Sociodemographic characteristics

The lack of data on other sociodemographic factors, particularly race/ethnicity, severely limits the generalisability of the findings and undermines research commitments to equality, diversity and inclusion (EDI). Despite the majority of studies including a measure of SES, there needs to be a better exploration of how SES mediates nature's effect through more detailed comparison across socioeconomic groups and more consistent data collection on both household and neighbourhood socioeconomic measures. Alderton et al^{Reference Alderton, Villanueva, O'Connor, Boulange and Badland38} found only three studies comparing findings across socioeconomic groups, with two of these finding greater benefits in those from poorer socioeconomic backgrounds (also supported by Zhang et al^{Reference Zhang, Mavoa, Zhao, Raphael and Smith13}), suggesting that those from poorer socioeconomic groups may benefit more.

Defining nature

Our nature research framework enables better testing of two prominent hypotheses regarding the mechanisms driving the relationships between nature interventions and mental health and well-being benefits. Attention restoration theory posits that the natural environment is particularly restorative to our attention.^{Reference Kaplan44} Stress reduction theory hypothesises that the natural environment reduces stress and promotes parasympathetic activity.^{Reference Ulrich, Simons, Losito, Fiorito, Miles and Zelson20} However, there is little disaggregation of what ‘natural environment’ means operationally; how natural the environment must be; or how much of it (dose) one needs to gain benefit. It is pressing that we understand these things, as with global urbanisation, people's ‘nature’ will most likely come from ‘human-designed environments with natural elements’ rather than wholly natural environments. It is important to know whether we can still see beneficial effects of nature using our urban parks, or whether more natural environments within urban settings are required to harvest nature's protective effects.

Some evidence is emerging, mostly from the adult literature, highlighting how the type and dose of nature may affect outcomes, such as nature connectedness (how connected people feel to nature)^{Reference Schultz and Tabanico45–Reference Wyles, White, Hattam, Pahl, King and Austen47} and psychological restoration.^{Reference Wyles, White, Hattam, Pahl, King and Austen47} Some evidence suggests that more natural environments (e.g. natural protected parks with increased biodiversity versus less biodiverse urban parks) and increased time in nature^{Reference Hatty, Mavondo, Goodwin and Smith46} have greater effects on nature connectedness^{Reference Schultz and Tabanico45,Reference Wyles, White, Hattam, Pahl, King and Austen47,Reference Mena-García, Olivos, Loureiro and Navarro48} and psychological restoration.^{Reference Wyles, White, Hattam, Pahl, King and Austen47} In a recent study, this relationship was more confused, with significant increases in nature connectedness seen also in urban parks as well as more natural environments.^{Reference Hatty, Mavondo, Goodwin and Smith46} There is a clear need for more research in this area, understanding how the type, quality and dose of nature, especially in the child and adolescent population, affects health.^{Reference Jimenez, DeVille, Elliott, Schiff, Wilt and Hart49} It is important to not simply extrapolate adult findings to the child and adolescent population, as childhood and adolescence may be a particularly sensitive and unique period, whereby nature may have long-lasting effects on mental health across the life-course. For example, one European study found that adults with low levels of childhood exposure to nature had significantly worse mental health outcomes^{Reference Preuß, Nieuwenhuijsen, Marquez, Cirach, Dadvand and Triguero-Mas50} and another large study (of 1 million people) with a 28-year follow-up period, found that high levels of the presence of continuous green space during childhood was associated with a lower risk of a multitude of psychiatric disorders later in life.^{Reference Engemann, Pedersen, Arge, Tsirogiannis, Mortensen and Svenning51} These findings are alarming, in light of the pattern of urbanisation and children spending less time in nature.^{Reference Oswald, Rumbold, Kedzior and Moore6} Finally, it is important to understand what the minimal effective dose of nature may be, to inform practice and policy-making. Recommendations from a Canadian national nature prescribing programme (PaRx) suggests a minimum of 2 h in nature per week;⁵² however, it is unclear what this recommendation is based on. Coventry et al,^{Reference Coventry, Brown, Pervin, Brabyn, Pateman and Breedvelt53} in a systematic review and meta-analysis examining nature-based interventions in community-based adults, put forward that the most effective nature-based interventions were offered for between 8 and 12 weeks, with an optimal ‘dose’ ranging from 20 to 90 min. Current nature prescriptions take place in diverse settings,^{Reference Nguyen, Astell-Burt, Rahimi-Ardabili and Feng54} and it would be helpful to know which settings or type of nature is most beneficial.

Confounders will differ along the continuum of the nature research framework, and air pollution becomes particularly salient in human-designed environments with natural elements (e.g. urban parks) and less so in purely natural settings. Some of these confounders are seen as mechanisms driving nature's beneficial effect.^{Reference Kruize, van Kamp, van den Berg, van Kempen, Wendel-Vos and Ruijsbroek14,Reference Nguyen, Astell-Burt, Rahimi-Ardabili and Feng21} Finally, using the biophilia hypothesis, which draws on our evolutional bond with nature, one may postulate that we may expect greater benefits of nature further up the nature research framework towards purely natural environments.

The nature intervention

The nature ‘intervention’ falls into three key areas: access, exposure, and engagement with nature, with quantity and quality of nature being relevant to all three areas. We call this the nature intervention framework. Six out of eighteen reviews and three out of five cohort studies aimed to examine what they defined as ‘exposure’ to nature. However, five of these six reviews included studies that were not directly measuring exposure, but rather using measures of surrounding green space as a proxy,^{Reference Vanaken and Danckaerts12,Reference Luque-Garcia, Corrales, Lertxundi, Diaz and Ibarluzea26,Reference de Keijzer, Gascon, Nieuwenhuijsen and Dadvand33,Reference Anabitarte, Ibarluzea, Garcia-Baquero, Santa Marina, Fernandez-Somoano and Tardon37} with the Normalized Difference Vegetation Index (NDVI) being one of the most common metrics used (Supplementary Table 1). Perhaps this is not surprising, as surrounding green space measured using the NDVI has been put forward as the most standardised metric of nature exposure.^{Reference Marviera, Kareiva, Desiree Felix, Ferrantec and Billington55} NDVI measures vegetation density.⁵⁶ An argument against this method is that it may not accurately capture someone's exposure, as the NDVI also includes measurements of non-accessible agricultural land. Some measures of surrounding nature, such as ‘school greenness’, may be more accurate measures of true exposure, if children play outside in their school breaks. Fewer studies measure direct exposure to nature, relying on subjective reported time in green space^{Reference Fleckney and Bentley25,Reference Zare Sakhvidi, Knobel, Bauwelinck, de Keijzer, Boll and Spano28} or on objective measures such as real-time Global Positioning System (GPS) technology to provide participants’ daily locations and movement patterns.^{Reference Vanaken and Danckaerts12,Reference Fleckney and Bentley25} One study^{Reference Mennis, Mason and Ambrus57} used geographic ecological momentary assessment (GEMA) technology (providing an objective measure of green space exposure) to assess whether being in urban green space is associated with reduced momentary psychological stress. Few reviews^{Reference Reece, Bray, Sinnett, Hayward and Martin27,Reference de Keijzer, Gascon, Nieuwenhuijsen and Dadvand33} looked at exposure to nature in terms of views of nature or having contact/touching materials from nature. Some reviews used the word ‘exposure’ loosely (Supplementary Table 2) and included studies measuring surrounding nature as well as access to nature in the neighbourhood.^{Reference Luque-Garcia, Corrales, Lertxundi, Diaz and Ibarluzea26,Reference Zare Sakhvidi, Knobel, Bauwelinck, de Keijzer, Boll and Spano28,Reference Anabitarte, Ibarluzea, Garcia-Baquero, Santa Marina, Fernandez-Somoano and Tardon37} Studies that looked at access to nature from the home or school did this in a variety of ways (Supplementary Table 1), for example measuring the time taken or distance from home to the nearest park or ‘park density’ in buffer areas around the home. Studies that examined a form of nature engagement (i.e. not merely being exposed to it but rather engaging or using it in some way) included, for example, studies looking at ‘nature play’,^{Reference Dankiw, Tsiros, Baldock and Kumar29} ‘nature-specific learning outside the classroom’,^{Reference Mann, Gray, Truong, Brymer, Passy and Ho34} ‘outdoor education programmes’,^{Reference Fang, Lu, Gill, Liu, Chyi and Chen32} ‘greening interventions’^{Reference Bikomeye, Balza and Beyer41} and the Scandinavian tradition friluftsliv (an immersive nature experience with an emphasis on the experience of closeness to nature during activities).^{Reference Mygind, Kjeldsted, Hartmeyer, Mygind, Bolling and Bentsen35} Very few studies reported the quantity and quality of nature.^{Reference Zhang, Mavoa, Zhao, Raphael and Smith13} Ensuring future studies clearly report on the quantity and quality of nature will also help us to understand the ‘dose’ required to reap its beneficial effects and what aspects of nature may be particularly beneficial or important to people's experience.

Outcome measures

Mental health and well-being outcomes appear to fit along a continuum, with ‘disorder’ at one end and ‘well-being’ at the other. Examining outcome measures in this way has highlighted the need for a greater focus on examining nature's effects on mental disorders, including depression and anxiety. Future studies should also focus on unexamined disorders, including eating disorders, ASD, bipolar affective disorder and psychosis in children and adolescents.

This meta-review has highlighted how the current literature has often merged mental health and well-being outcomes, not fully recognising that they are related but distinct entities with a separate literature base. Zhang et al^{Reference Zhang, Mavoa, Zhao, Raphael and Smith13} defined mental well-being as positive mental health and outcomes could include mood, stress, anxiety, depression, happiness, pleasure, emotional health, psychological health and mental health. In Fleckney & Bentley's review,^{Reference Fleckney and Bentley25} even though mental health and well-being outcomes were recognised as two different entities, they found that some studies used scales/tools to measure both, thereby blurring this distinction. Bloemsma et al^{Reference Bloemsma, Wijga, Klompmaker, Hoek, Janssen and Lebret39} used a validated tool to measure mental well-being – the Mental Health Inventory-5. Future research should better recognise the distinction between mental health and mental well-being, acknowledging them conceptually and operationally as different entities, as our understanding of them has evolved.^{Reference Huppert and So58,Reference Vaingankar, Subramaiam, Lim, Sherbourne, Luo and Ryan59}

Nature's effect

The evidence base is large, with the majority of studies finding significant positive results in support of nature's beneficial effect or positive results with unknown significance. Some studies found null results (not meeting significance); however, these have generally all been associated with a direction in support of a beneficial effect of nature. Only a small number of studies/findings suggested a potential negative effect, although the significance of these results is not clear. Despite the evidence base being large (in number of studies), it comes almost exclusively from HICs and there is medium to high risk of bias in the majority of studies, with few considering all potentially important confounders, such as air and noise pollution, social engagement and exercise. The mediating and moderating role of sociodemographic factors, such as ethnicity and SES, are not known or are poorly understood.

Implications

Despite the evidence base having its limitations, we are already seeing the widespread uptake of ‘green social prescribing’ within the UK's National Health Service (NHS) as part of the government's Green Social Prescribing Programme (launched April 2021).⁶⁰ Green social prescribing has been defined as ‘the practice of supporting people to engage in nature-based interventions and activities to improve their mental and physical health’.⁶⁰ This could involve a health professional prescribing a community garden project to a patient with the aim of improving their mental health.⁶⁰ Green social prescribing could therefore be thought of as a mechanism by which to implement the evidence base (on nature's effects on mental health and well-being) in practice. Despite the evidence base being larger for adults, with an attempt by Coventry et al^{Reference Coventry, Brown, Pervin, Brabyn, Pateman and Breedvelt53} at ascertaining the optimal ‘dose’ of nature one needs for nature-based interventions in adults, we must be mindful of the evidence base if or when we implement green prescribing to children and adolescents, understanding that even though current evidence suggests nature's beneficial effects, certainty is lacking. This is especially true for clinical populations. Intuitively, we may feel nature is good for us; however, it could be harmful (given the lack of and uncertainty in the evidence) to promote green prescribing to child and adolescent clinical populations as a form of ‘treatment’, especially if it is promoted as a stand-alone treatment and not as an adjunct to ‘treatment as usual’. However, if further down the line, more certain evidence does support its benefits, then ‘green prescribing’ could work not only for us humans (e.g. reducing the need for possible medications and unwanted side-effects) but also for the planet, in promoting nature's flourishing in our neighbourhoods and reducing the burden on mental health services (with associated carbon costs).

Limitations

The research field examining nature and its effects on mental health and well-being in children and adolescents is a rapidly developing, interdisciplinary and heterogeneous field, mostly made up of observational studies looking at surrounding green space as a proxy for exposure in general child and adolescent population samples or non-randomised poorly controlled experimental studies in more unwell/clinical child and adolescent populations. There are few studies examining mental disorders and no studies examining disorders such as ASD, eating disorders, bipolar affective disorder or psychosis in children and adolescents. Data are lacking examining the role of ethnicity and SES in children and aolescents; however, current evidence suggests that nature may be most beneficial to those from lower socioeconomic groups or marginalised groups.

Unfortunately, not all reviews gave details of their included studies (despite our attempts to contact the corresponding authors) and therefore we were unable to identify whether reviews had missed key primary studies. However, we felt this unlikely, owing to the large number of reviews. Therefore, we did not include primary studies published before the last identified review, to avoid the risk of double counting. We limited our search to the English language, which resulted in two study exclusions. We purposefully kept the scope of this meta-review broad in order to answer our study aims; however, this made summarising the heterogeneous evidence more challenging.

It was not in our scope to formally review and report mechanistic pathways, although this could have proved useful in supporting the evidence base more formally. Data were collected on this as detailed in the protocol. Studies often framed their results in the context of current mechanistic thinking; however, few studies examined mechanistic pathways in depth or contextualised them in the context of their specific results.

Strengths

This meta-review has synthesised a large multidisciplinary literature base that uses varying concepts and definitions and has produced two clear frameworks to guide future rigorous interdisciplinary research. Definitions and concepts of nature were broad to allow for the development of these frameworks, to ensure their relevance to this diverse field of research. This is a clear strength, as previous reviews have had a narrower focus of enquiry, which would inhibit the development of such frameworks relevant to the field at large. Studies were rigorously examined for their certainty of evidence, using validated assessment tools. A protocol was developed and made publicly available a priori.

Recommendations

To date, it is difficult to judge the strength of the evidence for nature interventions for mental health and well-being in children and adolescents. Our proposed nature research and nature intervention frameworks promote the design of future studies that will bring more certainty to the evidence base, by encouraging a clearer and more consistent approach to scientifically examining nature interventions. Accumulation of more certain evidence relies on clarity and consistency of measurement, as well as the use of validated measurement tools. These factors, in turn, enable replication, which informs evaluation of the strength of evidence. If we take ‘green prescribing’ through the NHS to be a formal reflection of an evidence-based clinical intervention, then it is not acceptable or ethical for prescribing to be conducted in the absence of standard evidence to inform dosing. ‘Dose’ requires knowledge of: intervention type, how much of it and for how long, with accompanying information about expected outcomes and side-effects. In this sense, we also need to be mindful of the ‘indication’ – appreciating the difference in examining mental health and mental well-being, which are separate but connected entities, and the difference in prescribing for clinical and non-clinical child and adolescent populations, in light of the evidence base. It is also important to be aware of mediating and moderating factors that can affect intervention outcomes, including sociodemographic factors such as access to green space and sociopolitical factors such as air pollution exposure.

To increase the amount of evidence about underserved groups in mental health research, future clinical trials should focus on more ethnically diverse child and adolescent populations and children and adolescents from low- and middle-income countries. We should also expand to a wider range of diagnoses. Of the clinical child and adolescent populations, current limited research has focused on ADHD, anxiety and depression, with no research focusing on children and adolescents with ASD, eating disorders, bipolar affective disorder or psychosis.