Exploring the role of exposure to green space in preventing anxiety and depression among young people aged 14-24 living in urban settings: a systematic review

Background Despite the growing problem of anxiety and depression amongst young people aged 14-24 years living in urban settings, reviews about the role of exposure to green space or nature in preventing anxiety and depression tend to focus on children, adults or sometimes adolescents. This review aims to explore whether, and if so how, exposure to green space reduces the risk of anxiety and depression among young people aged 14-24 living in urban settings. Methods Results 9,208 sources were identied and 48 were included. Experimental studies provided evidence that walking or being in a green space improves mood and state anxiety immediately following the intervention. Observational studies suggest that social interaction, physical activity, and mindfulness mediate the relationship between exposure to green space and mental health. We propose that the absence of noise and trac and the presence of nature promotes mindfulness and restoration. value assessed for quality, including risk of bias for each study, using the Effective Public Healthcare Panacea Project’s Quality Assessment Tool for Quantitative Studies ; the global score (strong/moderate/weak) is derived from rating six quality criteria and a summary score for the number of criteria scored as ‘Strong’ was (but exclude studies). For other study designs, CASP tools were used 21 , and a similar summary created. Two researchers completed quality assessments and data extractions for each paper, with benchmarking between each pair of assessors. If the global score differed, this was discussed until agreement was reached.


Study Design and Quality Assessment
The majority of studies (n=47, 98%) were quantitative. These included randomised trials (n=14), and non-randomised intervention studies (n=22), cohort studies (n=4) and cross-sectional surveys (n=7). There was strong initial agreement between the reviewers for global quality scores for these studies (90%). Areas of strength identi ed included approaches to dealing with confounders, use of validated outcome measures and low drop-out. This low attrition re ects that very few studies did follow-up post-intervention. Other areas in which quality was not generally highly rated were in representativeness of the sample (e.g. response rates were not reported), and blinding, which is important given the selfreported nature of outcomes. The one remaining paper was purely qualitative. Quality scores are reported for each paper in supplementary tables.

Evidence Synthesis
Studies that compare urban streets, urban parks and forests Thirteen of the studies described experimental designs that compared (walking or being in) urban settings with neighbourhood green space or forests. These consider different environments; over the last 10 years a body of research has developed comparing exposure to forests and urban environments 22,23,24,25,26,27,28,29,30 , complimented in more recent years by studies on urban parks. 31,32,33,34 These studies provide evidence that a brief exposure to a green environment elicits an immediate increase in mood and reduction in state anxiety (Table 1; scales used to measure outcomes are listed in Table 7). One study reported longer-lasting effects following exposure to a forest than a park 32 and estimated reductions in state anxiety are greater following visits to forests 22,27 than parks. 33,34 The dose of interventions ranged from 15 minutes 24,26,27,33,34 to 3 days 22,28 .
Studies that assess particular aspects of being in green/natural environments A further 8 studies (7 of which were experimental), assess particular aspects of being in green or natural environments: the colour green 35 , natural soundscapes 36,37 , natural scenery 38,39 , natural materials 40 , indoor exposure to plants 41,42 and to animals 41 , reported some positive changes in various outcome measures (see Table 2). For example, levels of anger were lower when viewing green (compared with red) scenery 35 , listening to natural sounds was shown to be more restorative than anthropogenic sounds 36 , and touching wood induced feelings of comfort (compared with marble). 40 Studies that compare physical activity in a green/blue environment and indoors, or evaluate physical activity programmes Three experimental studies compared exercise in a green or blue environment with doing the same exercise indoors. The rst intervention was a walking route along a canal. 43 Compared to an indoor walking route, this led to increased connectedness with nature, mediated by positive affect. Also, that participants tended to underestimate the positive hedonic effects of the outdoor walk. The second involved cycling on a static bike on the edge of a green eld and found greater increases in vigour, but not self-esteem, when exercising in the blue environment. 44 The third paper compared horse-trekking in a green environment with riding on an indoor simulator, so involved contact with the animal as part of the intervention, and reported improvements in feelings of anxiety and depression (Table 3). 45 Two further randomised trials assessed reported improvements in mood 46 and sleep following 47 outdoor running programmes. A before-and-after study reported reductions in social anxiety following a 2-month climbing programme. 48 Finally, one study compared the effects of doing outdoor exercise on mood in an induced goal-oriented state compared with baseline state; improvements in mood occurred only in the baseline group. 49 Studies that evaluate outdoor adventure programs, and education/training/employment in green environments A further set of observational studies (Table 3) consider the mental health bene ts of outdoor adventure opportunities and indicate that wilderness excursions of 9-10 days lead to increased mindfulness and reduced stress 50 and engender greater self-esteem through group belonging. 51 Studies evaluating longer (3-15 weeks) outdoor education programs also found measurable improvements in stress 52 , self-esteem 53 and social anxiety 54 . Much shorter interventions also resulted in positive changes in the autonomy, competence, relatedness and enjoyment subscales of the Intrinsic Motivation Inventory 55 and a reduction in anxiety after a one-day experiential training session in a local park. 56 In contrast to the experimental studies, some of these evaluations included follow-up. Although bene ts are not consistently sustained (intrinsic motivation one-month post-intervention 55 , there were some longer-term effects on general mood at 3 months 52 and depression symptoms (at approximately 9 months) 56 . A park-based prospective cohort study compared a violence prevention and mental health promotion after-school program with other after-school clubs, using youth crime rates as a proxy for violent behaviour 57 . They reported lower adjusted youth arrest rates three years post-intervention for the parkbased programme. A comparison of mindfulness training in a botanical garden greenhouse with conventional (indoor) mindfulness training concluded that the green environment particularly helped beginner meditators to overcome problems with concentration and stress. 58 Two studies also evaluated employment interventions in green or natural environments. 59,60 The rst is a qualitative study of a group of young people who completed a training programme and then undertook 2-3 months' work experience in an entry-level "green job". 59 More than half of them described a positive change in their attitude towards themselves during this time, and 94% of them cited gaining new knowledge and skills as a positive aspect. The second was a before-and-after study of a one-year conservation program. Quantitative ndings suggested improvements in anxiety and social functioning, while qualitative themes included new skills and training, developing friendships, co-operation and the bene ts of being outdoors in terms of health and happiness. 60 In contrast to the studies of tightly controlled interventions delivered to homogeneous samples (Tables 1-3), the observational studies reviewed above (Table 4) include more complex interventions, with multiple components potentially contributing towards outcomes. There is a common thread, of exposure to a green space or natural environment, but each intervention includes additional elements, comprising either education or training (50%) or outdoor adventure (50%). Some also involve re ection on these experiences 50,51,52 , which could be another important element.
Evaluations of such interventions have studied group dynamics but have less often included mental health and subjective wellbeing as outcomes 50 .
Studies of exposure to residential vegetation Five studies (Table 5) consider the relationship between exposure to vegetation and a variety of outcomes. Exposure to vegetation is commonly measured using NDVI, 'an indicator of greenness based on land surface re ection of visible (red) and near-infrared parts of the spectrum. 17 Dzhambov et al (2018) provides a longitudinal analysis of NDVI and mental health, including symptoms of depression and anxiety, at follow-up. 62 The authors found a relationship between exposure to neighbourhood green/blue space and mental health, as measured by the 12-item General Health Questionnaire. 63 Herrara et al (2018) also provide longitudinal evidence, which may offer an insight into causal pathways -they report that greater levels of residential greenness when participants were aged 16-18 years are associated with lower levels of work discontent and work overload (measures of work stress) when they reach age 20-23 years, suggesting a protective effect when transitioning to university or working life. In cross-sectional analyses the association between NDVI and mental health appears to be mediated by physical activity and restoration, though this does not hold in longitudinal analyses 62 . A later cross-sectional study found that NDVI was negatively associated with both anxiety and depression symptoms, and reported the following variables as mediators -perceived greenness, the restorative quality of the neighbourhood, dispositional mindfulness, rumination and resilience to stress. 64 Another cross-sectional study reported a negative association between NDVI and serious psychological distress (using Kessler 6 scale 66 , which measures symptoms of anxiety and depression), and found that this relationship was mediated by pollution and social cohesion. 65 There is further evidence that NDVI is associated with reduced symptoms of depression (Bezold et al 2018).

Studies of young people's perceptions of green spaces
Another group of studies provided insight into how green spaces are perceived by young people, and how this could mediate the relationship between green space and mental health (Table 6). An experimental study compared perceptions of bamboo forests with those of an urban environment. 29 The forest was rated as providing a better environmental experience in terms of sensory perception, atmosphere, climate, space and place. An observational study compared different views in Tokyo. 67 More trees in the view increased spaciousness and monotony. Oppressiveness was reduced in views with more trees and more sky, and increased in views with more buildings. Finally, increasing distance to trees reduced perceived oppressiveness and danger. This would suggest that the optimal environment in one in which trees and sky dominate rather than buildings, but with some distance to the trees. An experimental study by Wilson et al (2016) found that walking in an urban park is perceived as being more restorative than walking on a busy street near tra c, and that perceived noise mediates perceived restoration. 68 Yang et al (2011) showed that plants cause 'psychological noise reduction'; in other words, a green environment reduces perceived noise levels. 69 Alizadeh et al (2018) examined personal preferences for different green (forests, agricultural) and natural (mountains) landscapes and predictors of these, and found that personality type and subject studies are associated with preference, suggesting that it is important to consider personal preferences if the impacts of green spaces are reliant on their use. 70 Taken together, this set of studies suggest that greener environments are preferred, and are associated with restoration, partly by reducing perceived noise levels.

Narrative Synthesis
There is good evidence that a 15-minute walk in a forest or park can improve mood and state anxiety (compared to an urban environment). 26,27 It is not clear, however, whether this corresponds to a reduced risk of developing anxiety or depression. The impacts of different components of nature can be broken down, and such studies provide evidence of the restorative qualities of viewing or listening to nature, with associated improvements in mood and relaxation. 36,37,38,39 There is less good evidence about the social and psychological processes, such as activation or rumination, resulting from the opportunities for physical activity and restoration afforded by green space, or how these link to psychological disorders. There is limited evidence about how young people perceive green spaces, and how this affects their use of green space.
Insights from young people with lived experience Based on the literature and input from young people with lived experience of anxiety and depression, we suggest a tentative psychological pathway to explain how the restorative qualities of green spaces might lead to increased mindfulness and interrupted rumination 72 , and how a reduction in negative stimulation could enable restoration and possibly a reduction in the risk of anxiety disorder and depression. Our panel con rmed that the absence of technology in green spaces, which was mentioned in some of the evaluations of adventure programs, allowed mindfulness, interrupted rumination and restoration to take place. They also suggested that an absence of tra c encouraged mindful walking. Both observations demonstrate that it may be as much what is absent in green spaces as what is present (e.g. nature) that provides restoration. The panel related to the relaxing effect of being in green spaces, and felt that this enabled problem-solving, thereby providing a link to another factor which is bene cial for young people's mental health.

What works for different groups
Few studies compared results for males and females, despite clear differences in the epidemiology of mental health disorders in adulthood which emerge during adolescence. One exception, Kardjono (2017), suggests that a 4-week hiking programme reduced social anxiety for males, but induced preintervention anxiety for females. 54 In an evaluation of a climbing programme, while Ozen (2015) found that a climbing intervention reduced social anxiety overall, with no differences between men and women. 48 Of the thirteen experimental studies described in Table 1, eight included only males. The one study that included only females reported similar results, suggesting that exposure to forests or parks have similar effects for males and females. 27 In terms of age groups, most of the experimental studies (Tables 1-3) were carried out with student samples of very similar ages, so results cannot be compared by age. Educational or adventure programmes were positively evaluated for both school-age teenagers and students (Table 4).
Few studies in this review considered ethnicity as a moderator. While 61% of the experimental studies (Tables 1-3) were carried out in Asia, the predominantly observational studies (Tables 4-6) have a global spread. Included studies did not consider effectiveness in different socio-economic groups. Many of the experimental studies used students as participants, who are unlikely to represent all socio-economic groups. Two US studies reported ethnic and sociodemographic diversity within their sample, but neither of these studies report results for different groups. 57,59 It is notable that much of the experimental research considered in this review focuses on forests as the exposure or intervention. Some studies reported elevated state-anxiety immediately prior to a walk in a forest, which suggests that this can be an unfamiliar or even threatening environment.
One study found some evidence that young people's preferences for mountains, forest and agricultural landscapes is linked to personality type. 70 We also found some evidence that the impacts of exposure to green spaces and outdoor exercise might be moderated by personality type 26 or by psychological state. 49 For example, those with high-trait anxiety levels experienced a greater reduction in feeling of "depression-dejection" after walking through forest areas than those with normal and low-trait anxiety levels 26 , and differences in goal-orientation explain why individuals do not all respond in the same way to exercise. 49

Discussion
We synthesised a wide range of evidence about the role of access to green space in preventing anxiety and depression amongst 14-24 year olds. We found evidence that exposure to forest environments leads to greater momentary mental wellbeing compared to being on an urban street, and that urban parks can deliver similar bene ts to forests. These studies did not provide evidence about longer-term outcomes such as symptoms or diagnoses of anxiety or depression. However, the evidence from observational studies that residential exposure to vegetation is associated with reduced risk of depressive symptoms for young people is crucial in terms of implementing change that will have broad reach and lasting bene ts. 61 Observational studies also provide some insights into causal mechanisms, such as links between green space and restorative properties leading to reduced rumination. 62,64 While adolescents spend more of their time further from home than children, young people are less likely to own cars or have access to private gardens. Neighbourhood green space and vegetation is therefore crucial to their well-being (as recognised in the Sustainable Development Goals). 73 The importance of green space is partly because it enables many other activities that bene t mental health. It is hard to disentangle the impacts of green space and mediators such as physical activity or social interaction. The psychological processes by which green space may reduce the risk of anxiety or depression are unclear. Many of the included studies cite Kaplan and Kaplan's Attention Restoration Theory (1989). 74 This contested theory 75 suggests green space engages indirect attention and thus provides rest for directed attentional capacity, and therefore presumably interrupts unhelpful rumination or worry. The papers reviewed suggest that green spaces promote mindfulness, mediated by physical activity and restorative qualities. We hypothesize that this in turn reduces rumination 71 and improves psychological outcomes. Others have suggested that greenspace reduces the tendency to ruminate and thereby increases adaptive coping through enhanced psychological resilience. 76 Similar bene ts might be afforded by the practice of mindfulness 77 , but this requires training and effort. It seems that green environments encourage 'effortless mindfulness', and this is supported by one study that found a green setting when learning mindfulness was particularly helpful for beginners. 58 An evaluation of a hiking programme encouraged participants to re ect on the natural surroundings and engage in 'active mindfulness'. It has been found that regular mindfulness can promote trait mindfulness 78 , providing a possible pathway to a sustained reduction in the risk of psychological disorders.

Recommendations for research
In order to understand if exposure to green space prevents anxiety and depression amongst people aged 14-24 years, it is essential that more studies examine longer-term follow-up. Further research must explore the "dose" and frequency of exposure associated with longer-lasting impacts indicative of prevention of anxiety and/or depression. Another recommendation for future research is to measure changes in psychological processes such as rumination and activation that are key to anxiety and depression, and to utilise psychological theory to underpin explorations of the impact of green space on mental health.
The studies used a wide variation of interventions/exposures, considering different types of green space, different durations and varying activities. To allow an improved dialogue with mental health science, these interventions could adopt a more structured way of reporting the contents of the exposure, similar to those laid out in the TIDieR guidelines. 79 Systematic reporting would then allow investigations into impacts of green space on mental health to be more readily evaluated, replicated and potentially implemented.
We identi ed a gap in the literature on [14][15][16][17][18][19][20][21][22][23][24] year olds in terms of factors, such as perceptions, that promote use of local green spaces; such evidence is crucial to ensure that built environment professionals take into account the needs of this age group in both policy and decision-making. More generally, there seems to be a lack of qualitative work with this age group to understand the bene ts of green spaces for mental health, and what the causal mechanisms might be. Although it was included in our search terms, we found few studies about the bene ts of blue spaces for young people's mental health. This re ects the nding of our earlier scoping review, in which only 2% of sources related to blue spaces, but this is a rapidly growing area of research, and its importance was underscored by comments from our PPI panel about the bene cial effects of blue spaces. Future studies should also consider the role of demographic variables, previous experiences (e.g. with outdoor activities) and individual preferences (for different environments).

Recommendations for practice
Recommendations for practice should be interpreted with some caution, as there is limited evidence regarding prevention itself. Recommendations might include consideration of social prescribing of adventure interventions for young people at risk of anxiety and depression. Although evaluations of such interventions provide some limited evidence of sustained effects on general mood and feelings of depression 52,56 , there is as yet no clear evidence about the dose or frequency required to have a lasting impact, which may be a barrier to prescribing. Outdoor adventure opportunities should also be integrated into educational curricula. High-quality longitudinal studies are still needed to estimate the long-term effects of regular exposure to green space on clinical measures of anxiety and depression. However, based on the studies we reviewed and the insights of our panel, we conclude that access to green space is likely to enhance other interventions to improve mental health, such as physical activity 43,45 , mindfulness practice 58 and problem-solving. It also has multiple additional bene ts 80 in terms of health and wellbeing for the rest of the urban population, improving thermal comfort and biodiversity in cities, and reducing pollution and risk of ooding, all of which have implications for the future health of today's young people.

Limitations
We have taken a novel approach to reviewing the evidence about exposure to green spaces and mental health for young people, drawing on a wider range of literature than previous reviews, and including the views of young people themselves. While this approach has helped bridge a gap (from immediate psychological responses to changes in clinically measurable depression or anxiety disorders), we acknowledge several limitations. Firstly, the search terms and databases we used may have missed relevant papers. Secondly, screening was carried out by one researcher. Thirdly, though we set out to include all study designs, the review is heavily weighted towards quantitative research. This is partly due to the limited qualitative research on this topic with young people, but also partly due to the additional exclusion criteria at the full text screening stage, which sought to select the most relevant sources from a large number of eligible studies. Nevertheless, it is worth noting that many of the included studies applied quantitative approaches to qualitative descriptions of attitudes or feelings, and some used mixed methods.

Declarations
Ethics approval and consent to participate Not applicable

Consent for publication
Not applicable Availability of data and materials Not applicable.

Competing interests
The authors declare that they have no competing interests.

Funding
This work was funded by a Wellcome Trust Mental Health Priority Area 'Active Ingredients' commission awarded to IB at the University of the West of England, Bristol. For the purpose of open access, the author has applied a CC BY public copyright licence to any Author Accepted Manuscript version arising from this submission. The funder did not in uence the reporting of results.
Authors' contributions IB, DS and FM developed the aims and research questions. IB, DS, FM and RH developed the inclusion and exclusion criteria. The literature search was completed by RR and DS. Title screening was completed by IB following piloting by IB and FM. Full text screening, quality assessment and data extraction were completed by RR, IB, DS and RH. Data synthesis was carried out by IB. RR and IB wrote the rst draft of the paper. All authors contributed to editing the manuscript. The authors had the nal responsibility for the decision to submit. the urban park walk than those following the city area walk for the following three adjectives: "comfortable", "natural", and "relaxed" (p <0.01). The total STAI score was 19.3% lower after the urban park walk than after the city area walk (urban park: 39.0 ± 6.3; city area: 48.4 ± 7.5; p <0.01). Differences were also detected in the POMS scores for the negative subscales of tensionanxiety, angerhostility, fatigue, and confusion being significantly lower after walking in the urban park than after walking in the city area (p <0.05).
walking speed. For the POMS scores, viewing the scenery in the urban areas increased tensionanxiety (p =0.00). The landscapes of the forests were evaluated as being significantly more "comfortable" (p =0.00, r =0.51), "soothing" (p=0.00, r =0.53), and "natural" (p =0.00, r =0.59). Viewing the scenery in the forests induced significantly higher refreshment (p=0.00, r =0.55).      The models for anxiety and depression indicated that higher greenspace, both objective and perceived, was consistently associated with better mental health. The associations between NDVI and both anxiety and depression showed a pattern of increasing magnitude from smaller to larger buffers, while the opposite pattern held for relationship between tree cover and depression. More greenspace was consistently associated with reduced scores on the anxiety and depression scales. . Quality assessment score out of 6. GAD-7, Generalized Anxiety Disorder 7-item scale; GHQ-12, General Health Questionnaire -Bulgarian translation; K6, The Kessler 6 Scale; MAAS, Mindful Attention Awareness Scale; NDVI, The Normalized Difference Vegetation; PRS, Perceived Restorativeness Scale. Quality assessment score out of 6.
16 PF, Cattell Sixteen Personality Factor Questionnaire; SRSS, Short-version Revised Restoration Scale. Table 7. References for outcome measurement tools.