A bibliometric and visualized analysis of research on air pollution and cardiovascular diseases

A large number of studies have shown that air pollution has a great impact on cardiovascular diseases (CVD). However, there are few bibliometric studies or visual analyses in this field. The objective of this study was to research trends and hotspots of air pollution and CVD. We used CiteSpace and VOSviewer software to retrieve relevant studies from the Web of Science Core Collection (WoSCC) over the past decade. Amount to 4284 documents on air pollution and CVD were included in this study. The past decade saw an upward trend in the number of studies. The analysis of national publications showed that the United States had the highest academic contribution in this field. Peking University, the University of Washington and Harvard University were the main institutions studying the effect of air pollution on CVD. The cooperation among institutions with high publications was very close. Cluster analysis of the keywords listed four categories as follow: (1) oxidative stress and the cardiovascular effects of air pollution; (2) the cardiovascular effects of pollution exposure sources; (3) the relationship between environmental stressors and CVD; (4) personal-level interventions. This study puts forward a comprehensive summary of the trends and development of air pollution and CVD, confirms the research frontier and hotspot direction and could give a meaningful reference for researchers in this field.


Introduction
With rapid industrialization and urbanization, air pollution is becoming increasingly serious.Air pollution is the contamination of the natural air environment by harmful chemicals.[1] According to the report by the World Health Organization (WHO), almost everyone in the world(99%) breathes air with unhealthy levels of fine particulate matter and nitrogen dioxide, especially in low-income and middle-income countries [2].Exposure to ambient air pollution can increase mortality and morbidity, shortening life expectancy while contributing to the global burden of disease [3].Epidemiological investigations and experimental studies at home and abroad have clearly confirmed that air pollution can cause CVD, even leading to death in severe cases [3].Oxidative stress [4], endothelial dysfunction [5], and systemic inflammation [6] have been proven to be important factors involved in the pathogenesis of diseases.Although much evidence has shown the impact of air pollution on cardiovascular health, the research trends and hotspots in this field are unclear.
Bibliometric methods have been a scientific specialty, part of research evaluation methods, and are widely used in the fields of science and application [7].Bibliometric mapping can be used to incorporate different data and show networks and relationships among authors, publications, citations, and so on.The findings of the analysis define its key terminology and concepts, provide important information about their related strengths and weaknesses, and propose potential future research directions.In recent years, although a lot of studies have proven the effect of air pollution on CVD [8], there has been no research using bibliometric methods or any bibliometric software (CiteSpace, VOSviewer, etc) to explore the hotspots and trends of these fields.Therefore, it is imperative to comprehensively sort and summarize the current research trends and hotspots of air pollution and CVD.Our study aims to analyze the trends, authors, institutions, cooperation, agencies and the keywords on air pollution and CVD from 2013 to 2022 so that researchers can have a better understanding of the current research trends and hotspots in this field.

Data sources and retrieval methods
We chose to use the WoSCC as the data source.This study used topic ('atmospheric pollution' OR 'air pollution') and ('cardiovascular disease') to search and collect research articles.Then we selected English documents that were articles and reviews.Finally, we obtained 4284 documents.After CiteSpace duplication analysis, 4284 documents remained, and we analyzed these documents.

Bibliometric analysis methods
In this study, we used both CiteSpace and VOSviewer to draw knowledge maps.Each software has its own merits.The software CiteSpace, which was developed by Chen Chaomei [9], can be used to forecast research hotspots, investigate topics trends through citation analysis, cluster analysis, co-word analysis, and other methods.Firstly, the retrieved documents were exported in plain text file format from WoSCC and then imported into the CiteSpace software.Secondly, the timespan was set to '2013-2022', the years per slice was set to 1 and the selection criteria was set to Top 50.Then, we used CiteSpace to perform cooperation network analysis of countries, keyword co-occurrence, keyword clustering, keyword emergence, and timeline maps.VOSviewer is a program that can construct and view bibliometric maps [10].Larger nodes in the map indicate more related articles; thicker lines and closer distances between nodes indicate stronger associations.In the end, we applied VOSviewer to perform the network analysis of authors, institutions, journals, and co-citation analysis.

Results
The change in the number of publications is shown in figure 1.The maximum number of publications is 648 in 2021, and the minimum number is 253 in 2014.The average number of publications is approximately 428 per year.The overall number of publications basically increases year by year.
Of the 4284 articles screened, there were 18127 authors.The highest number of publications was by Schwartz Joel, with 67 relevant articles from 2013 to 2022, which received 2679 citations, with an average citation number of approximately 40 per article.In terms of visual analysis, Schwartz Joel, Brook Robert, Kan Haidong, Burnett Richard and other core research teams were formed.The author co-occurrence analysis obtained by VOSviewer showed (figure 2) that there was strong intrateam collaboration and strong interteam collaboration.VOSviewer was used to draw a co-citation map of study references.If two or more papers are simultaneously cited by one or more subsequent papers, it is said that these two papers form a co-citation relationship.Thus, when some highly cited articles are frequently co-cited, this can reflect the great influence of these articles.We set the minimum number of total co-citations of the studies to 20, and present the map as shown in figure 3. The top 10 co-cited references in the co-citation network of references and their total number of citations are shown in table 1.
The institutions affiliated with the authors of the 4284 articles were visually analyzed and mapped using VOSviewer, as shown in figure 4. Research institutions with a minimum frequency of 5 articles were included in the figure, with a total of 601 nodes.Figure 5 shows the top 20 institutions with more than 65 articles each.8 of the 20 institutions belong to the U.S., 7 to China, 2 to Switzerland, and the remaining 3 to the UK, Canada and Australia.The largest number of articles was published by Peking University in China, with 165 articles.Figure 6 illustrates the institutions among these 20 institutions that have a relative upward trend in publications.We found that the number of publications in the fields of air pollution and CVD at Peking University, Harvard T.H. Chan School of Public Health, Fudan University, Chinese Center For Disease Control And Prevention and    Keywords can summarize the main content of an article and reflect the research focus on a field.We used CiteSpace for keyword co-occurrence analysis on the obtained publications and drew a keyword co-occurrence map, which is shown in figure 9.The larger the ring, the more frequently the keyword appears, and the more representative it is of research highlights in this field.In the figure, 'air pollution' and 'cardiovascular disease' have the highest frequency of occurrence.Apart from the relevant keywords of the topic, the top five keywords in terms of frequency are 'specific matter', 'mobility', 'association', 'exposure', and 'long term exposure'.
Each color block in the keyword clustering map represents a category and can reflect a hot topic.The Q and S values in the cluster map show that modularity =0.3451 > 0.300, and mean silhouette = 0.6277 > 0.500,     indicating a good clustering effect and high reliability.The keywords in the literature are divided into 7 clusters, among which cluster # 0 refers to the research on the mechanism of the impact of air pollution on CVD and it mainly emphasizes the cardiovascular effects of air pollution, including keywords such as 'insulin resistance', 'DNA metabolism', 'occupational exposure', 'coronary artery disease', 'endothelial dysfunction' and so on; Cluster # 1 # 4 # 6 focus on the cardiovascular effects of pollution exposure sources, including keywords such as 'fine particulate matter', 'black carbon', 'ultra fine particle', and'PM2.5';Cluster # 2 # 5 are studies on the association between environmental stressors and CVD, consisting of 'time series', 'temperature', 'traffic noise', and 'residential exposure'; Cluster # 3 is about personal-level interventions, including keywords such as 'stress', 'obesity', and 'health outcome'.(figure 10).
The keyword cluster timeline map can be used to explore the relationship between the development time span of each cluster and research progress.The node size of a keyword reflects its frequency of occurrence, and the number of nodes can reflect whether the research situation is becoming cooler or hotter.For example, the number of nodes in keyword cluster # 6 is gradually decreasing, and the research trend is gradually cooling.(figure 12).

Discussion
Over the past decade, the number of publications on air pollution and CVD has gradually increased over time, indicating that the heat of this field is gradually increasing.The analysis of national publication and institutions showed that the United States was the highest academic contribution in this field.Among the authors with more publications, Schwartz Joel and Brook Robert were both from the United States.However, among the top 20 institutions in terms of the number of publications, most of them with an upward trend in the number of publications were from China.This reflected China's increasing emphasis on the impact of air pollution on CVD and the growing number of studies being conducted in this direction.Moreover, multiple research teams have performed research in this field from different perspectives.Schwartz Joel, with more than 67 published articles, is the most published writer.Brook Robert, Kan Haidong, Burnett Richard ranked second, third, and fourth.Schwartz Joel's team mainly studied the correlation between ambient particulate air pollution and daily mortality.They found an independent association between short-term exposure to coarse particles (PM10), fine particles (PM2.5) and cardiovascular mortality in over 600 cities worldwide [21].Brook Robert's team found that metabolic characteristics are altered and plasma amino acid levels are reduced with short-term exposure to ambient air pollution, which is believed to be part of the reason for the impact of air pollutants on vascular function [22].Kan Haidong's team found that exposure to PM2.5, nitrogen dioxide, carbon monoxide, and sulfur dioxide can quickly trigger acute coronary syndrome, and the greatest risk occurs within the first hour of exposure to pollutants [23].Burnett Richard's team conducted a prospective cohort study on the relationship between PM2.5 exposure and cardiovascular mortality in China, as well as a cohort study on the relationship  between ambient air pollution [24,25].Those research teams focus on the exposure to pollutants and the risk of CVD, reflecting that this field is worthy of further research.
Among the top 10 co-cited references in the co-citation network of references, we found that the number of co-cited references and their total number of citations were not positively correlated.For example, the article by Pope et al didn't have the highest co-citation number, but it had the highest total citation number.Articles with a high total number of citations were likely to have a higher impact, while articles with a high number of cocitations indicated that more research had been conducted in areas related to them and that there was a higher level of heat.Among these ten articles, research on particulate matter also accounted for the majority.These studies included an exploration of the causal relationship between PM2.5 exposure and cardiovascular morbidity and mortality [11], the use of Cox proportional risk regression models to estimate the association between PM mortality and specific causes of death [14], an assessment of the risk of cardiovascular and respiratory disease among Medicare enrollees admitted to hospitals for short-term PM2.5 exposure [17] and an assessment of the association between long-term exposure to fine particulate air pollution and all-cause, lung cancer and cardiopulmonary mortality [18].On the other hand, there are many studies on the effects of air pollution on cardiovascular risk and mortality, which include studies of spatial and temporal trends in mortality and disease burden due to air pollution [13] and the estimation of risk ratios for first cardiovascular events in postmenopausal women without CVD [15].All of these observations indicated the current concern about the impact of air pollution on cardiovascular morbidity and mortality.
Analysis of research institutions showed that Peking University, the University of Washington and Harvard University were major leaders among institutions studying the effect of air pollution on CVD.Moreover, the cooperation among institutions with high publications was very close, showing the active collaboration between institutions in the same region, and also between different regions and countries.In a word, Scholarly communications are helpful to jointly promote the research progress, and make the research more diverse.Notably, the number of published articles on air pollution and CVD are mainly focused on the environmental field.Additionally, most of the authors with substantial publications are from the School of Environmental Health or the School of Public Health, very few are clinicians, which presume that clinicians should be more attentive to the adverse effects of air pollution on CVD.
Cluster analysis of the keywords revealed the following main categories: (1) oxidative stress and the cardiovascular effects of air pollution; (2) the cardiovascular effects of pollution exposure sources; (3) the relationship between environmental stressors and CVD; (4) personal-level interventions; All these categories reflect the research hotspot and trends in the near future.
According to the stable publication numbers during these years, oxidative stress has always been a hotspot.Numerous epidemiological and experimental studies have demonstrated that air pollution has a promoting effect on a systemic vascular oxidative stress reaction, which emerges as a key pathway in all of these detrimental cardiovascular actions.From the research trends, early studies showed that oxidative stress biomarkers, such as Malondialdehyde (MDA), 8-hydroxy-2′-deoxyguanosine (8-OHdG), and Superoxide Dismutase (SOD) can be detected in blood or urine [26][27][28][29].Different air pollutant exposure condition or different occupation were also associated with a significantly increased level of MDA,8-OHdG, and SOD, indicating that increased oxidative stress appears to be a valuable translational biomarker linking air pollutant exposures to cardiovascular outcomes [26,30].After analyzing the publication highlights, the research hotspots for pollutant types were ultra-fine particles and coarse particles in 2013-2014, and over time, the hotspots gradually shifted to PM2.5 in 2018-2020.So, studies have concentrated on the exposure to PM2.5, which leads to increased oxidative stress, then might be more susceptible to adverse cardiovascular effects [26,31].Moreover, many studies have also explored other pathophysiological mechanisms through the disturbance of the pulmonary autonomic nervous system and the direct permeation of environmental toxicants into the tissue [32].
Particulate matter (PM) is categorized according to size into ultra-fine particles, PM2.5, and PM10.Nowadays, numerous studies focused on PM2.5 and ultra-fine particles.Since smaller sizes of PM can penetrate more deeply into the tissue and higher particle toxicity [33].Another reason is that epidemiologic studies have identified a link between PM2.5 exposure and an increase in adverse effects of CVD [34,35].This means CVD influences of environmental exposures on cardiovascular health are being given increasing attention.However, the heat research about the influence of different pollution exposure sources evolves constantly, from initially focusing on outdoor(ambient) pollution to indoor(household) air pollution.Current epidemiologic evidence suggests an association between higher short-term and long-term ambient air pollution exposure and all-cause CVD mortality and morbidity [36].Interestingly, 'Cooking' has become a prominent word in the past two years, systematic review and meta-analysis have demonstrated that household cooking was positively associated with CVD, especially in low-income and middle-income countries [37].Therefore, it is able to reasonably presume that researchers are placing more emphasis on the discovery and control of environmental risk factors in cardiovascular risk.
Numerous epidemiological and clinical research provides compelling evidence that exposure to fine particulate matter air pollution contributes to the development of CVD, as well as cardiovascular mortality risk [38].The dysfunction of the cardiovascular system due to PM leads to an increased prevalence of CVD, such as arrhythmia [34], heart failure [39], hypertension [40], and atherosclerosis [41].Meanwhile, Weather exposure in terms of changing temperature, relative humidity, rainfall, and other weather patterns exhibits a range of direct and indirect effects on human health.Air pollution and weather exposure exceeding certain thresholds can have negative impacts on human health.Many studies have shown that the effect of cold temperature, hot temperature, and air temperature were a risk factor for cardiovascular disorders [42].In addition, more recent evidence indicates that air pollution and noise contribute dramatically to the global burden of CVD [43].Since the association of air pollution and environmental stressors, such as weather patterns and noise, is often missed in the literature, weather exposures and noise should be given more attention by future research institutes.
Cluster analysis showed that physical activity was the research hotspot.There is growing evidence indicating an interaction between sports activities and air pollution, which reductions in PM2.5 levels can result in demonstrable benefits to CVD [44,45].A growing collection of studies have reported that several personal-level approaches, such as portable air cleaners (PACs) and/or face masks, may help to mitigate CVD events [46].Along with the significant personal health benefits, the evaluation of a potential individual-level intervention to reduce PM2.5 exposure and improve subclinical and clinical cardiovascular outcome is a noteworthy trend and deserve further research.

Limitations
We have analyzed all the relevant papers from WoSCC on air pollution and CVD and conducted bibliometric and visual analysis of trends and hot spots of the research.However, several limitations remain.First and foremost, we only analyzed English language literatures, and non-English literatures were not included.In addition, all documents were only derived from one database, WoSCC.

Conclusion
Air pollution is a major risk factor for morbidity and mortality globally, of which cardiovascular events are the single largest contributor.contributor.The impact of air pollution and CVD is gaining increasing attention from clinicians worldwide.Therefore, it is necessary to have a deep understanding of the current status of research and search after the research directions and hotspots.In conclusion, our bibliometric analysis first provides information on research trends and hotspots.This may help researchers conduct more in-depth research on the fields of air pollution and CVD.

Figure 1 .
Figure 1.Publication trend on air pollution and CVD.

Figure 2 .
Figure 2. The cooperation network of authors.

Figure 3 .
Figure 3.The co-citation network of references.

Figure 4 .
Figure 4.The cooperation network of institutions.

Figure 6 .
Figure 6.Institutions with an upward trend in the number of publications among the top 20 most productive institutions.(a).Peking University, (b).Harvard T.H.Chan School of Public Health, (c).Fudan University, (d).Sun Yat-sen University, (e).Chinese Center For Disease Control And Prevention, f.Monash University).

Figure 7 .
Figure 7.The cooperation network of countries.

Figure 8 .
Figure 8.The network visualization map of analysis for journals.

Table 1 .
[11]10 co-cited references in the co-citation network of references.Brook et alParticulate Matter Air Pollution and Cardiovascular Disease An Update to the Scientific Statement From the American Heart Association[11] Pope et al Lung cancer, cardiopulmonary mortality, and long-term exposure to fine particulate air pollution [18] 316 5915 Judith et al Health Effects of Fine Particulate Air Pollution: Lines that Connect [19] 303 3641 Shah et al Global association of air pollution and heart failure: a systematic review and meta-analysis [20] 269 776