It is remarkable that when you talk to people about green space they tend to have positive experiences to tell. They remember their childhood climbing a tree, a long hike in the forest, a barbeque with friends in the garden, or some time spent with their beloved in a park (Figure 1). Perhaps this is not so surprising as for tens of thousands of years humans have lived in forests and savannahs surrounded by nature, and only during the past few thousand years they have moved into cities, where nature is often less available. Our bodies and minds may be best adapted to living with nature, a concept Wilson1 described with the term biophilia: people’s innate affinity for the natural world.
Currently half the world’s population live in cities, and this proportion is expected to increase rapidly to 70% during the next 20 to 30 years.2 This rapid increase is particularly happening in low- and medium-income countries; 80% to 90% of people already live in cities in high-income countries.
Cities have long been known to be society’s predominant engine of innovation and wealth creation, yet they are also its main source of crime, pollution, and disease.3 Over the years, we have created large, mostly grey cities with many high-rise buildings and little green space. (Shenzhen, China; Beirut, Lebanon; Buenos Aires, Argentina; Kuwait City, Kuwait; and Athens, Greece are a few examples.) It is therefore not surprising that only 23% of the residents of Athens are very or rather satisfied with available green space.4 The amount of green space available to people in cities also varies considerably from, for example, 1.9 m2 per person in Buenos Aires, Argentina to 52.0 m2 in Curitiba, Brazil.5 Rates of diseases such as mental illnesses tend to be higher in urban areas compared with rural areas. For example, in a meta-analysis, Peen et al.6 found that the risk of mood disorders was 39% higher in urban areas than in rural areas.
People value green space and are increasingly willing to pay for it. Donovan and Butry7 found that house prices in Portland, OR, were on average US $8,870 higher if there were street trees and they sold 1.7 days faster. A small 15 m garden in London sold recently for 1.2 million pound sterling.
This topic area is a rapidly evolving field with a substantial number of studies and reviews emerging in the past few years, but a broad overview of the current state-of-research is lacking. We therefore aim to describe the evidence linking green space in cities to health and to make recommendations for further research. We have three main premises. The first is that green cities are healthy cities, but generally there is lack of sufficient green space in at least parts of many cities. The second is that green space is beneficial for healthy psychophysiological functioning, health, and well-being. The third is that there are still many important open questions in terms of when, where, or how much green space is needed and what are the underlying mechanisms of the relation of green space to health.
This review is not comprehensive; there have been a number of recent extensive reviews on this topic (Table).8–22 Instead, we aim to provide a holistic and narrative overview of the available reviews to highlight the current state of the evidence base, pointing out key reviews, strength of evidence, and influential studies on the importance for healthy green urban living.
In the “Introduction,” we set the context, and in the “Results” section, we describe a conceptual framework and proposed mechanisms, followed by the description of existing green space indicators, health effects of green space, and the contribution of these mechanisms. Finally, we provide recommendations for further research.
In this review, we adopt the United States Environmental Protection Agency definition of green spaces as “land that is partly or completely covered with grass, trees, shrubs, or other vegetation,” which can include parks, community gardens, and cemeteries.23 All studies we review here define green space consistently with this definition.
We use a modified version of the conceptual framework recently proposal by Hartig et al.11 In this framework, the relationship between contact with the natural environment and health is mediated through a number of possible mechanisms including air quality, physical activity, social contacts, stress, and restoration. These mechanisms have a number of possible modifiers, such as distance to green space, accessibility factors, perceived safety in the green space, societal context, cultural context, gender, age, and socioeconomic status (Figure 2). Improvement in air quality, increase in social contacts, physical activity, and a reduction in stress are all well known to be associated with improved health. Below, we briefly describe the possible mechanisms for these associations and give some key examples.
Green Space Indicators
Before describing possible mechanisms and health effects of green space, we briefly discuss the current methods for assessment of green space indicators in epidemiologic studies. Generally surrounding greenness and/or access to green space are used in epidemiological studies.14,18 With surrounding greenness (an indicator of general greenness of living environment), a buffer is built around the home or another location and the percentage of green space or the amount of greenness is estimated within the buffer using satellite-derived indices, such as the normalized different vegetation index. For access to green space, the Euclidean distance to a park, or the network distance to a park based on the closest route using street networks, or the presence of park within a given area around the home is estimated. At times the estimation is not only done for the home but also for work or school and commuting route. Furthermore, some epidemiologic studies obtain information on the use, perception of, and visual access to green space by questionnaire. Other epidemiologic studies use tracking devices and motion sensors, such as GPS, accelerometers, and smartphones with applications for measuring location, ecological momentary assessment, and physical activity.14 Finally, some epidemiologic studies perform audits to assess the quality of the green space.
Tallis et al.24 found that the tree canopy of the urban forest in the Greater London Area removed between 852 and 2,121 tones of PM10 annually, which equates to between 0.7% and 1.4% PM10 air-quality improvement. Regional plans to increase tree cover from the current 20% to 30% are expected to remove 1.1% to 2.6% of PM10 by 2050. Dadvand et al.25 found a reduction of around 5 μg/m3 in personal exposure to PM2.5 and NO2 in pregnant women in Barcelona per interquartile range of the normalized difference vegetation index (NDVI). The former study is an example of the possible filtering effect of green space, and the effect is generally fairly small. The latter shows that the combined effects of filtering and replacement of mobile pollution sources by green space can be larger.
Green space also reduces temperature in cities, and therefore reduces the urban heat island effect. In a meta-analysis, Bowler et al.26 found a 1°C reduction in temperature when comparing temperatures in parks with urban areas.
More than 50 studies have been conducted aiming to relate access to green space to physical activity levels, but the results have been inconsistent.8 A recent systematic review of the association of proximity and density of parks and objectively measured physical activity in the US and found that the associations varied, possibly due to the heterogeneity of exposure measurements. However, there are examples of beneficial associations, such as a recent large Australian study (N = 203,883), which found that with an increase in the percentage of green space, the odds of moderate to vigorous physical activity increased by up to 30%.27
Social Contacts and Cohesion
The cover of the July 20, 2015 issue of the New Yorker28 showed people drinking tea under trees. “Sitting together for tea is what friends do” says Jean-Jacques Sempé, the designer of the cover, and he goes on to say that “Time spent with friends is one of the greatest pleasures of my life.” Relatively few studies have evaluated the relationship between green space and social contacts. Some of the classic studies in the Robert Taylor Homes, Chicago, have shown that apartment greenness and building greenness improve neighborhood social ties.29
The newly promoted biodiversity hypothesis suggests that reduced contact with nature may adversely affect the human commensal microbiota and its immunomodulatory capacity. Hanski et al.30 found a relationship between generic composition of skin microbiota and land use type around the home. They also found a reduced level of allergic disease, mediated through acinetobacter and gammaproteobacteria. However, very few studies so far have focused on this mechanism.
Moore31 recently suggested the “biogenics” hypothesis, suggesting that regular exposure to low concentrations of mixtures of natural compounds and toxins in natural environments confers pleiotropic health benefits by inhibiting the activities of interconnected cell signaling systems, particularly PI3K/Akt/mTORC1. When overactive, Akt and mTOR (mTORC1) can lead to pathological processes resulting in cancers, diabetes, inflammation, immunosuppression, and neurodegenerative diseases. Future evaluation of this mechanism is required.
Restoration and Stress Reduction
The more consistent and promising pathways are stress reduction and restoration. The Stress Reduction Theory suggests natural that environments promote recovery from stress and help to lessen states of arousal and negative thoughts through psychophysiological pathways. Natural elements with characteristics such as a level of ground surface, spatial openness, curving sightlines, and the presence of water may induce recovery from any form of stress, even mild short-term stress, via an unconscious and innate response.32 The Attention Restoration Theory suggests that nature can replenish directed attention fatigue. Natural environments abound with “soft fascinations” that a person can reflect upon in “effortless attention,” such as clouds moving across the sky, leaves rustling in a breeze or water bubbling over rocks in a stream.33 For example, Ulrich32 found that patients who underwent a gall bladder operation and had a view from a window with trees recovered faster and used fewer potent analgesics than patients with a view of a brick building wall. Ulrich et al.34 also found that subjects recovered from stress faster, measured in terms of skin conductance and pulse transit time, when they viewed photos from nature compared with traffic and pedestrian malls. In subjects who initially underwent a stressful interview, Jiang et al.35 found that the percentage of tree cover in videos (ranging from 2%–61%) was directly related in an exposure response fashion with stress recovery. The Japanese have developed a practice called “Shinrin Yoku” or forest bathing to relieve stress.36
The recent use of technology such as electroencephalography, in which electroencephalography signals are translated and classified in different emotional states, showed different responses for volunteers walking around in different neighborhoods (urban shopping streets, green space, and busy commercial districts).37 Volunteers showed lower frustration, engagement, and arousal, and higher meditation when moving into the green space zones. Bratman et al.38 showed that after a walk in nature, subjects showed reduced rumination, as measured by questionnaire and reduced blood perfusion in the subgenual prefrontal cortex compared with a walk in an urban area. Reduced activity in the subgenual prefrontal cortex has been associated with improved symptoms in people with depression and regulation of cortisol levels, which is related to reduced stress.
Mental Health and Cognitive Function
A recent systematic review found limited evidence for a causal relationship between surrounding greenness and mental health status in adults, but little or no relationship with access to green space.18 Although the review found many associations, the main limitation was the cross-sectional nature of most of the studies. For example, a recent cross-sectional study by Triguero-Mas et al.39 found a 10% to 20% reduction in perceived risk of poor mental health, perceived depression and/or anxiety, and intake of tranquilizers or sedatives, antidepressants, and sleeping medications per interquartile range increase in average normalized difference vegetation index around the homes, suggesting that substantial health benefits are possible. Alcock et al.40 used a longitudinal design by following up the mental health status of people moving house. They found an improvement of mental health scores for those moving to greener areas, with effects lasting at least 3 years, and deterioration in mental scores for those moving to less green areas. Green space may reduce health inequalities emanating from differences in socioeconomic status. Mitchell et al.41 found a 20% difference in mental well-being when comparing the most and least financially strained among those who had great difficulty with access to green space, while the difference was only 11.9% for those with very easy access to green space. Finally, Dadvand et al.42 measured the cognitive development of children 7 to 10 years old using repeated computer tests over a year and found a 6% better development of the working memory and superior working memory among those in the highest tertiles of greenness in school compared with those in the lowest tertile of greenness.
A seminal study on the relationship between green space and all-cause mortality in the United Kingdom found that all-cause mortality was around 6% lower in the highest quintile of green space compared with the lowest quintile.43 Furthermore, the difference in all-cause and circulatory disease mortality between different socioeconomic classes was much smaller among those in the highest quintile of green space compared with the lowest quintile, further suggesting that green space may reduce health inequalities. A natural experiment reported by Donovan et al.,44 in which they evaluated the effect of the loss of 100 million trees to the emerald ash borer, an invasive forest pest on mortality, found that the presence of the borer was associated with 6.8 additional deaths per 100,000 adults and 16.7 additional cardiovascular deaths per 100,000 adults. In meta-analyses of green space and mortality, Gascon et al.18 found an 8% reduction in all-cause mortality and a 4% reduction in cardiovascular mortality when comparing the highest with the lowest exposure group of green space.
Other beneficial impacts of access to green space have been reported, such as a reduction in crime22 and obesity8,17 and an increase in birth weight.12 But there are also risks with green space, such as physical risks when trees fall on people or property, the spread of Lyme disease, and an increase in skin cancer, allergies, and asthma.11,21
Contribution of Mechanisms
Very few studies have evaluated the contribution of the different mediators between green space and morbidity and mortality. De Vries et al.45 estimated the contribution of different potential mediators of green space on health (stress, social cohesion, and green physical activity) to general health, acute complaints, and mental health and found that stress and social cohesion contributed the most to the outcomes (around 20%–40%), while green physical activity contributed less than 10%.
The recent surge in studies of green space and health has strengthened the evidence base considerably. Many of these new and emerging studies have shown evidence of beneficial associations with health outcomes, but some concerns has been raised about potential residual confounding by socioeconomic status. Green space is valued by people and they are willing to pay more for having greener residences. There is also some evidence showing differential access to green spaces among racial and ethnic minorities.46 There may therefore be a relationship between green space and socioeconomic position47 making socioeconomic position a potential confounder; however, the impact of the socioeconomic position on the findings of studies of the health effects of green spaces depends on local context and the type of health outcome and can vary from minimal to moderate.48,49 Furthermore, people with poorer mental health may be more inclined to move to greener areas, but there is little evidence this happens so far.40
Even though it appears that green space is associated with benefits to mental health, mortality, and other outcomes, questions remain in terms of where, when, how much, and what type of green space is required—if causal—for those benefits, the contribution of various mechanisms, and the effectiveness of green prescriptions. The epidemiologic studies so far have used fairly crude green space indicators, which may not be enough to guide transport and urban planners, landscape architects and policy makers to start planning or refitting our cities and investing more in green infrastructure. For example, can we get our “dose” of green space when we visit a large natural area outside the city once in a while, or can we get it when commuting to work in our car, or do we need local parks and trees in our streets? Is blue space as important? So what more is needed, particularly in urban environments?
Agenda for Improved Research Designs
To date, most epidemiologic studies have been cross-sectional in design and we need more longitudinal studies and intervention studies to ensure that access to green space, or the lack of it, precedes the measured health outcomes. For cost effectiveness, green space exposure indicators could be assigned to subjects in existing cohort studies that have good data on individual and area level or individual socioeconomic status information and other covariates.14 Pooled analyses that exploit the wealth of data available within existing cohorts also provide opportunities to strengthen the evidence base. These pooled cohorts have been used to investigate other exposures such as air pollution at larger scales than would have been possible if only relying on the individual cohort.50 Informations on potential mediators would be desirable.
Although easily obtained green space measures such as normalized difference vegetation index or percentage green space are attractive, researchers should go beyond these measures and include more information on type and quality of the green space. This type of information is critical for urban planning, so must be provided to translate research results into interventions. Although exposure estimates are traditionally assigned to the home of study participants, it may be important to assign green space estimates to commuting routes, as there is evidence to show that only short duration of green space exposure—such as that obtained during a commute—may be needed to obtain beneficial effects. Assigning green space indicators to work or school is also important as these places are attended during daylight hours and subjects can see green space. People within a cohort or cohorts who change residence may be an interesting subgroup to study as their exposure to green space may change, allowing estimates of longitudinal effects.
Also, cohort studies may provide the opportunity to look at the relation between green space and health for multiple and new outcomes and to examine the underlying biological pathways by using -omics technologies that these studies often already apply.51 Various studies have shown that green space may be able to reduce health inequalities and this needs replication and confirmation. Setting up a new cohort for green space and health research only may be prohibitively expensive, but if new cohorts are set up for other purposes, green space should be considered as a potential exposure of interest from the early stages of design and planning so that the required data will be available.
In general it appears that there are health benefits associated with an increase in green space, especially around the home, and even short-term exposure to green space may offer improvement in acute psychophysiological parameters.14 However, it is still unclear how much, where, when, and what type of green space is needed to improve and/or sustain long-term health benefits. For example, is green space needed around the home, or around work, or is exposure to green space during commuting sufficient? Is there an exposure–response relationship, and if so what is the shape and is there some minimum level? Further article is needed on these questions, using appropriate study designs to inform transport and urban planners, landscape architects, and policy makers.
Quality assessments in future epidemiologic studies are therefore also essential to better address these questions. Quality characteristics of green spaces, such as aesthetics, biodiversity, walkability, sport and play facilities, safety, and organized social events, have been suggested to affect the use of green spaces for physical activity.52 Badly maintained and unsafe parks with few attractions may be a deterrent to park visits. This deterrence may partly explain the inconsistent evidence for the association of green space with physical activity as generally the quality aspect of green spaces has not been considered. Having highly allergenic trees in the street may lead to an increase in allergies and asthma. A number of GIS-based studies have started to include quality or perception measures in the research. These have not been widely used in epidemiologic studies, but are likely to be important. Future studies should include audits to assess the quality of green space or subjects’ satisfaction with green space.
Agenda for Research Topics
Many cities have recently started to introduce or improve green space or green space programs; such efforts lend themselves to evaluation of effects on health and other indicators.53,54 Changes to the urban environment—such as introducing new parks or upgrading existing ones, greening of vacant lots, and introducing green corridors, urban gardens, or green exercise programs—provide opportunities for evaluation of effects on health. The main challenges are to find good populations to study before and after the changes and to find a valid control group as cities change continuously. Improvements to the urban environment often lead to gentrification with a large influx of wealthier residents and outgoing of the traditional residents, which makes it difficult to have valid before and after comparisons. Furthermore, these types of studies lend themselves to both quantitative and qualitative approaches to evaluation. Collaboration with social scientists is essential.
Furthermore, there are very few studies on the magnitude of the contribution of various mechanisms of the benefits of green space (e.g., air quality, physical activity, social contacts and cohesion, stress, and restoration) to health45 and some of these mechanisms have been very poorly studied. Foremost among the understudied mechanisms is the importance of biogenics and biodiversity. More studies on the contribution of these mechanisms are needed to plan effective interventions in cities. The problem is often that, in existing epidemiologic studies, either some information on the mechanisms or on health outcomes exists, but rarely, if ever, information on both. Therefore, studies on the health benefits of green space rarely comprise complete information to answer research questions. Some attempts are made with specially designed studies to evaluate the contribution of the various mechanisms.14 In the foreseeable future, it is most likely that we can only estimate the contribution of some mediators, unless new cohort studies are set up in which they can be incorporated from the beginning, but as mentioned above, these are expensive.
There is some indication in the literature that some groups of the population may benefit more than others, such as people with lower social economic positions and women, including pregnant women, and these results need further confirmation.43,46,49 A group of particular interest may be racial or ethnic minorities moving into cities, for whom there is little data.
In addition to their ability to improve mental and physical health, green spaces have also been suggested to have “therapeutic effects” on a range of health problems for people with diseases, such as mental health problems, pulmonary diseases, and Alzheimer’s disease. Evaluations of these green prescriptions are generally small in size or lacking entirely. Further study is needed in this area as it is a promising aspect of clinical practice, but a good evidence base is needed.
Cities can be healthier for people, not by painting trees on walls, but by having a nearby park, planting trees in the streets, and introducing urban gardens. Urban gardens may have additional benefits in terms of local food production and economy and, if done at a sufficiently large scale, can contribute to the sustainability and self-sufficiency of cities. Many cities need more parks, which can also become part of the identity and attraction of cities. Think, for example, of the iconic Central Park in New York. Finally, green roofs may transform the city, not only in terms of resilience but also in terms of visual attractiveness. More research is needed to tease out the important aspects, but in general to strengthen the evidence that more green space leads to better health.
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