Annotated Literature Review: Health Benefits of Urban Greenspace

EXECUTIVE SUMMARY

Goal: There is a significant body of research dating back many decades investigating the various health benefits of greenspace1, many of which can be categorized into measured and perceived mental, physical, or social benefits. In order to gain an understanding of this evidence base, a structured literature on health benefits of urban greenspace was conducted and relevant articles summarized.

Findings: There is a general consensus that urban greenspace influences human health in a positive manner, but in some cases, there was insufficient evidence to generalize about specific health outcomes including diabetes (Kondo et al., 2018), cancer (Kondo et al., 2018; Ekkel and Vries 2017) or birth outcomes (Kondo et al., 2018; Wolf et al., 2020; Fong et al., 2018; Ekkel and Vries 2017). Most original research studies utilized a cross-sectional study design (Kondo et al., 2018), with many researchers controlling for socio-economic status in their analyses (Wolf et al., 2020). In general, studies focused more on short-term health benefits of greenspace (e.g., improved blood pressure, stress reduction & cognitive performance) (Wolf et al., 2020). The lack of randomized control trials made it difficult to distinguish whether or not reported results were attributable to an intervention (i.e., introduction of physical activity) or to the urban greenspace itself, particularly in studies of short-term improvements in physical and psychological health (Saitta et al., 2019). Studies included in this review exhibited variation in sample size (e.g., eight to a combined 290+million) (Wolf et al., 2020; Twohig-Bennett et al., 2018), with the majority of studies having been conducted in Europe and the United States. The full range of the human life span has been represented in studies to-date: adults (71%) young adults (13%), children (13%) & older adults (3%) (Wolf et al., 2020). Measured outcomes have been shown to vary by geographical location and/or type of natural and built environment examined (Roberts et al., 2020). It is worth noting that few studies focused on health inequities or the impact or urban greenspace specifically on disadvantaged for vulnerable populations (Kondo et al., 2018).

Overall findings were positive associations between urban greenspace and attention, mood, and physical activity (Kondo et al., 2018) and negative associations between urban greenspace and mortality, short-term cardiovascular markers (e.g., heart rate, blood pressure) and violence (Kondo et al., 2018). Generally, exposure to greenspace resulted in improvement of overall well-being, reduced symptoms of depression, anxiety, attention deficit/hyperactivity disorder (Younan et al., 2016), and a boost in immune function (Capaldi et al., 2015; Rook 2013). Other findings of interest are presented below:

Mental Health Benefits:

Exposure to urban greenspace can mitigate psychological stress by providing opportunity for physical activity, social interaction, and engendering cognitive and physiological responses associated with stress reduction and attention restoration, which may be pronounced for urban populations (Mennis et al., 2018; Jennings et al., 2016; Knecht 2004) more so for Black, indigenous, or people of color, which are subject to additional contextual societal (e.g., systemic racism) and environmental factors (e.g., proximity to sources of air pollution or major roadways) (Mennis et al., 2018).

Study participants were better able to perform a test of working memory after walking through an arboretum compared to those who walked on roads with heavy traffic and those who walked through the arboretum had 20% improvement in working memory (Hall and Knuth 2019).

A 10-20% reduction in perceived risk of poor mental health, depression and/or anxiety and intake of medication was found per interquartile range increase in average NDVI around homes, suggesting that there are substantial mental health benefits from nearby urban greenspace (Nieuwenhuijsen et al., 2017).

1 It is important to note that while ‘greenspace’ may take on different meanings, the term has been used extensively to refer to areas or urban vegetation such as public and private parks, gardens, residential landscapes, urban forests or other municipal landscapes (Hall & Knuth 2019). Some studies focused on a specific type of green measure (e.g., tree canopy measure) or nature-immersive experience (e.g., outdoor education program intervention), but a common metric used to indicate amount of greenspace in a study area was normalized difference vegetation index NDVI) (Kondo et al., 2018). Outcomes can and have been shown to vary by type of greenspace analyzed (MacBride-Stewart et al., 2016).

Meta-analysis of studies showed that increased greenspace exposure was associated with decreased salivary cortisol, a marker of stress, with effect mean difference (EMD) -0.05 [95% confidence interval (CI) -0.07, 0.04] (Twohig- Bennett and Jones 2018).

Physiological measures of stress (e.g., electromyography, skin conductance response, pulse transit time, cardiac response, partial thromboplastin time) indicated quicker and more complete recovery in a natural environment, even when measured over 10-minute period, and initial few minutes of recovery showed parasympathetic response which slows heart rate, increases intestinal and gland activity, and relaxes muscles in the gastrointestinal tract, while no such response was evident for urban settings (Berto 2014).

Physical Health Benefits:

Lower body mass index (BMI) among adults was observed in higher greenspace neighborhoods with more destinations for walking than in less green neighborhoods (Tsai et al., 2016) and proximity and access to urban greenspaces have been linked to healthier weights and lower BMIs in children (Chawla 2015; Hall and Knuth 2019), while perceived lack of greenspace and playground space among pre-school children have been independently associated with being overweight (Douglas et al., 2017).

Increased greenspace exposure has been associated with heart rate [EMD -2.57 (95% CI -4.30, -0.83)], diastolic blood pressure [EMD -1.97 (95% CI -3.45, -0.19)], high-density lipoprotein (HDL) cholesterol [EMD -0.03 (95% CI -0.05, <-0.01)], low frequency heart rate variability (HRV) [EMD -0.06 (95% CI -0.08, -0.03)], increased high frequency HRV [EMD 91.87 (95% CI 50.92, 132.82)], and cardiovascular mortality [odds ratio (OR) 0.84 (95% CI 0.76, 0.93)] (Twohig-Bennett et al., 2018).

Walk in nature have positive short-term effects on cardiovascular system and as measured in hypertensive patients, walks in nature decreased serum levels associated with high blood pressure, including endothelin-1, homocysteine, renin, angiotensin II type 1 receptor, and angiotensin II type 2 receptor, suggesting that walks in nature lower blood pressure in young, middle-aged, and older adults (Hall and Knuth 2019).

Regular exposure to low concentrations of mixtures of natural compounds and toxins in natural environments confers pleotropic health benefits by inhibiting activities of interconnected cell-signaling systems (e.g., PI3K/Akt/mTORCI) and when overactive can lead to pathological processes resulting in cancer, diabetes, inflammation, immunosuppression, and/or neurodegenerative diseases (Nieuwenhuijsen et al., 2017).

Social Health Benefits:

Survey data revealed that across the United States, there is a significant positive relationship between attractive physical settings (e.g., trails, parks) and community satisfaction, encouraging social interactions across diverse populations, which can remedy decline of social relationships in urban areas (Jennings et al., 2016).

Participants from multiple studies reported that accessible, usable parks enabled social interaction and development of social skills in addition to exposing people without disabilities to those with disability to foster community learning and improve tolerance and acceptance in society, while at the same time empowering social confidence of people with disabilities (Saitta et al., 2019).

Urban greenery can help people develop a sense of community and neighborhood attachment, increase social contacts, and reduce feelings of social isolation (Nesbitt et al., 2017). Positive correlations between use of public urban greenspace and strength of neighborhood ties and sense of community have been reported, in addition to more deaths for older adult residents recorded in urban areas with low greenness (Douglas et al., 2017).

Health Benefits of Access:

Disadvantaged communities, children, older people and people with mental health problems and pregnant women usually have less access to urban greenspace compared to more affluent populations but may be the greatest beneficiaries, suggesting that provision of access in a safe manner is important for reducing health inequities and buffer effects of some stressors (Kruize et al., 2019; Gascon et al., 2015).

Distance to destination, suitability of infrastructure (e.g., walking paths), and safety are very important factors motivating people to visit urban greenspaces (Kruize et al., 2019; Saitta et al., 2019; Hartig et al., 2014), and factors such as encountering busy roads may deter use of greenspace (Lachowycz and Jones 2013; Lee and Maheswaran 2011).

Across North American cities, underprivileged populations have disproportionately less access to vegetation including greenspace than affluent groups, with disparities more pronounced on public versus private land (Hall and Knuth 2019).

In the United States, typical standard suggested for ‘walkable’ is a destination within 0.25 miles or 400 meters at most (suggested by Sturm & Cohen 2014 in Ekkel & Vries 2017), but it has been shown that a distance of 100 to 300 meters (Ekkel and Vries 2017) is the threshold distance after which use of greenspace declines rapidly. Reported mental health measured using MHI-5 tool was better among residents having access to a park within 400 meters compared to those at 800+ meters (Ekkel and Vries 2017).

In addition to measured and perceived mental, physical, and social health benefits from urban greenspace, there are other areas of research that offer important findings on additional health benefits from urban greenspace. These include some less tangible and therefore harder to measure aspects (e.g., spirituality, feelings of vitality) as well as restoration from such practices as the Japanese art of shinrin-yoku (translated into English as ‘forest bathing’). A few notable findings are included below:

Other Health Benefits:

Aspects of human-nature interaction important to well-being that are not well understood but may be important include sense of place, aesthetics and preference, spiritual beliefs (Dickinson and Hobbs 2017); these views could have an impact on the use of urban greenspace (Rakhshandehroo et al., 2015; Shanahan, Fuller et al., 2015).

Health benefits of greenspace found to be larger in some studies involving larger areas with more biodiversity compared to urban park environments (Roberts et al., 2019) and in studies including biodiversity as a measure of interest, it (biodiversity) has emerged as an important positive element for people using urban greenspaces (Carrus et al., 2015; Zhou and Rana 2012); variation in greenspace itself has important role in enhancing population health (Shanahan, Lin et al., 2015).

Functional magnetic resonance imaging (fMRI) scans showed that spending time in forest settings prompted the brain area activations related to involuntary attention and cognitive restoration and one study found that short 15- minute sessions of ‘forest bathing’ produced enhanced subjective feelings of vigor, recovery, and vitality (Wolf et al., 2020).

Conclusion: Urban greenspace is an important multi-faceted influence on the health of urban populations, with respect to mental, physical, social and other benefits. However, more research utilizing a randomized control trial approach is needed to best assess such health benefits, particularly with respect to specific types of greenspace or micro-environments (e.g., street tree canopy, arboreta, mixed use park). In addition, some research uncovered here has suggested that underrepresented and/or vulnerable social groups may benefit more from exposure and access to urban greenspaces, so further research in this area is needed to add to the current peer-reviewed literature evidence base. Accessibility to urban greenspace emerged as a recurring barrier for underrepresented populations, so attention to providing safe and inclusive accessible greenspace should be of priority for urban areas.