Currently, 58% of the global population and 84% of the UK population are estimated to live in an urban area (Ritchie et al., 2024; Worldometer, 2024). Moreover, the United Nations projects that 68% of the global population will live in an urban area by 2050 (Kundu & Pandey, 2020). This is significant as stressors and harmful pollutants associated with living in urban environments (e.g., air and noise pollution, higher incidence of crime, urban noise etc) in conjunction with a more ‘nature-deficit’ urban space are associated with poorer mental health outcomes (McMichael & Beaglehole, 2000). This is further exacerbated by contemporary lifestyles, which are characterised by increased time spent indoors, sedentary behaviours, and a corresponding reduction in outdoor activities and interactions with nature (Rojas‐Carvajal et al., 2022).

The World Health Organisation recognises our natural environment as a significant determinant of our health and wellbeing (WHO, 2017; WHO, 2019). Green spaces, including urban greenspaces (i.e., parks, allotments, urban greening), have potential to act as health-supporting environments (Frisara, 2020), where three main pathways have been identified as central to supporting positive mental health outcomes (Markevych et al., 2017; Zhang et al., 2021). These include:

• reducing harmful impacts (e.g. reducing exposure to pollutants and stressors)
• restoring capacities (e.g., supporting recovery from psycho-physiological stress)
• developing capacities (e.g., indirectly supporting positive health outcomes through the provision of physical and social opportunities).

Previous systematic reviews have examined greenspace exposure on mental health outcomes in both children and adults, although studies have been mostly cross-sectional (Fyfe-Johnson et al., 2021; Gianfredi et al., 2021). Other research has demonstrated the protective effects of greenspace exposure on specific mental health conditions (Sarkar et al, 2018; Engemann et al., 2019), in addition to the real-time effects of nature exposure on mental health and wellbeing outcomes (Bakolis et al., 2018; Hammound et al., 2024).

The aim of this specific review by Bolanis et al (2024) was to systematically examine the evidence-base (from inception to January 2023) on the relationship between green space exposure and various suicide-related outcomes, to include suicide mortality, self-harm, and suicidal ideation.

Methods

A systematic review was an appropriate way to synthesise the evidence in relation to the specific research question (Siddaway et al., 2019). The authors followed a structured research process appropriate for a systematic review by adhering to the PRISMA 2020 guidelines (Page et al., 2021). Electronic searches (i.e., PyscINFO, MEDLINE, and Web of Science) and manual searches (e.g., reference lists of included studies) were implemented to identify relevant studies that met the review criteria.

Studies were included if they reported associations between outdoor green space and suicide mortality, self-harm, or suicidal ideation, were published in peer-reviewed journals in either French or English, included original data, and employed subjective or objective measures of green space. Studies were excluded if they focused on blue space, indoor vegetation (e.g., houseplants), representations of green space (e.g., images or videos), sensory aspects of greenery (e.g., sound, audio clips, or smells), or employed qualitative methods.

Green space exposure was defined using both objective and subjective measures of green space. Objective measures included those that assessed the proximity (e.g., distance to), quantity (e.g., total area of greenspace within a specified region), and visibility (e.g., views of green space from a particular location or area) of green spaces. Subjective measures included measures that captured an individual’s experience of greenspaces, such as subjective evaluations of quality (e.g., characteristics of greenspaces) and quantification of visits or activities undertaken in greenspaces (e.g., duration and frequency).

The Mixed Methods Appraisal Tool was employed to assess and describe the research quality of each included study (i.e., an assessment of methodological limitations and risk of bias of the evidence), where both observational and experimental studies formed part of this review.

However, due to heterogeneity of green space exposure indicators and buffer sizes, in addition to heterogeneity of data at an ecological and individual level, a meta-analysis was not considered suitable, and the findings were reported narratively.

Results

A total of 23 eligible studies (mostly observational) were included in this systematic review. Study participants included youth and adults from the general population. The majority of studies were conducted in high-income countries in Europe (n=10), North America (n=5), and Asia (n=7), while one study was conducted worldwide.

Just under two thirds of the included studies on greenspace exposure and suicide mortality (9 / 14) found protective effects of greenspace exposure. Similarly, two thirds of studies (6 / 9) reported protective effects for greenspace exposure on suicidal behaviour (i.e., self-harm, suicidal ideation). Larger protective associations for greenspace exposure and suicide mortality were found for females (n = 7) than for males (n = 4).

Conclusions

Green space exposure may have a protective effect across various suicide-related outcomes, including suicide mortality, self-harm, and suicidal ideation. This review found that green space exposure (defined using both subjective and objective measures was associated with reduced suicide-related outcomes. The authors concluded that:

Greenspace exposure may have a protective role across the entire spectrum of suicide outcomes, including suicide mortality, self-harm, and suicidal ideation, with larger putative benefits observed among females.

Strengths and limitations

Systematic procedures were adhered to in this review to ensure transparency and reproducibility, including pre-registering the review protocol on PROSPERO. Studies published both in English and French were also considered. However, studies from low- and middle-income countries, more diverse cultures and indigenous populations, who may have different experiences and understanding of nature, were not included and may result in different findings.

A strength of the present review is that both objective measures and subjective measures of greenspace exposure were included. While previous research provides support for proximal green space exposure (using objective measures) and mental health outcomes, subjective measures may provide a more accurate assessment of green space exposure (Darcy et al., 2022; White et al., 2021). Urban greenspaces can be underutilised or perceived as inaccessible (Hunter et at., 2015; Kou et al., 2021), and provision of greenspace alone does not necessarily result in engagement (Ball et al., 2009).

Finally, as this is the first review of the published evidence on the associations between green space exposure and suicide-related outcomes, this review adds to the evidence-base in this field and will be of relevance to clinicians, urban planners and policymakers alike.

Implications for practice

The findings of this review point to public natural greenspace as not only having an important role in supporting positive mental health outcomes, but also potentially having a protective effect for a range of suicide-related outcomes. While further research is required to strengthen the evidence-base (due to heterogeneity of study designs and methodologies employed), the current findings have potential implications for enhancing access to and engagement with greenspaces, particularly for vulnerable groups who stand to benefit the most.

This includes implications for the design of public natural spaces, where provision of greenspace alone does may always transform into access, use and engagement (Lee & Maheswaran., 2011). Specific qualities of public natural spaces, such as accessibility, safety, availability of facilities, privacy, and cleanliness, are integral to supporting access, use, and engagement (Petrunoff et al., 2022; Stanhope et al., 2024), particularly as vulnerable groups may have distinct needs or face various barriers and challenges when accessing or engaging with these spaces (Burnett et al., 2023; Burrell et al., 2024).

Therefore a more nuanced approach to understanding the relationship between greenspaces and human health is needed, one that takes into account the specific qualities of natural environments and urban characteristics, while also embracing a more inclusive approach to greenspace design (e.g., inclusion of vulnerable groups in the decision-making process). These considerations are crucial for maximising the health benefits associated with greenspaces and for widening access to greenspaces for population groups who stand to benefit the most (Bell et al., 2014; Stanhope et al., 2024).

In addition to public natural spaces, near-home nature contact (e.g., private domestic gardens and home-based nature activities) may be an important consideration. Near-home nature exposure can act as a viable mental health strategy where it may be substituted over public greenspace by those who are home-bound or more vulnerable (Darcy et al., 2022; Lenda et al., 2023). Research indicates indirect nature exposure (e.g., nature views through a window) is a common way to engage with nature for those living in urban settings (Cox et al., 2017; Lenda et al., 2023), and regular doses of nearby nature can contribute to improved mental health (Cox et al., 2017). This has policy implications for the provision of private greenspace as a priority in new urban developments (Chalmin-Pui et al., 2021).

In summary, a stronger evidence base which demonstrates the protective effects of greenspace on suicide-related outcomes can inform both policy development and urban planning, thus helping to widen access, use and engagement of greenspaces for those population groups who would benefit most.

Statement of interests

Trish Darcy is a Research Associate with the Mental Health & Addiction Research Group (MHARG) in the Department of Health Sciences at the University of York and is funded by the NIHR Applied Research Collaboration Yorkshire and Humber. Trish contributes to the ‘Natural Environments and Mental Health’ and ‘Mental Health and Physical Multimorbidity’ research themes within MHARG. She holds an MSc in Health Psychology and is currently in the final stages of her PhD which is exploring how nature can support those living with a long-term health condition.  The views expressed are those of the author, and not necessarily those of the NIHR or the Department of Health and Social Care.

Links

Primary paper

Bolanis, D., Vergunst, F., Mavoa, S., Schmelefske, E., Khoury, B., Turecki, G., … & Geoffroy, M. C. (2024). Association between greenspace exposure and suicide-related outcomes across the lifespan: A systematic review. Science of the total environment, 906, 167451. https://doi.org/10.1016/j.scitotenv.2023.167451

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