Links between green space and public health: a bibliometric review of global research trends and future prospects from 1901 to 2019

The publication output for G-H research from 1901 to 2019 is summarized in figure 2. The first article in this field appeared in 1901, but there were only 11 documents retrieved from 1901 to 1989, which indicate a stagnation trend and very little development prior to the 1990s (which is the reason we did not consider this period in depth in the next analysis). After 1990, the annual number of publications started to rise steadily and has significantly increased since 2000, reaching its highest peak, 2372 publications, in 2018. According to this growing trend, it could be predicted that the number of G-H research papers in 2019 will exceed the number of publications in 2018.

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The number of active journals covering G-H research has also increased significantly over time, from only one in 1901 to the highest of 943 in 2018. In addition to the fact that the G-H study has attracted more attention from researchers in a variety of fields, the general increase of new scientific journals is probably also a major reason why there were more journals publishing studies on public health. There were only a small number of journals, such as Environmental Health Perspectives, which published G-H related studies in the pre-1990s. In the 1990s and 2000s, the journal with the largest number of articles is Social Science & Medicine. In the 2010s, PLoS One, became the journal with the largest number of publications (table 1).

According to WoS's classification system for subjects, a paper may belong to more than one subject area. The latest version of WoS classifies journals into 249 subject categories. Between 1901 and 2019, G-H research has been associated with approximately 50% of all subject categories, which suggests it has been involved in a very broad range of research areas. In the pre-1990s, early studies mainly described the role of parks in promoting public health. After 1990, the number of WoS categories has gradually increased. Figure 3 reveals the significant structural changes in Web of Science categories for G-H studies. During the 1990s, ecology ranked first in terms of the number of publications, followed by environmental science. Since the 2000s, environmental science has ranked the highest on the list of WoS categories for G-H research and has occupied a gradually larger proportion across the 2010s. In addition to environmental science, other categories related to the environmental field (environmental research) have developed rapidly in the last three decades. The presence of many categories related to the environment indicates that environmental impacts are closely related to the processes linking green spaces to public health. Urban studies, multidisciplinary science, and geography newly appeared on the top 10 list of subject categories associated with publications in the 2010s. Moreover, urban studies became the fifth most frequently occurring subject category in the 2010s, which suggests, to a certain degree, that health factors were increasingly considered in urban planning and design (Rydin et al 2012, Wolch et al 2014).

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With respect to publication performance by country, almost all the papers that were published in the pre-1990s came from the USA. Moreover, the USA has always been the leader in G-H research, contributing the most publications in the other three time periods and accounting for 42.3% (1990s), 34.7% (2000 s), and 22.8% (2010 s) of total publications respectively. Table 2 shows the top 10 countries (number of publications) that participated in G-H research from the 1990s onward. The top 10 countries in terms of numbers of publications contributed 76.3% (1990s), 68.9% (2000s), and 61.6% (2010s) of total publications, respectively, during the 30-year period. In addition, the top 10 countries in the three stages are mostly developed countries, which account for 68.42%, 59.40%, and 57.44% of the word's gross domestic product (GDP) respectively in the three decades. This observation indicates that the stronger a country's economic capacity, the richer its research in the green spaces and health field is likely to be. However, the total population of these countries accounts for only 12.75%, 29.59%, and 27.95% of the world's population, respectively. It also suggests that developing countries with large populations, especially low-income groups, which are the most vulnerable groups to health issues, have insufficient theoretical and practical results regarding the G-H research. G-H research has developed rapidly in China, and the volume of publications from China has grown from only 14 in the 1990 s to 1422 in the 2010 s.

Institutions in the USA were always the most productive in the pre-1990s period. However, they do not always rank in the leading positions in the other three decades. Table 3 lists the top 10 most productive institutions. In these three periods, US institutions only ranked first in the 2000s whilst in the other two periods, their positions were taken by Australia and China. In China, the institutional rankings of the Chinese Academy of Sciences rose sharply from the 2000s to the 2010s and became the most productive institutions in the 2010s.

Geographic maps were applied to indicate the spatial distribution of institutional publication activity, which refers to the frequency by which an institution was extracted from the author's affiliations. According to the definition proposed by (Wu and Ren 2018)), low, middle, and high activity correspond to ⩽5, 5–10, and ⩾10 publications for each period, respectively. In the pre-1990s, G-H research institutions were few and mainly distributed in big cities of the USA, such as New York (this period is not shown on the geographic maps). In the 1990s, only a few US institutions showed high publishing activity, some institutions in Europe showed a middle level of publishing activity, and almost all other institutions showed low publishing activity levels. In the 2000s, institutions involved in G-H research increased rapidly and publication activity reached a high level. However, high G-H research publication activity was mainly restricted to the USA, Europe, and Australia. There were only several high-activity institutions in East Asia. In the 2010s, G-H research activity expanded significantly in East Asia, and G-H research institutions had now expanded to six continents (figure 4).

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Analyzing keywords can uncover the most interesting areas or issues in specific research fields. Unfortunately, documents retrieved from WoS in the pre-1990s period were excluded, as their keywords are not available. Here, 1737, 10 294, and 52 307 keywords were obtained for the 1990s, 2000s, and 2010s, respectively (appendix C). The dramatic increase in the number of keywords indicates a broadening emphasis of G-H research. Duplicate detection and merging were performed in Excel, followed by a manual check. The frequency of co-occurrence of keywords can reflect, to a certain extent, hotspots within a research theme, the methods, and the content of retrieved documents. High-frequency keywords that appear in all three decades suggest that the research terms represented by those words remain a continued focus. Changes in high-frequency words indicate a shift in research emphases, representing the trends of the G-H research to some extent.

The relationship between green space and stress has been a major focus of attention during recent decades (table 4). Stress is the process by which an individual responds psychologically, physiologically, and often with behaviors, to a situation that challenges or threatens their well-being (Baum et al 1985). The vast majority of research to date has suggested that exposure to green space can relieve the pressure that residents feel and help with recovery from stress (Ulrich et al 1991). Another topic that has been a hotspot and continues to be important is the association between green space and mortality. For example, (Gascon et al 2016)) reviewed the available evidence on the relationship between long-term exposure to residential green spaces and mortality in adults, and found support for the hypothesis that living in areas with more green space reduces mortality, mainly cardiovascular disease mortality. Additionally, vulnerable groups, in particular, children have been considered as the main target in G-H research during the three decades. For instance, (Christian et al 2015) reported a positive relationship between neighborhood green spaces and early child health development and related behaviors. Other researchers have found that green spaces are crucial for young children's mental health (Engemann et al 2019), attention (Faber Taylor and Kuo 2009), and motor development (Fjørtoft and Sageie 2000).

Regarding newly appearing high-frequency words, 'physical activity,' which first became a high-frequency word (ranked 11th) in the 2000s, has now dramatically risen to be ranked in the top 2 in the 2010s. Physical activity, which has been considered as a critical pathway for linking nature to human health, promotes physical and mental health across the life span (Hunter et al 2015). Other emerging high-frequency words mainly originate from environmental science, such as 'climate change,' 'biodiversity,' 'ecosystem services,' and 'air pollution.' Due to rapid population growth and urbanization, environmental problems that are not conducive to health, such as heat hazards, air pollution, and climate change, received the highest attention. Ecosystem services afforded by green spaces as nature-based solutions for improving public health (e.g. improving the above-mentioned environmental problems) was also becoming a significant focus since the 2010s (van den Bosch and Sang 2017, Barnes et al 2018).

To identify the main knowledge domains, VOSviewer that has unique advantages in mapping knowledge domain label structure (Van Eck and Waltman 2009) was adopted to indicate the co-occurrence relationships of keywords, which can clearly indicate the internal composition or structure of a field. A total of 38 637 author keywords appeared in the retrieved documents were identified by VOSviewer (appendix D). Furthermore, keywords with a frequency of more than 25 appearances were collected, and small clusters (less than 30) were merged acquiescently. Finally, a total of 284 qualified keywords were successfully identified and classified into three groups (blue, green, red). As shown in figure 5, each circle represents a keyword, and the size of the circle denotes the frequency of that keyword. Lines connecting the circles indicate relationships between the two keywords, and colors identify the different clusters, which correspond to the main specific areas of G-H research. Hence, based on the clustering group and the important keyword (high-frequency keyword), we can clearly identify the research main domains and focus.

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Furthermore, in order to determine the specific name of each domain, we referred to the definition of health by World Health Organization(WHO), that is, ' health is a state of complete physical, mental and social well-being and not merely the absence of disease or infirmity' (World Health 1995). Conceived in this way, health encompasses physical health, mental health, social health (well-being), and the absence of disease. Additionally, green spaces provide a wide range of ecosystem services (ESs), in particular, regulating services (e.g. reduction of heat) (Yu et al 2019, 2020a, 2020b, Yang et al 2020), and supporting services (e.g. maintenance of biodiversity) (Kong et al 2010), many of which are positively linked to reducing risk of disease (e.g. heat morbidity) (Hartig et al 2014, Markevych et al 2017, Bratman et al 2019). Hence, an increasing number of scholars are considering ecosystem health (eco-health) (Su et al 2010, O'Brien et al 2016, de Jesus Crespo and Fulford 2018).

Combining these concepts with the keyword clustering analysis, we could finally determine the specific domains in G-H research. Due to the lack of 'social health (well-being)' related keyword, we deliberately proposed and defined the three clustering groups as green spaces and physical health (G-PH), green spaces and mental health (G-MH), and green spaces and ecosystem health (G-EH). In addition, according to the number of clustering terms (appendix E), we can roughly estimate that the G-EH studies occupy the largest proportion of retrieved literature, accounting for about 48.8%, followed by the G-MH related literature, accounting for about 30.3%, and G-PH researches, accounting for about 20.9%.

In the blue cluster, the keywords 'health,' 'public health,' 'well-being,' 'green space,' and 'parks' are the nodes of research term that are highlighted. In addition, cultural ecosystem services provided by green spaces also presented in this cluster, such as the nodes 'leisure' and 'recreation.' However, the biggest node in this cluster is 'physical activity,' strongly linking with above-mentioned research terms, indicating that green spaces (e.g. parks) promote certain types of physical activity ('exercise,' 'walking,' and 'cycling'), and thus has a significant contribution to physical health (e.g. 'obesity'). Notably, there is some evidence that nature-based physical activity (green exercise) achieves more physical health benefits than equivalent exertion in indoor or constructed settings (Thompson Coon et al 2011, Barton et al 2016, Frumkin et al 2017).

The second research cluster refers to the linkage between green space and mental health. Mental health is defined by the WHO as 'a state of well-being in which an individual realizes his or her own potential, can cope with the normal stresses of life....' With regard to this conceptualization, mental health encompasses the presence of psychological well-being and the absence of mental illness. The largest circle, 'mental health,' mainly links to 'stress,' 'depression,' and some disease-related keywords, such as 'mortality,' 'epidemiology,' and 'hypertension.' Many studies have demonstrated that exposure to green space is positively associated with mental health, including mental illness and the presence of psychological well-being (Bratman et al 2019).

The red cluster is the largest of the three clusters, and corresponds to the link between green space and ecosystem health, with a focus on environmentally relevant hotspots. The largest node, 'ecosystem services,' mainly links to 'climate change,' 'air pollution,' 'green infrastructure,' 'drought,' 'urban heat island,' 'sustainability,' 'conservation,' and 'biodiversity.' Green space plays a crucial role in advancing the ecological sustainability of cities and promoting human health by providing a wide range of ecosystem services, such as water cleansing, mitigation of water hazards, improving air quality, reducing noise, maintaining biodiversity, and preventing heat stress.

Living near (or having easily accessible) green spaces, with safe and attractive areas for activity, promotes physical health: the underlying mechanism of this effect is physical activity levels (Pretty et al 2003). A large body of literature has confirmed that physical activity in green spaces (green sports) produces greater health benefits than sports activities in the built environment (Triguero-Mas et al 2015, Dadvand et al 2016, van den Berg et al 2019). Over three broad activity domains (work, active transport, and leisure), green spaces are considered essential for promoting physical activity via active transport (walking or cycling) and leisure (sport or recreation) (Hartig et al 2014). For example, higher levels of greenness in the environment may lead people to favor walking or cycling over other transport modes by making routes to the destinations more attractive, although other factors such as distance, infrastructure, and safety are also important (Heinen et al 2010). Nevertheless, there are potential negative effects associated with physical activity (e.g. sports injuries). Green space can offer opportunities for most beneficial 'green exercises,' which are positively associated with a lower risk of chronic diseases such as obesity, type II diabetes, and hypertension. Therefore, physical activity is a crucial intermediate factor in establishing the links between green space and public health, especially for physical health (Triguero-Mas et al 2015, Hunter et al 2019).

Exposure to green spaces can have a positive impact on reported psychological well-being. Although the mechanisms underlying these links remain unclear, perceptions and experiences that are mainly immaterially related to green space, are probably important mechanistic connections that promote better mental health. Knowing and viewing are two specific ways people perceive green spaces (Russell et al 2013). Knowledge of belonging to a community rather than being alone, derived through experiences of nature, is argued to be a plausible reason for the broadly positive psychological benefits of nature (Mayer et al 2009). Views of green spaces have been repeatedly associated with health benefits, especially for reducing stress levels, such as perceived stress, job stress, and driving stress (Parsons et al 1998, Bringslimark et al 2011). Another mechanistic channel experiences, which represents an interaction with green spaces (Bratman et al 2012). To date, around 150 human-nature interaction patterns have been generated and cataloged (Bratman et al 2019). Long-term interaction with green spaces has been extensively documented to have a positive effect on mental health, suggesting improved happiness and subjective well-being. For example, experiencing green spaces promotes restoration (e.g. increased attention, and reduced stress, fatigue, and irritability) better than does other experiences that have been studied (such as interacting with friends) (Ottosson and Grahn 2005). In addition, exposure to green space has been associated with reductions in mental illness, due to the benefits that are perceived or experienced regarding green spaces in terms of improving sleep, relieving stress, reducing rumination (a maladaptive pattern of self-referential thought), and subgenual prefrontal cortex activation, which are the major risk factors for mental illness, especially depression (Bratman et al 2015).

Many studies have shown that there is an intermediate link between the existence of ecosystems and public health through buffering ecosystem services (ESs), which refers to the ability of ecosystems to 'buffer' health effects (Bratman et al 2012, Jackson et al 2013, Guerry et al 2015, de Jesus Crespo and Fulford 2018). The processes of buffering ESs that are most relevant to public health can be broadly classified into the following categories: clean air and water; noise reduction; flood mitigation; heat effect reduction; climate stability; biodiversity conservation, etc Therefore, green space can modify health outcomes to a certain extent and reduce the risk of clinical diseases such as gastrointestinal diseases, respiratory diseases, and cardiovascular diseases, through the causal pathway of buffering ESs. For example, green space 'water cleansing' can help prevent the gastrointestinal disease from drinking or recreational use of infectious or toxic water (Katukiza et al 2014). Also, green space 'mitigation of heat effects' can help avoid extreme temperatures through shade and evapotranspiration, lowering heat morbidities during heatwaves (Bouchama and Knochel 2002). Another focus for plausible causal pathways of buffering ESs (supporting services) and important concerns, is the role of greenspace biodiversity in promoting eco-health. One explanation is that exposure to diverse microbiomes in green spaces (e.g. soils, plants, water) helps train the immune system to accurately distinguish dangerous from helpful bacteria and modulates immune function, which can affect a number of health outcomes, such as in the areas of cancer, allergies, and autoimmune diseases (Hanski et al 2012, Ruokolainen et al 2015, Von Hertzen et al 2015).

In addition to the causal mechanisms and pathways of the three domains mentioned above, some extra explanations need to be outlined about the proposed framework. First, although we distinguished clusters of causal pathways with different colors (figure 6), the actual pathways are complexly intertwined, and multiple pathways are likely to be engaged simultaneously. For example, ecosystem services (cultural services) can also provide better aesthetic environments and recreational opportunities to promote physical activity, which is positively associated with physiological and psychological health. In addition, the strongest evidence suggests that the greatest benefits of physical activity are in the prevention and treatment of cardiovascular disease and related mortality as well as all-cause mortality (Thompson et al 2003, Van den Berg et al 2015). Second, all these beneficial health outcomes of green spaces may be affected by potential moderating and mediating factors, such as socio-economic status (SES), gender, age, preferences, occupation, culture, and individual perceptions (Triguero-Mas et al 2015, Akpinar et al 2016, Dadvand et al 2016, Sugiyama et al 2016). Third, the social benefits of interacting with green spaces (e.g. increased social cohesion), which can be regarded as 'social health' with respect to neighborhoods and communities, have received less attention than the other three health categories (Keniger et al 2013, Hunter et al 2019). Fourth, nature-based planning and health-based design should be considered when designers and planners advance the programmatic and policy goals for green spaces in order to achieve increased health benefits (Barnes et al 2018).

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