals and the kinds of activities supported by the setting). Alternatively, Ulrich’s psycho-physiological model (Ulrich et al., 1991; Ulrich, 1983) posits that stress reduction arises, not from cognitive recovery as in the above model, but from an affective and aesthetic response to the visual stimulus of a natural environment (called “preferenda”) that spark a psycho-physiological response that can produce significant recovery from stress (Ulrich, 1983). Psychological and physiological stress recovery is a therefore posited as a key pathway believed to underline the relationship between green space and health outcomes (see Figure 1 below).

The Salutogenic Model

Antonovsky, a medical sociologist, has posited the theory of salutogenesis (origins of health) as a necessary complement to the dominant pathogenic view of health (i.e., the etiology of disease) (Antonovsky, 1979). He conceived of health as a dynamic “steady state,” a sliding or movable point on the ease–dis-ease continuum. Working mostly with Holocaust survivors, he developed a scale by which to operationalize and measure salutogenesis, the Sense of Coherence (SOC), which has three components: comprehensibility, manageability, and meaningfulness. Antonovsky postulated that individuals and/or groups mobilize their “generalized resistance resources” (GRR) in order to manage stress and maintain a “dynamic steady state” of health. A strong SOC is associated with less stress and tension and with greater coping resources and health resilience in which an individual has the capacity to meet the demands posed by everyday stressors. Whilst salutogenesis is frequently referred to in the green health literature (Ward Thompson, Aspinall, & Roe, 2014), this model has, as yet, to be operationalized and properly tested in the field.

Green Space and Mental Health Pathways

A number of nature–health models have identified the key pathways by which green space impacts on health (Hartig, Mitchell, de Vries, & Frumkin, 2014; Lachowycz & Jones, 2013; Kuo, 2015). These include air quality, physical activity, social contact and psychological restoration. All four pathways operate by direct contact with nature (i.e., by viewing nature or walking/being active in nature) and are dependent on the frequency and duration of contact. Hartig et al. (2014) posit that two of these pathways also operate via indirect contact, that is, air quality and stress recovery operate through the natural environment without an individual or group consciously engaging with nature. All four pathways share a reciprocal relationship: for example, the presence of street trees may improve air quality in a neighborhood, which in turn generates a more conducive environment for active travel (e.g., walking/cycling activities), in turn offering affordances for social contact (impromptu or organized). This, combined with the aesthetic affordances of the trees themselves—or soft fascination—in turn facilitates psychological and physiological restoration. A further posited pathway in green space and health outcomes is that of enhanced immune function and the role of inflammatory cytokines known to impact on depression, arising from biodiversity and microbiota in natural settings. However, to date, the evidence is weak (see Kuo, 2015; WHO/CBD, 2015 for reviews of the evidence).

A critical pathway in relation to mental well-being is the posited link between green space and social well-being. Social company is a co-determinant of both physical activity (Giles-Corti et al., 2005) and better mental well-being (Sugiyama, Leslie, Giles-Corti, & Owen, 2008). Community open space and natural settings have been found to enhance social ties and sense of community in older adults (Kweon, Sullivan, & Wiley, 1998; Sullivan, Kuo, & Depooter, 2004); in residents of American urban communities (Kim & Kaplan, 2004); and in residents of a large public housing development in Chicago (Sullivan et al., 2004). Maas et al. (2009) found that less green space in people’s living environment coincided with lower indicators of social well-being. But few studies have explored the impact of green space on social cohesion in people with serious mental health problems, in older people, or migrants, populations who all experience isolation and lack of opportunities to engage socially.

Cities, Green Space, and Mental Well-BeingClick to view larger

Figure 1. Green health: some pathways by which green space is posited to impact on mental health outcomes

The Form of Contact with Nature

Little is known about differences in the form of contact (i.e., viewing or being in nature) or length of contact time or the quality of nature being experienced. Both being in and viewing nature from a window or virtually via photos on a computer screen appear to be beneficial to psychological restoration (see Africa et al., 2014 for a recent review of the evidence). Recently, as little as a 40-second micro-break viewing a nature scene was shown to improve concentration levels in the work place (Lee, Williams, Sargent, Williams, & Johnson, 2015) but little is known about “dose” response in the field. The current research is mostly all cross-sectional in design, making causal pathways difficult to establish, and longitudinal evaluation of sustained benefits of contact with nature is limited. An exception is a study by Ward Thompson et al. (2014) exploring quality of life benefits arising from woodland interventions in deprived urban communities in Scotland over time. This protocol has subsequently been developed to explore the effectiveness of social and physical interventions in urban woodlands on stress regulation in deprived urban communities in Scotland, with results pending (Silveirinha de Oliveira et al., 2013). Another rare longitudinal study analyzed mental health outcomes (i.e., General Health Questionnaire scores) from British Household Panel Survey data over five consecutive years (Alcock et al., 2014). The authors found that moving to a greener area is not only associated with immediate improvements in people’s mental health, but that the effect continued three years post-move.

Green Space and Mental Health: The Evidence

Depression in the General Population

Depression is the most common mental disorder that occurs in people of all ages across the world (Ferrari et al., 2013). Depression is described as “an overwhelming feeling of sadness and hopelessness that can last for months or years” (editor’s summary, Ferrari et al., 2013) which impacts on people’s ability to pursue purposeful lives, and is associated with increased anxiety, decreased energy, feelings of guilt and/or low self-esteem, disturbed sleep or appetite, and poor concentration.

The evidence relating green space to outcome measures of depression at a population level is limited to a handful of epidemiological studies, mostly exploring quantity and residential proximity to city green space. Beyer et al. (2014), in a study in Wisconsin, found higher levels of neighborhood green space were associated with significantly lower levels of symptomology for depression, anxiety and stress, after controlling for a wide range of confounding factors (n=2,479); Maas et al. (2009), exploring morbidity data in Danish medical practices serving a population of 345,143, found the prevalence of psychological morbidities (i.e., for anxiety disorder and depression) was lower in neighborhoods with more green space (i.e., in a 1 km radius around the home); in another Dutch study, Van den Berg and Van den Berg (2010) found relationships between quantity of green space (as measured by a 300km radius around the home) and the impact of perceived stressful life event (n= 4529) but found no statistically significant relationships between mental health benefits as measured by General Health Questionnaire (GHQ-12) (a self-report measure of psychological morbidity) and green space quantity. In a New Zealand study, Nutsford, Pearson, and Kingham (2013) (n=7552) found statistically significant relationships between the quantity of green space (i.e., both total and useable green space within a 3 km radius around a home and distance to nearest useable green space) and decreased anxiety/mood disorder treatment counts in an urban setting. Lower risk of depression was strongly associated with green space quantity measures, including a higher percentage of neighborhood tree canopy. As flagged above, Alcock et al. (2014) found mental health improvements in individuals relocating to areas with more green space in the UK and that these benefits were sustained three years post move (n=594).

Exploring the incidence of depression in pregnant women in Bradford, UK, McEachan et al. (2015) found higher residential greenness was associated with a reduced likelihood of depressive symptoms in pregnant women, with associations stronger for more disadvantaged groups and for those who are already physically active. Since research suggests 12%–23% of women experience depression during pregnancy (Bennett, Einarson, Taddio, Koren, & Einarson, 2004; Gaynes et al., 2005)—and given that depression in pregnancy is the highest risk factor for postnatal depression—access to green space may offer a promising intervention to reduce risk of depression in this population.

In summary, green space quantity is associated with lower levels of depression in high-income countries, but very few studies have replicated experimental designs or used comparable measures of green space and depression outcome indicators.

Stress and Anxiety in the General Population

Stress levels are rising worldwide. A recent snapshot of stress in the USA shows that 43% of adults reported higher levels of stress in 2014 than in the proceeding year (APA, 2015), with results higher in women (e.g., 44% experienced symptoms of stress in the last month), and 1 in 5 of the population failing to engage in any activity to relieve stress (APA, 2015). This is accompanied by a rise in workplace stress globally; with a staggering 86% of workers in China reporting workplace stress (Global Organization for Stress, 2016).

Stress involves activation of neurobiological systems that preserve stability (or viability) or change via allostasis (Gunnar & Quevedo, 2007). Allostasis describes the body’s ability to respond to distress by changing activity level and maintaining it at the new level for as long as necessary. Psychological stress occurs when external demands exceed the adaptive capacity of an individual, in turn exerting physiological processes. Two endocrine response systems are particularly reactive to psychological stress: the hypothalamic-pituitary-adrenocortical axis (HPA) and the sympathetic-adrenal-medullary (SAM) system. Any threat triggers the release of hormones such as cortisol and adrenaline, raising levels of sugar in the blood, redistributing blood flow to muscles and lungs, and triggering the “fight or flight” response to stressors. This stress mechanism has helped us survive from an evolutionary standpoint. The problem arises when the stress response doesn’t switch off. The stress hormone levels stay too high for too long, causing allostatic load, the cumulative “wear and tear” on the body that the body experiences owing to repeated cycles of allostatis.

The effect of stress on the regulation of immune and inflammatory processes has the potential to influence depression as well as cardiovascular disease (CVD), human immuno-deficiencies such as HIV/AIDS, and cancer (Cohen, Janicki-Deverts, & Miller, 2007). Generally, stressful events are thought to influence the development of physical disease by causing negative affective states (e.g., feelings of anxiety and depression), which in turn exert direct effects on biological processes or behavioral patterns that influence disease risk. Exposure to chronic stress (i.e., experienced over a prolonged period of time) is considered to be most toxic (as compared to shorter-term acute stress) because it is more likely to result in long-term or permanent changes in the emotional, physiological, and behavioral responses that influence susceptibility to disease. Approximately 20% to 25% of persons who experience major stressful events will subsequently develop depression (van Praag, de Koet, & van Os, 2004).

The science of urban stress is being explored with new mobile technologies that map how people feel as they move through a city environment. For example, a novel study in Scotland has shown different patterns in brain activity from walking in a busy commercial urban area versus an urban park (Aspinall et al., 2013), with higher levels of meditation (as measured by electroencephalography (EEG) software developed by Emotiv) experienced when moving in to the city green space. Further testing is ongoing in older adults aged 65+ analyzing the raw EEG data. The researchers hope to pinpoint those areas in a city that cause older people most stress—such as traffic junctions—and make recommendations for design improvements to improve access and mobility (Mood, Mobility, and Place, n.d.)

Lederbogen et al. (2011) have shown that the brains of city-dwellers respond to social stress differently depending on their histories of urban living. In a laboratory experiment, the team applied social stressors (in the form of negative feedback to a series of arithmetic tests) and found differences in the amygdala—the brain’s emotional processor—depending on whether the participants had been raised in the city or the countryside. In addition, the cingulate cortex, which helps to regulate the amygdala and processes negative emotion, responded more strongly in those brought up in large cities than in those brought up in the countryside.

Studies monitoring the stress hormone cortisol and relationships to green space are limited to handful of studies. A study in Dundee, Scotland explored differences in stress physiology and quantity of green space using diurnal cortisol patterns (as measured in human saliva four times daily and synchronized to wake times) in adults experiencing life stressors, that is, unemployed or not in work for some other reason (Roe et al., 2013; Ward Thompson et al., 2012). Controlling for confounders such as income deprivation, the study found that residents living in areas with higher levels of green space showed better stress regulation, as indicated by a steeper slope cortisol gradient over the day. The effect was stronger for women, whose diurnal patterns of cortisol showed greater hypocortisolemia (i.e., unusually low levels of cortisol resulting in a flat diurnal cortisol slope indicative of chronic stress and exhaustion) for those living in areas with little green space compared to those living with higher levels of green space (>43%). This is an important result because dysregulation of the daily pattern of cortisol secretion, part of our circadian rhythms, is associated with an array of negative health outcomes including major depressive disorders. The patterns in cortisol were also reflected in data capturing perceived levels of stress, with both men and women reporting lower perceived stress in areas with more green space. These findings on perceived stress are echoed in a Danish study, which showed increases in perceived stress in individuals living more than 1 meter away from a green space (n=11,200) (Stigsdotter et al., 2010).

Some of the most interesting evidence of the health benefits of nature to stress and immune function is coming from Japan, and revolves around the popular practice of shinrinyoku or “forest bathing,” a lifestyle practice introduced in 1982 by the Forest Agency of Japan. A series of studies have shown the effect of walking in forests and natural environments on psycho-neuro-endocrino-immunology, including cortisol levels, pulse rate, blood pressure and heart rate variability (Park et al., 2010; Toda, Den, Hasegawa-Ohira, & Morimoto, 2013). Forest bathing on two consecutive days has been shown to increase the number and activity of anti-cancer NK cells by 50% and 56% respectively and to sustain numbers up to month later (Li, 2010). In addition, extended time in a forest decreased inflammatory cytokine implicated in chronic disease by roughly one half (Mao et al., 2012). The chemical compounds derived from plants—phytoncides—have shown to boost immune function (Li et al., 2009), and certain tree species—including Japanese cypress, Japanese cedar, Japanese beech, and Japanese oak—are all proven to be effective in raising NK activity.

In summary, there is growing evidence showing links between green space and the psycho-neuro-endocrine system and immunology, which has important implications for chronic diseases associated with stress including depression, CVD, stroke, and cancer. The stress–green space pathway may account for the lower reported incidence of specific morbidities associated with hypertension (the main risk factor for heart disease) such as ischemic stroke (Wilker et al., 2014) and circulatory disease (including cardiovascular diseases, such as heart disease and stroke) (Mitchell & Popham, 2008) but causal links have yet to be established.

Specific Mental Health Problems

Serious mental illness is a term used to classify persistent psychiatric conditions that can greatly affect a person’s behavior, thinking, emotions, and relationships (Kloos, 2005). Diagnoses considered to be serious mental illnesses include schizophrenia, bipolar disorder, and severe and persistent depression.

As long ago as 1998, Freeman (1998) stated a need for “more studies on individual environmental factors in specific population groups and in relation to specific psychiatric disorders,” but research in groups with specific mental health problems and relationships with the built environment is still virtually nonexistent. Yet rates of psychosis in urban contexts are rising (Boydell et al., 2003). People with severe mental illness have poorer diets, take less exercise, smoke more, and often gain weight due to their medication, with an increased risk of obesity, heart disease, and diabetes. For example, people with schizophrenia, on average, die fifteen to twenty years younger than the general population (Schizophrenia Commission, 2012). This is a health inequity that requires urgent attention, and particularly interventions to decrease sedentary behavior, which is a distinct risk for multiple adverse health outcomes in people with schizophrenia. Promoting physical activity in people with severe mental illness is therefore very important, but little is known about the types of environments that can best support physical activity in people with specific conditions, nor what types of environment alleviate or aggravate symptoms. Below, we look in turn at specific mental health problems that are rising globally—schizophrenia, PTSD, autism, ADHD, and dementia—and explore the evidence for green space and nature therapy (including outdoor exercise) as a rehabilitation aid.

Schizophrenia

Research has shown the detrimental effect of a busy urban environment (i.e., a Camberwell UK shopping street) on people with diagnosed psychotic disorders as compared to a brief mindfulness relaxation task (Ellett, Freeman, & Garety, 2008). Exposure to the urban environment increased levels of anxiety, negative beliefs about others, jumping to conclusions, and levels of paranoia. By contrast, walking in natural environments has been shown to improve mood, reflection, and sense of purpose on life goals in people with psychosis and schizophrenia (Roe & Aspinall, 2011a). In the same study, an urban walk rich in historic fascination generated similar effects, suggesting that walking in carefully chosen urban environments (for example with interesting building facades) can promote mental well-being in people with severe mental health problems.

Recent research has shown relationships between sedentary behavior (as measured by daily sitting time) in people with schizophrenia (n=123) and perceived satisfaction of built environment characteristics, notably the aesthetics of the neighborhood environment, including the presence of street trees, which promoted more active living (Vancampfort et al., 2014). Sedentary behaviors were most notable in patients who perceived the neighborhood built environment to be less enjoyable and attractive. The study is consistent with the authors’ earlier study (Vancampfort et al., 2013) showing a relationship between aesthetic satisfaction with the neighborhood and walking activity. Participants with a higher body mass index reported significantly less access to nearby recreational parks.

This series of studies warrants replication in a wider international context and further exploration of environmental attributes, including quantity and quality of green space. Currently, none of the research takes into account sensory qualities of environment that can impact on symptoms of schizophrenia, such as traffic noise and the dynamics of street life (e.g., numbers of people, presence of dogs, scooters, and bikes).

Post-Traumatic Stress Disorder (PTSD)

Post-Traumatic Stress Disorder (PTSD) is a psychiatric condition that causes serious physical, mental, and social changes in an individual’s life. Its core symptoms comprise of (1) re-experiencing traumatic content, (2) persistent avoidance of associated traumatic feelings and thoughts, and (3) hyper-arousal (sleep disruption, nightmares, irritability, or anger) (Varning Poulsen, Stigsdotter, & Refshage, 2015). It may be a result of a genetic vulnerability to anxiety, being trapped in a war zone, or being a survivor of a natural disaster (e.g., a hurricane). More than half a million American war veterans sought PTSD care at a cost of three billion dollars in 2014 (Institute of Medicine, 2014).

To date research has explored the impact of nature-based activity on war veterans and emotional well-being outcomes. Varning Poulsen et al. (2015), has recently synthesized the literature showing the physical and mental well-being benefits that arise from nature-assisted therapy (NAT) in war veterans with PTSD symptoms. The review flags a range of interventions (from gardens to wilderness contexts) including three randomly controlled trials (RCTs) that have shown the benefit of nature as a therapeutic resource in the treatment of PTSD. Fostering reflection and psychological restoration, building community with others, the ability to plan and perform a task, and assisting with the challenge of returning to work are some of the reported benefits emerging from the literature.

Findings from five further studies (Dustin, Bricker, Arave, & Wall, 2011; Hyer, Boyd, Scurfield, Smith, & Burke, 1996; Mowatt & Bennett, 2011; Gelkopf, Hasson-Ohayon, Bikman, & Kravetz, 2013; Caddick & Smith, 2014) show that nature-based physical activity (e.g., kayaking, fly fishing, sailing) in the natural environment has therapeutic outcomes for veterans including: enhanced coping skills, improved confidence, sense of control and self-efficacy, reductions in symptoms of hyper-arousal, relief from the trauma of nightmares and flashbacks, and from the numbing of emotional responsiveness that characterizes PTSD. Participating in nature-based physical activity was shown to enhance motivation for living (Caddick & Smith, 2014) and helped facilitate a comforting sense of normality in veterans’ lives (Dustin et al., 2011). Dustin et al. (2011) also suggests that interacting with the “bigness” of nature helped veterans diagnosed with PTSD to put their own lives and problems into perspective as they became immersed in the world around them. Giving meaning and purpose to life (e.g., place within the world and sense of perspective) is a trend in PTSD and other research on specific mental health conditions (e.g., Roe & Aspinall, 2011a).

Collectively, these studies suggest that nature-based activity can significantly reduce PTSD symptomatology and depression. But few studies have followed veterans over time—or indeed explored other PTSD sufferers in different contexts. Owing to few RCT studies, and the prevalence of qualitative studies, the evidence level is low and unable to separate out the effects of environmental setting from the outcomes of the activity per se. Future research should address the experimental design and measurement tools to consistently measure outcomes and build the evidence base more robustly.

Dementia

Dementia in an umbrella term used to describe a group of brain disorders that involve progressive deterioration in cognitive functioning. There are many different types of dementia, but they are all characterized by a set of similar symptoms including loss of memory, mood changes, and problems with communication and reasoning (Alzheimer’s Society, 2013). Dementia is a global public health priority, with reports suggesting that each year 7.7 million new cases of dementia are identified (WHO, 2016). The financial costs of dementia on a nation’s economy are greater than the economic cost of cancer and heart disease combined (Luengo-Fernandez, Leal, & Gray, 2010). So there is a pressing need to identify cost-effective interventions.

One of the biggest single benefits to increasing cognitive functioning in older age is walking. Exercise has been linked with higher volume white brain matter—the connective tissues that wire the brain—in older people aged 75 and over, which in turn supports healthy cognitive functioning (Gow et al., 2012). One of the most pressing areas in aging research is therefore to understand what might promote physical activity in older people to support cognitive functioning, and which environments might best support physical activity.

In the general population, there is now considerable evidence to indicate that “green exercise,” that is, walking in green spaces including parks and streets with trees, generates significant mental well-being benefits over and above walking in nongreen environments, including increased mood, self-esteem, and energy (Roe & Aspinall, 2011a; systematic reviews by Thompson Coon et al., 2011; Bowler et al., 2010). For older people specifically, walkable green space in an urban neighborhood was associated with increased life longevity (Takano, Nakamura, & Watanabe, 2002), but there are few studies comparing walking in green versus built environments and the cognitive attention outcomes.

Specifically for dementia, there is emerging evidence of the benefits of nature-led activities. However, this evidence is fragmented (Natural England, 2013a). The strongest evidence relates to reduced levels of agitation and anger, as identified by a recent systematic review (Whear et al., 2014). For residents in care homes—half of whom, on average, have some form of dementia or cognitive impairment—access to gardens can reduce the levels of agitation experienced by older people with dementia. De Bruin et al. (2009), found that day care residents in Dutch care farms set in a natural environment experienced improved nutrition, physical exercise, and social interaction compared to a control group measured in a regular urban day care setting. There is virtually no evidence of improvements to cognitive function or emotional well-being from time spent in natural settings amongst people with dementia.

In summary, the evidence is limited, over-biased towards qualitative studies, and lacks the robust design of RCTs. Whear et al. (2014) indicate that future research should focus on study designs that measure outcomes in comparable ways and scales, such as levels of verbal and physical agitation, pacing and walking behaviors (including exit-seeking and trespassing) and emotional outcomes such as anger, mood, or anxiety. Like much of the research on specific mental health disorders, a consistent set of tools to measure common mood and behavior-related outcomes is needed.

Attention Deficit/Hyperactivity Disorder (ADHD) and Behavioral Problems in Children and Young People

Attention Deficit/Hyperactivity Disorder (ADHD) is a condition that makes people inattentive, impulsive, and hyperactive. It is one of the most common psychiatric disorders of children and young people, and affects between 2 and 7 per cent of children worldwide (Bruchmüller, Margraf, & Schneider, 2012).

ADHD is associated with poor performance in schools and high rates of drop-out in high school education (Barkley, Fischer, Smallish, & Fletcher, 2002; Loe & Feldman, 2007). A significant number of children with ADHD also suffer from impairments in social functioning, experiencing difficulties in social interaction with their peers (Nijmeijer et al., 2008). In this respect, young people with ADHD share similar features to those exhibited by children on the autistic spectrum. ADHD—and the relationship to green space—has been the focus of considerably more research than autism to date.

Faber Taylor and Kuo have consistently demonstrated over a series of studies carried out in the USA, that the symptom severity of ADHD is reduced when playing in “green” settings as compared to play sessions in hard landscapes (Faber Taylor, Kuo, & Sullivan, 2001; Kuo & Faber Taylor, 2004). This team have also investigated “dose” effects of nature in children with ADHD, comparing 20-minute walks through a park, with walks through less green settings. It has found that children with ADHD performed significantly better after walks in the park, exhibiting higher levels of concentration than after walks in the other settings. The authors suggest that a regular “dose” of daily nature might serve as a safe, inexpensive, and easily accessible tool for managing ADHD (Faber Taylor & Kuo, 2009).

Research in Europe has also reported higher levels of functioning of children with ADHD from time spent in woodland versus urban settings (Van Den Berg & Van Den Berg, 2010). A further study in Scotland explored psychological restorative outcomes of time spent in forest versus indoor classroom activities in young people with and without behavioral problems (which included ADHD) (Roe & Aspinall, 2011b). Across both groups the forest setting delivered positive benefits to mood, anger, and stress, but the rate of improvement—in the short term—was greater amongst the young people with behavioral problems. This is the first study (in conjunction with Roe & Aspinall, 2011a) to identify that the rate of psychological restoration from exposure to green settings is potentially greater in people with mental health problems than those with good mental health.

Autism

Autism is a complex, lifelong condition, often referred to as autism spectrum disorder (ASD), and is characterized by difficulties in social communication (i.e., verbal and nonverbal language), social skills (i.e understanding and relating to other people), and social imagination (i.e., understanding and predicting other people’s behavior) (Delahaye et al., 2014), alongside difficulties in navigating the world owing to sensitivities in sound, touch, taste, light, and color. Consequently children with ASD are more susceptible to developing anxiety (Mannion, Leader, & Healy, 2013). About 1% of the global population has ASD with the prevalence increasing: autism is the fastest-growing developmental disability in the USA (Center for Chronic Disease, 2016). This pattern is reflected in the UK: the number of school children diagnosed with ASD has increased by over 50% in the last five years (2011 UK census data), although it is important to note that this does not mean the condition has become more widespread, but simply that it is more easily detected (Gotham, Risi, Pickles, & Lord, 2007). People with autism tend to describe the world as one in which the experience of people, places, and events causes considerable anxiety.

Current evidence, though limited, suggests that engagement with the natural environment can be beneficial for children on the autistic spectrum, particularly from nature-based activities such as gardening projects, summer camps, field visits, and animal therapy (Natural England, 2013b; Rickinson 2010). A new and promising field of research is the benefit of being outdoors to circadian entrainment and sleep quality in children with ASD (Nioi, 2016, unpublished). Children with ASD experience sleep problems at a much higher rate than typically-developing peers and respond to melatonin as a sleep aid. An early pilot study in young people with ASD (Nioi, 2016, unpublished) has shown the benefits of regular exposure to blue light (which regulates melatonin production naturally) in a residential school setting (via exposure to natural daylight via regular outdoor play) in addition to good sleep hygiene (i.e., limited blue light exposure from gadgets prior to sleeping) is promising in terms of delivering better-quality sleep.

However, a recent synthesis of the evidence indicates a lack of robust evidence quantifying the benefits of engaging children on the autistic spectrum with the natural environment (Natural England, 2013a). The report flags the lack of robust experimental design, poor consistency in outcome measures, small participant numbers, and any exploration of outcomes in relation to autism characteristics by gender.

Challenges for Future Research

One of the challenges in evidencing the relationship between nature and mental health and well-being is in getting reliable mental health data at a population level by which to carry out analyses; most of the research relies on self-reported population health data or on localized studies using physiological and self-report indicators in small samples. Reliable data on the prevalence of psychiatric disease is especially hard to find because diagnoses are often imprecise or incompletely recorded. Therefore, it is difficult to show exactly how the dynamics of mental health is changing in our cities over time and to show relationships to environmental attributes such as green space. Replicability in experimental design and in sets of outcome measures remains a problem for the field and for psychology generally (Nosek et al., 2015).

The global mental health challenge is set to increase as world crises and PTSD experiences rise, for example, from rising trends in modern terrorism, the rise in natural disasters and displacement caused by flooding and forest fires, and rising trends in international lifestyle and forced migration from war conflict, causing the displacement of entire cultures. There is some limited evidence to suggest that socially cohesive and physically connected communities—including place-based attributes, such as green space, that foster connectivity in the environment—are more resilient to natural disasters (Marín et al., 2010), but further research is needed to establish such trends. One recommendation, therefore, is for future research to address how nature can alleviate the mental health burden that arises from man-made and natural disasters.

Conclusion

The “No Health without Mental Health” mandate (Prince et al., 2007) adopted by the World Health Organization (WHO) and leading health organizations (e.g., the UK’s National Health Service) recognizes the need for new approaches to mental health care and “whole place” approaches. This article highlights the contribution that natural environments can make to alleviating mental health problems, both in terms of supporting good mental health maintenance and recovery. It has argued for more robust evidence and consistency in experimental design and outcome measures, and for nature to be more widely explored in relation to specific mental health and psychiatric problems. It has flagged new levels of urgency to the global burden of mental disorders in light of increasing urbanization and an increasingly aging population, and argued that the benefits of natural interventions to mental health be costed so that city leaders can see the value of investing in green infrastructure. Good mental health is fundamental to physical health, but does not receive the same level of attention as physical health care promotion. In addition, access to good-quality urban green space is not equally distributed: whilst people living in deprived urban communities often experience poorer mental well-being and greater exposure to chronic stress, these are also the populations most likely to have less access to quality green space for stress regulation compared with those of higher socio-economic status. So, increasing the amount and quality of green space in our cities—and facilitating and improving access to it—offers a promising and cost-effective solution to the growing burden of mental health problems, at the same time as delivering additional benefits to physical health and chronic disease.

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