A Systematic Visualization Review of Green Environment and Public Health for 2003-2019 Based on The Co-Citation Bibliometric Analysis Theory


 Currently, the world is facing challenges of environmental pollution and public health owing to increasing urbanization. Therefore, many researchers from developed and developing countries are considering environmental pollution and public health to be the most important topics for sustainable development alongside a healthy and green environment. Although in the literature many researchers have investigated a pictorial view of green environment by defining the urban green space and blue space effects on public health, the green environments and public health research trend remains unclear. Thus, this study aimed to contribute to the literature by visualizing the bibliometric for green environments and public health, and to identify the missing research pathway. Data for this study was collected from the Web of Science from 2003-2019 in order to facilitate a visualization and bibliometric analysis carried out by CiteSpace. The visualization results reveal the most influential studies, institutions, authors, countries, keywords, and category cloud in the green environments and public health research field. Furthermore, this study suggests that researchers need to pay attention to how the genome changes due to climate change, as well as environmental pollution and its effect on human health. Mental health and research related to green environment and social health is also missing. In addition, there is also a missing link regarding green environment, underground water and public health. Additionally, this study could help authors and publishers make decisions concerning research on green environments and public health and planning for future perspectives to contribute to both academic development and applied methodology.


Introduction
The world is facing challenges regarding increasing public health and environmental issues owing to rising urbanization. The total urban population recorded in 2018 was 55%, while the estimation of the United Nations was approximately 68% by 2050 (U.N 2018). Urbanization is the key to development in any country but also endorses the negative effects on human wellbeing ( Hartig et al. 2007) and also helps in the healing process after surgical mediation (Ulrich 1984). Similar patterns were found for a wide range of health bene ts including mortality ).
Among all the health bene ts, mental health bene t is of particular attention in the literature, highlighting stress responses as a major connection among the health effects and neighborhood conditions. In recent studies, the green environment was directly linked to biomarkers of attention and stress (diurnal disparity of salivary cortisol) (Thompson, Roe et al. 2012, Roe, Thompson et al. 2013) and brain waves as restrained by electroencephalography (EEG) devices, signifying the link among stress reduction and fatigue with exposure to green environments , Aspinall, Mavros et al. 2015. The Attention Restoration Theory (ART) suggests that green environments can decrease mental fatigue and help to reinstate an individual's ability to focus their attention. To maintain this focus, humans require mental effort, resulting in mental stress. In order to overcome this stress, humans need the chance to relax their focused attention. The way to overcome mental stress is to engage the brain in different kinds of attention like fascination attention; this can happen spontaneously without the same mental efforts used for focused attention. Indeed, researchers claim that the green environment has the ability to fascinate the attention and help to reduce mental stress, particularly in fast-paced, urban environments. (Kaplan 1995).
According to Mass et al (2006) and De et al (2003), health and green spaces have a stronger relationship within lower-status socioeconomic groups. They stated that the elderly and other individuals staying home during the day supports the hypothesis that the extent of exposure to green space on health is directly or indirectly related to their exposure to the local environment. They suggest that it could be a cost-effective strategy to recover health with an increase in access to green environmental spaces. On the other hand, these lower-status socioeconomic groups have limited traveling ability to other places, making them dependent on the local environment in the neighborhood for their healthy exposures (Kuo, Bacaicoa et al. 1998, Taylor, Wiley et al. 1998, Mitchell and Popham 2008. This evidence suggests that green spaces have stronger health effects between ethnic, racial, and socioeconomic groups; green environmental spaces might be "systematically organized to diminish health disparities" besides improving health generally (Hartig 2008).
Besides health bene ts, the green environment also contributes to a great extent in the improvement of the environment by decreasing emissions, energy saving, providing cooling effects, mitigating heat islands, and decreasing air temperature due to urbanization (Zhang, Gao et al. 2014). These environmental challenges are the most serious problems of the 21st century. Green spaces in urban areas like trees, gardens, lakes, and vegetable plots have a considerable effect on the microclimate and lessens urban heat island growth by reducing air temperature (Dimoudi and Nikolopoulou 2003). Vegetation in urban areas has a great impact on temperature reduction in two ways: shading and evapotranspiration. In the shading process, the solar radiations are intercepted by leaves and branches of trees and cannot reach the ground below the plant or tree. A study in Australia demonstrated that trees can reduce wall surface temperature by 9 o C and air temperature by 1 o C (Berry, Livesley et al. 2013). On the other hand, the evapotranspiration is the sum of plant transpiration and evaporation, indirectly effecting air temperature by the consumption of energy required by plants for transpiration. It was con rmed by a study in Greece that evapotranspiration can reduce air temperature by 3.1 o C through a green environment (Georgi and Dimitriou 2010).
In addition, trees can promote a green environment and its bene cial effects. The placement of trees near apartments and residential areas can reduce energy consumption in relation to heating and cooling purposes because trees will provide shade in the summer and keep the environment cooler around itself, while providing lesser shade in winter and letting sunlight through. Furthermore, they help to reduce dust and make the environment better by reducing pollution, regulating air movement, and produce humidity (Prevezanos 2007).
Nowadays, bibliometrics is a widely used statistical method to analyze scienti c publications and understand the past, present, and future trends in certain elds of research. Bibliometrics uses a citation analysis technique, as opposed to the existing review of survey papers, and constructs a citation graph, which is a network or graphical representation of the citations between documents to measure the impact of their eld, the in uence of a group of researchers/countries/institutes, and the intensity of a speci c research study, journal, or author. Usually, such research was done on a co-citation nexus, keyword linkages, and an association between co-authors. Considering the association of co-citation, if there are two studies and a third paper published and cited the earlier papers, this is a so-called co-citation, as per Sun et al. (Sun, Zhou et al. 2020). This technique is easier and more reliable to aid aspirant researchers in understanding a certain research eld with heavy literature data. The bibliometric mapping technique not only provides a pictorial description of the state of the sculpture in the world of research but also delivers direction for scientists in their emerging area of research interest on a speci c topic, hypothetically signifying stimulus for future theoretical and practical approaches to enlarge a speci c eld of the existing literature.
The bibliometric visualization method has previously been widely used in a number of studies to nd research pathways and understand the trend of the speci c research area. In the current dearth of literature, one researcher use bibliometrics to investigate the research trend for a local climate change classi cation scheme in order to improve the sustainability of the urban area ( and scholars have carried out visualization and bibliometric review analysis for different elds, still the green environments and public health eld was undiscovered. Consequently, this study tried to investigate the conceptual missing research pathway, the trend of the research, research links, active research countries, and active research institutes who are executing research related to green environments and public health. This study will contribute to the literature so researchers can understand the green environments and public health research area and provides a pathway for future research. This study was conducted based on the literature data of about 5916 documents; there is no author who claims to have done similar research.

Data source
For this study, data was collected from Web of Science (WoS) published papers in the indexing of scienti c citation index (SCI), social sciences citation index (SSCI) and science citation index expended (SCIE) database by searching for green environments and public health. By avoiding the bias and errors in analysis, the data screening were carried out by following the guidelines of Professor Chaomei Chen.
The suitable designed formula was applied in order to make sure that all the papers available on WoS core collection were accurately and correctly gathered and the results cover the green environments and public health research eld very well. For this study, a time span from 1950-2020 was selected and data were downloaded, while the screening data results found that before 2003 there was no research related to green environments and public health. Therefore, this study continues to visualize Scientometric analysis from 2003-2019 by xing the slice=1.

Research method
Visualization network mapping is the analysis of the co-occurrence method, which provides to an author the missing link of a certain research area, research pathway, hotspot, process of evaluation, and trend towards the development of a speci c research eld. The visualization method takes the literature (published papers) as a sample for the visualization of the time trend in a certain research area; this paper, for example, covers green environments and public health. The study used the CiteSpace software in order to visualize the research pathway, and knowledge innovation for green environments and public health developed by Professor Chen Chaomei (12). This study will analyze the visualization network map of co-citation, network of authors, mapping network of countries, mapping network of institutions research links, literature published in the journals network, and keywords. Using visualization, within green environments and public health new pathways of research and missing links in previous research can be found, allowing researchers to clearly understand the research trend development. Co-citation indicates that two research articles are cited to each other in publication, which reveals the degree of nexus between the cited publications (Zhenjiang, Lin et al. 2012). A set of co-citations re ects the set of research articles received, which are signi cantly and semantically correlated. Two widely used techniques were applied while collecting data from WoS: rstly, research article citation analysis (RACA), which permits the documents to identify research papers with peer citations. Each research article contains a speci c node and links acquire the relationship between nodes. Secondly, the analysis of the author correlation (AAC), which connects the links between the author's nodes and institutions.
The RACA and AAC agree to cluster the mapping into periodic themes or theoretical hotspots. Generally, the cluster depends on the research work composed and connected with another cluster. Clusters are also known as the label based main keywords. CiteSpace regulates the grading of the key terms by different algorithms. The maximum algorithm level is denoted with long-likelihood ratio (LLR) as it takes the exclusivity of the cluster (Chen, Ibekwe-Sanjuan et al. 2014). Apart from LLR, this study also considers the Latent semantic index (LSI) because it seizes the frequency of the cluster term. The visualized results of both RACA and AAC are denoted with a timeline view and network mapping view. In the former, it shows the structure of nodes, location, and speci cation with time lag, while the timeline layout distributes the nodes and clusters with linkages to each other in a particular axis. The timeline layout gives a clear understanding of co-citation association links between each other with different time durations. In RACA, however, when the cluster is generated then the nodes are merged with each cluster, which allows one to observe the research gap with a time trend.
When analyzing the RACA and AAC, two important parametric points were xed at the g-index for the node criteria section and over the selected time period. The G-index was selected because it de nes the most cited article and quanti es signi cant productivity. The setting of the g-index was manually xed at 25 and the time duration was selected for 16 years. Setting the g-index and time duration helped CiteSpace to read the literature co-citation links for 16 years and show the most highly cited co-reference work to aid researchers in comprehensively understanding the data.

Temporal structure metrics
In the case of burstness, the temporal matric is the only singular matric which signi es the citation count of literature in the short-term; citation counts help the researcher to take into consideration the research interest around the globe (Wang, Cruse et al. 2012). CiteSpace software reports the burst strength from the starting period and duration. Similarly to the total citation count, this matric is vulnerable to selfcitation.
In the network mapping, betweenness certainty shows the measure of certainty of the structure matric of the extent to which a node is connected with two or more nodes (Freeman 1977, Brandes 2001. Nodes with high certainty are connected with a big node as well as a number of nodes together, meaning that it has a highly signi cant in uence on the green environments and public health research eld (Chen, Ibekwe-Sanjuan et al. 2014). Citation count certainty, on the other hand, denotes the relevance of node structure in the mapping network.

Results And Discussion
3.1 References network Table 2 shows the top ten reference citations, including the number of citations, author details, paper published in journal, and DOI for easy access to the paper, and for the top 10 cited references a mapping network is given in Figure 1. Among the top ten citation count references is a paper entitled nature and health which was published in the Annual Review of Public Health journal published by Terry & Hartig (2014), with a citation count of 170, the highest found in the data. The author investigated that due to increasing urbanization, resource degradation and social life changes had a signi cant effect on nature as well as human health. Furthermore, this study takes nature as the physical environment owing to the consideration of increasing urbanization and population demand for natural resources in order to keep a sustainable green environment to aid public health. The 2nd position was taken by Wolch et al (2014), with a citation count of 92. Wolch investigated environmental justice, public health, and urban green space and aimed to make cities green enough. This study followed the literature of Anglo-American green urban space, especially considering public parks, and compared the USA with large Chinese cities. In the USA, many cities have adopted a policy to increase the supply of green urban environments to make neighborhoods healthy, as well as implementing policies to reform land into green urban environments. Similar policies were followed by China in many big cities, while the Chinese land is state owned; regardless, the Chinese government took the initiative to make urban areas green and reduce environmental pollution to aid sustainable public health. The third is Mireia et al (Mireia, Margarita et al. 2015), with a citation count of 88. This study conducted a systematic review of 28 studies and investigated green and blue environments and their long-term mental health exploitation. The conclusion of this study reveals that there is limited evidence that green spaces positively and signi cantly improve the mental health of adults, but in the case of children the literature has many studies whose results found highly positive and signi cant improvements in their mental health due to green environments. Further, this study suggests that green urban environmental planning is compulsory to produce clever children for the future.
With a citation count of 79, Lee & Maheswaran (Lee and Maheswaran 2011) ranked fourth among the top ten citation counts. Lee & Maheswaran performed a literature review on the health bene ts of green urban environments, and their conclusion is that there is not enough strong correlation between urban space, mental health, and wellbeing. They also found that natural environmental resources such as green environments positively in uence physical activity. Although many studies have concluded that green environments have a positive in uence on public health, this remains hard to overcome with strong correlation. The fth study, with 58 citation counts, was conducted by Markeyach et al (Markevych, Schoierer et al. 2017); this study explored the green environment pathway linkages, and concluded that although there is enough evidence that the green environment effect is correlated with public health, the pathway of green environment remains, including functions such as how green environments affect public health, age groups, and what health outcomes result from a green environment. This study gives a clear missing link of research needing to be done.  Furthermore, from the co-citation mapping the top 20 burst references were detected with strong burstness, as well as a sudden increase in citation counts; this indicates the peer review interest in the research eld of green environments and public health. Figure 2 shows the results of the top 20 strong bursts references.

Journals research mapping network
The journal visualization mapping network is another feature to visualize the green environments and public health eld. For this analysis, the based on the node the setting of CiteSpace was xed to cite journals, for the journal visualization network mapping the 5916 documents record were analyzed. The merged network nodes resulted by CiteSpace were 1078 and the links density form one node to another node found 9767, then top 10 highly cited journal were visualized. The results of Table 3 and Figure 3 represent the visualization network of highly cited journals and their frequency. The highly cited journals are sorted with large nodes and according to place the auto node size was xed. Among the top 10 journals, the top ranked journal was Environmental Science and Technology (ES&T), with citation counts of 2072 for the publication of green environments and public health. ES&T is a multidisciplinary journal who published the paper environmental related audience work, as well as providing authors, researchers, and policy makers with information upon which to make their decisions. ES&T has been a foundational focus for thought-leading, policy-changing contributions and will continue to serve as the home for signi cant, broadly relevant, and generalizable research that serves to inform decision-making.
The second ranked journal is the Science of the Total Environment (STOTEN), with a citation count of 1738. STOTEN is also an international multidisciplinary journal publishing research related to hypothesis driven, novel work and has a high impact on the environment, particularly in regards to the lithosphere, astrosphere, hydrosphere, biosphere, and atmosphere elds. With a citation count of 1717, the Chemosphere journal ranked third among the top 10 journals. Chemosphere was originally designed for communication related papers and literature reviews. It also publishes high impact research related to the environment and engineering. The fourth journal is Environmental Pollution, with a citation count of 1517. Environmental Pollution publishes high quality research and review papers in the discipline of environmental pollution and employs a peer review publication process. The fth journal recognized is Water Research, with a citation count of 1320. Water Research publishes papers related to science and technology, including anthropogenic, water quality, and water recycling research. The Journal of Hazardous Material (JHAZMAT), with a citation count of 1157, ranked sixth. JHAZMAT publishes papers related to public health impacted by environmental effects, environmental science, environmental engineering, risk mitigation, and hazardous material. The seventh journal is Science, a multidisciplinary and highly signi cant research publication journal related to the environment and other disciplines. The Environmental International journal covers health impact assessments, environmental epidemiology, and environmental health risk assessments. The Environmental International journal is ranked eighth, with a citation count of 987. The ninth journal is PLOS One, with a citation count of 894. PLOS One is also a multidisciplinary journal which publishes high impact research related to medicine, social sciences, all science related research, and engineering. Environmental Toxicology and Chemistry is ranked tenth and publishes papers related to theoretical work, original experiments, and signi cantly advanced research related to environmental toxicology.

Country and institutes visualization mapping network
Country visualization is also an important feature to aid in understanding which countries are actively carrying out research related to green environments and public health. The Citespace selection of node type was xed to country and the top 10 countries were selected from the merged nodes 568, as shown in In the case of country mapping visualization, the USA, Republic of China, England, India, Spain, Australia, Germany, Canada, and Italy were selected as the top countries doing research on green environments and public health. China ranks rst, followed by the USA. The USA has a history of being a developed country, therefore in earlier years focused on green environments and public health. China is still running towards the pathway of development, but is rapidly developing. This speed of development, however, brings about environmental pollution in China. Therefore, latterly China also considers green environments and public health for the sustainable development of the nation. By the literature's visualization, England and India are also quite concerned with green environments and public health, England is also a developed country but India belongs to the list of developing countries. Due to high population pressure and industry, India is also actively researching green environments and public health. Australia, Canada, Germany, and Spain are also developed countries with less population; therefore, their contribution in green environment research has been less compared to the USA, China, and India.
The visualization networking map presented in Figure 5 is for the most in uential institutes focusing on green environmental and public health. The most active institute is from China, the Chinese Academy of Sciences (CAS). This is the national academy for natural sciences and natural resources, and the headquarters of CAS is located in Beijing, the capital city of China. Since 2016, the CAS ranked as the number one academy around the world in the nature index. It is also the world's largest academy, where more than 60,000 researchers are working in different research directions. The University of Exeter, located in Devon, United Kingdom, is also an active institute and ranked second.

Visualization of keywords and categories
The research focus was investigated by examining the keywords to nd the pathway of research in a certain period of time, which indicates the discipline of large scale publications, the attentiveness of research concepts, and appearance of a great number of researchers who are doing research in parallel (Leung, Sun et al. 2017). From previous literature, one scholar argues that scienti c development is an alteration of traditional science and modern science (Kuhn and Hacking 2012). This means that scienti c research is changing over time and income insurability among old and contemporary approaches, because vocabulary changes accordingly. Therefore, we can observe that the existence of a revolution occurred by the changes in vocabulary. The statistics of the gure of keywords that occurred in the previous literature imitates the signi cance of keywords in this period to a speci c research area (Wang, Guo et al. 2019). Thus, keyword co-citations can show a missing link of research in a certain speci c research area. The co-word approach rst proposed by Callon et al. (1983) is widely used in the research eld of information science. The concept of co-citation ideas in bibliometrics is driven by the co-word examination. Therefore, it is scienti cally proven that in two scienti c terms an "inscription of keywords" exists in a published research work in the same time, indicating that there is a signi cant nexus between these two papers with similar keywords, if the words appear more times, each time brings them closer together (Zhang and Xu 2008). The bibliometric analysis is the sum of all co-citations, co-words, and coterm analysis.
For the co-word or keywords, an occurrence analysis was retrieved from the merged nodes of 709 associated links, with 6479 for green environments and public health. In Figure 7 are the top co-occurring keywords. From Figure 5, we can see that there are different groups which connect to each other and describe how environment affects health. Group one describes the environmental pollution, drinking water, contamination of heavy metal, toxicity, and group two describes the effect on physical health, mental condition, mortality, etc. Although the keywords are different from each other, the main objective of each study presented de ned how green environments affects public health, physiology, and mental stress.
The most in uential node is presented with the thickness, and the colour changes over time.
For a clear understanding, there is a cloud of each keyword occurrence, as well as identi cation of a missing research pathway. This study analyzes the frequency of keywords by category because it is more e cient to understand and nd the missing path nder research for green environments and public health. Figure 8 is summarized in four groups: (1) environmental science and ecology, (2) public, environmental and occupational health, (3) marine and freshwater biology, and (4) biotechnology and applied microbiology. These four groups appeared twice with high thickness and colour strati ed nodes.

Environmental science and ecology
Here, environmental sciences re ect all environmental aspects of research which directly and indirectly connect to the health keywords such as water contamination, environmental pollution, carbon emission, and air quality. When environmental pollution increases, it directly affects human physical activities and mental health (Barbara 2014). From Figures 7 and 8, it is clear that not only is there a large proportion of keywords going to the environment but also going from various categories with a high frequency cloud. It is also worth noting that big data and visualization analysis identi es the correlation between green environments and public health. It is con rmed that ecosystem and environment pathways of research trends are connected such as in ecosystem cultural activities, which can also provide signi cant aesthetic environmental and recreational opportunities to improve physical activities. These, in turn, are directly connected with physiological and sociological health conditions (Zhang, Yu et al. 2020 Considering the dearth of literature, it has already considered physical and mental health in relation to urban green spaces and concluded that the physical social contact with green environments can overcome many physical bene ts (

Marine and freshwater biology
The cloud of marine and freshwater biology is an interesting and important research area related to microorganisms available in the water, whether surface water or underground water which can be directly contaminated due to climate changes and environmental pollution; this directly affects public health (Kamble 2014). Green environment, freshwater, and public health related research need to be done in the near future because it also has a frequency of nodes both in keywords and category. The ocean has a direct effect on public health (Fleming, Broad et al. 2006). Due to climate change and environmental pollution, microbes and contaminated seafood are consumed by humans and so directly affect human health. Therefore, it is urgent to consider green environments to reduce pollution for marine species, as well as maintain seafood safety to aid the increasing population. Furthermore, marine and freshwater organisms can cause death, injury, or long-term health problems, so is worthy of consideration in future research.
Biotechnology and applied microbiology 3. In the case of country and instate visualization, there are many European countries such as Germany, Australia, Canada, and Spain, from Asia China and India are considered to be the active countries regards doing research related to green environment, green urban space, and public health. But the USA and China are the top two countries with the high frequency of node and thickness of node.
From the colours of node, it is clear that the USA has earlier considered the importance of green environments and public health and it has a long history of development, but China has a very short history of considering the importance of green environments and public health, and is a developing countries but the speed of its development is very fast in global terms. The Chinese Academy of Sciences is the top institute doing research related to green environments and public health and is located in Beijing, China.
4. Considering the co-occurring keywords and category of research are merged in are in four clouds, environmental science and ecology, public, environmental & occupational health, marine and freshwater biology, and biotechnology & applied microbiology, selection go clouds were done basis on the density frequency of nodes. These clouds are most important clouds for green environments and public because it covers the research related to social science issues, science and biological issues.
Although this study is overcome with clear pictorial gures, it is also overcome with the missing pathway of research needing to be done in the future for a sustainable green environment and to keep the public healthy. Therefore, this study recommends that researchers need to pay attention regarding how the genome changes due to climate change, as well as environmental pollution and its effect on human health, including mental health. In addition, there is also a missing link to connect green environments, freshwater, and public health. In future research of green environments and public health, each topic cluster needs to be visualized and more analysis needs to be done such as overlay map, earth networking map, and concept tree.

Declarations
Ethical Approval and Consent to Participate Not applicable.

Consent for Publication
Not applicable Competing Interest Figure 1 Visualization mapping network citation count references Figure 2 Visualization of citation count reference burstness Visualization mapping network of categories