After removal of duplicates, we screened 9,208 titles and abstracts, and 702 sources were eligible for full text screening (Figure 2). After further exclusions (76% of which were due to the age range not being relevant), 89 sources were considered against four key criteria (these additional inclusion criteria are listed under ‘Full-Text Screening’ in Figure 1). They relate to the target age range (14-24 years), study design (experimental), exposure (neighbourhood green space) and outcome (anxiety disorder or depression). 48 sources met at least two of these criteria and were included in the review.
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 identified included approaches to dealing with confounders, use of validated outcome measures and low drop-out. This low attrition reflects 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 self-reported 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 environments22,23,24,25,26,27,28,29,30, 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 park32 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 days22,28.
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 green35, natural soundscapes36,37, natural scenery38,39, natural materials40, indoor exposure to plants41,42 and to animals41, reported some positive changes in various outcome measures (see Table 2). For example, levels of anger were lower when viewing green (compared with red) scenery35, listening to natural sounds was shown to be more restorative than anthropogenic sounds36, 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 first 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 field 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 mood46 and sleep following47 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 benefits of outdoor adventure opportunities and indicate that wilderness excursions of 9-10 days lead to increased mindfulness and reduced stress50 and engender greater self-esteem through group belonging.51 Studies evaluating longer (3-15 weeks) outdoor education programs also found measurable improvements in stress52, self-esteem53 and social anxiety54. Much shorter interventions also resulted in positive changes in the autonomy, competence, relatedness and enjoyment subscales of the Intrinsic Motivation Inventory55 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 benefits are not consistently sustained (intrinsic motivation one-month post-intervention55, there were some longer-term effects on general mood at 3 months52 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 behaviour57. They reported lower adjusted youth arrest rates three years post-intervention for the park-based 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 first 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 findings suggested improvements in anxiety and social functioning, while qualitative themes included new skills and training, developing friendships, co-operation and the benefits 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 reflection 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 outcomes50.
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 reflection 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 analyses62. 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 scale66, 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 traffic, 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 rumination72, and how a reduction in negative stimulation could enable restoration and possibly a reduction in the risk of anxiety disorder and depression. Our panel confirmed 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 traffic 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 beneficial 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 socio-demographic 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 type26 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 levels26, and differences in goal-orientation explain why individuals do not all respond in the same way to exercise.49