Air pollution and chronic obstructive pulmonary disease-related health outcomes: a bibliometric and visualization analysis

Introduction

Chronic obstructive pulmonary disease (COPD) is a chronic lung disease characterized by persistent and progressive airflow obstruction (1). Epidemiological studies have indicated that the incidence of COPD is increasing globally, affecting approximately 300 million people worldwide. COPD is the third leading cause of death and the fifth leading cause of disability globally (2). The prevalence of COPD in people aged 40 years and above in China reaches as high as 13.6% (3,4). Therefore, it is urgent to explore the risk factors for COPD in depth and develop effective prevention and control measures.

Air pollution constitutes the most important environmental factor contributing to disease, disability, and premature death worldwide (5). Studies have indicated that in 2019, air pollution caused an estimated 6.7 million deaths in low- and middle-income countries (LMICs) (6). GOLD 2023 also indicated that COPD is a result of the interaction between environmental and genetic factors (1). Studies have revealed that short-term exposure to fine particulate matter (PM2.5) significantly increases the burden of acute exacerbation of COPD (AECOPD) (7,8). Similarly, ambient inhalable particulate matter (PM10) is associated with hospitalization and mortality in patients with COPD (9). The effect of air pollution on respiratory health has been studied extensively (10-13). Therefore, with the advancement of theoretical knowledge and the development of new knowledge frontiers, analysing the research status and identifying research frontiers in the field of the effect of air pollution on COPD-related health outcomes are important.

Bibliometrics is a widely employed method for evaluating the quality, research status, and development trends in academic research in various fields (14-16). It aims to quantitatively analyse published literature through mathematical and statistical methods and to more intuitively reveal the knowledge structure, research status, and development trends in a given field. Compared with conventional review studies, comprehensive analyses of literature on the basis of bibliometrics can more accurately reveal certain patterns and trends in scientific and technological research, thereby providing a basis for scientific decision-making and management (17,18).

However, to date, no bibliometric study on the effect of air pollution on COPD-related health outcomes has aimed to explore the research status, hotspots, and development trends in this field. Therefore, in this study, CiteSpace, VOSviewer, R language, and the bibliometric package of RStudio software were employed to conduct bibliometric and visualization analyses in this field. This research aimed to reveal the existing research hotspots and potential future research directions in the field of the effect of air pollution on COPD-related health outcomes both domestically and internationally, identify potential research gaps, and provide a theoretical reference for researchers and research institutions in this field. We present this article in accordance with the BIBLIO reporting checklist (available at https://jtd.amegroups.com/article/view/10.21037/jtd-2026-0784/rc).

Methods

Data sources and search strategy

In this article, the Web of Science Core Collection (WOSCC) was chosen as the data source. As the world’s largest comprehensive academic information resource, the WOSCC covers almost all disciplines globally, with the most comprehensive data resources from more than 8,700 core academic journals, including those in fields such as natural sciences, biomedicine, and engineering and technology. The WOSCC has been widely adopted in bibliometric research and visualization analysis in recent years (19-21). Although the reliance on a single database may introduce selection bias, the WOSCC includes the vast majority of high-impact, peer-reviewed core journals in this field, with highly standardized, updated, and complete bibliographic data. The use of this database effectively ensures the representativeness of the study sample and the reliability of the analysis results. Compared with other databases, the WOSCC offers distinct advantages in terms of the completeness and consistency of bibliometric indicators (e.g., citation relationships, collaboration networks, and keyword co-occurrence). The use of this database can largely mitigate analytical bias resulting from database heterogeneity, incomplete coverage, or inconsistent data formatting, making it particularly suitable for systematic, comprehensive analyses of research trends in a given field. A subject term search was adopted, with the search period set from the establishment of the database to May 31, 2025. The search was completed within one day to avoid deviations caused by daily database updates. The definition of health outcomes in this study refers to the published literature by Ma et al. (22). In summary, the search strategy is as follows (the detailed search strategy is summarized in Table S1): TS = (Air Pollution OR Air Pollutions, etc.) AND TS = (Pulmonary Disease, Chronic Obstructive OR Chronic Obstructive Pulmonary Diseases OR COPD, etc.) AND TS = (Death OR Mortality OR Emergency Room OR Hospital Emergency Service OR Hospital Admission, etc.). Document types were limited to articles and reviews. Exclusions included studies outside the defined scope; records with incomplete metadata (e.g., missing authors or affiliations); non-peer-reviewed materials (conference abstracts, editorials, letters, notes, news items, and news reports); and duplicate entries. In addition, only English-language publications were included. After the implementation of manual deduplication and relevance screening, the eligible records were downloaded in full record and cited references format and saved as plain text files to facilitate the subsequent bibliometric analysis. The overall framework of this bibliometric study is shown in Figure 1.

Figure 1 Research framework of bibliometric analysis.

Statistical analysis

The literature data in the field of research on the effect of air pollution on COPD-related health outcomes include the annual number of publications, annual cumulative number of publications, funding support, journals and co-cited journals, authors and co-cited authors, countries, institutions, cited literature and co-cited literature, keywords, and themes. Four bibliometric tools were applied in this study for data analysis, namely, Microsoft Excel 2021, VOSviewer 1.6.20, CiteSpace 6.2.R3, and the bibliometric package of R language and RStudio software.

VOSviewer 1.6.20 is a software tool developed by van Eck and Waltman from Leiden University in the Netherlands. It is a Java-based application specifically designed for constructing and visualizing bibliometric networks (23,24). The visual maps generated in VOSviewer 1.6.20 comprise nodes and lines, in which the nodes represent specific elements, the size of a node indicates the frequency of occurrence of a given element, and the frequency of occurrence is positively correlated with the node size. The lines between nodes represent collaborative relationships, and the thicker the line is, the stronger the collaborative relationship (25). In this study, VOSviewer 1.6.20 was employed for the clustering and visualization of co-cited journals, the extraction and clustering visualization of author and co-cited author data, the clustering visualization of country collaborations, and the clustering visualization of institutional collaborations. In bibliometric analysis, two main types of analyses were conducted in VOSviewer 1.6.20, namely, co-authorship analysis and co-citation analysis. The units of analysis included countries, institutions, authors, cited references, and cited authors. The following thresholds were applied: a minimum of 10 publications for countries; a minimum of 50 publications for institutions; a minimum of 15 publications for authors; a minimum of 600 co-citation occurrences for co-cited journals; and a minimum of 200 co-citation occurrences for co-cited authors.

CiteSpace software, developed by Professor Chaomei Chen from the College of Computing and Informatics at Drexel University, is an effective data analysis and visualization tool that can generate multidimensional, time series, and dynamic visual maps. This tool has been widely applied in the fields of bibliometrics and scientometrics in recent years (26-28). In this study, CiteSpace 6.2. R3 was employed to conduct co-cited reference clustering and to generate keyword co-occurrence networks, keyword clusters, and keyword burst analyses. In the analysis in CiteSpace 6.2.R3, the time slicing was set to 1 year per slice. Two types of node types were analysed separately, i.e., reference and keywords. The threshold of the top N nodes per slice was set to the top 50. A modularity Q value (Q) >0.4 indicates a valid clustering structure, and a mean silhouette value (S) >0.5 indicates reasonable clustering results. Labels were extracted via the Label [Label-Linked Reaction (LLR)] method. In burst detection, the top 25 keywords and the top 25 references with the strongest citation bursts were identified.

Bibliometrix, developed in R, can be used for statistical analysis of literature data and supports visualization, clustering, and collaboration analysis (29). In this study, the bibliometric package in R and RStudio was chosen to extract the publication volume of journals, plot the temporal trends in journal publications, obtain national publication outputs, visualize international collaboration networks, derive cited reference data, and determine keywords.

To verify the robustness of the analytical results, sensitivity analyses were conducted for the threshold settings. In VOSviewer 1.6.20, the thresholds for country publications, institutional publications, author publications, co-citation counts of journals, and co-citation frequencies of authors were sequentially adjusted, and collaboration network and co-citation clustering analyses were again conducted. In CiteSpace 6.2.R3, the top N nodes per slice threshold was adjusted from the top 50 to the top 100, and all burst settings were applied for both keywords and references to regenerate clustering and burst maps. All sensitivity analysis results were compared with the original findings to confirm the stability of the conclusions.

Results

Analysis of the number of publications

The number of publications is an important indicator for measuring the development of an academic field. The analysis of the number of publications included the annual number of publications and the annual cumulative number of publications. The annual number of publications and the annual cumulative number of publications in this research field are shown in Figure 2. A total of 11,167 studies were obtained from this research field. The number of relevant studies has generally increased over time. The earliest literature in this field appeared in 1964. Based on the changes in the number of published studies, the research in this field can be approximately divided into the following two stages:

• 1964–1990: at this stage, the annual number of publications was smaller than 10 (except for 12 publications in 1989), and the annual cumulative number of publications was smaller than 100, which indicates that this period represents the initial stage of research on the effect of air pollution on COPD-related health outcomes.
• 1991–2025: at this stage, the annual number of publications increased notably. The number of publications exceeded 500 from 2018 to 2024, reaching a peak value of 945 in 2021. Although there was a decrease in 2023 and 2024, the number of publications still remained at 822 and 785, respectively. The curve of the annual cumulative number of publications in this field follows a cubic polynomial function with a goodness of fit R² of 0.9984, indicating that this stage represents a medium- to high-speed growth period of research.

Figure 2 Annual number of publications and annual cumulative number of publications in the field of research on the effect of air pollution on COPD-related health outcomes. COPD, chronic obstructive pulmonary disease.

Funding support

The status of funding support can reflect the degree of attention and focus given by a country/region to research in this field. The statistical analysis results provided in Table 1 reveal that 3,887 research papers in the field of the effect of air pollution on COPD-related health outcomes have received funding support, accounting for 34.81% of the total number of publications. Among them, 1,901 papers were funded by various institutions in the USA, accounting for 48.91% of the total, followed by 1,281 papers funded by various institutions in the UK, accounting for 32.96% of the total. The National Natural Science Foundation of China has provided notable support for research in this field.

Journals and co-cited journals

A total of 11,167 studies were published in 1,903 journals. The top 10 journals by number of publications in the field of the effect of air pollution on COPD-related health outcomes are shown in Figure 3A. The top 10 journals are listed in Table 2. Summarizing the journals with the highest publication volumes can directly reflect the main journal sources of relevant research literature in this field.

Figure 3 Journals and co-cited journals in the field of the impact of air pollution on COPD health outcomes. (A) Top 10 journals by number of publications; (B) trend in the number of publications in the top 5 journals over time; (C) co-citation journal cooperation network diagram; (D) dual-map overlay of journals in the field of the effect of air pollution on COPD-related health outcomes. COPD, chronic obstructive pulmonary disease.

The top 10 journals published a total of 2,327 publications (accounting for 20.84% of the total), indicating that there is no highly concentrated group of core journals in this field. Among these journals, the International Journal of Chronic Obstructive Pulmonary Disease accounted for the greatest number of publications, with 480 papers (4.30%). However, its average impact factor over the past 5 years, Journal Citation Reports (JCR) category, total citation frequency, average citation frequency per paper, and H-index all rank low, suggesting that this journal generates relatively limited academic influence. Among the top 10 journals, another 8 belong to the professional field of respiratory disease research. The journal Chest ranks second in terms of the number of publications, with 362 papers (3.24%). Its average impact factor over the past 5 years is 9.1, and this journal is classified in JCR category Q1. Moreover, both its total citation frequency and H-index rank first. Although the number of publications in the European Respiratory Journal, American Journal of Respiratory and Critical Care Medicine, and Thorax was relatively small, their average impact factor over the past 5 years, average citation frequency per paper, and H-index were the highest. These findings indicate that these journals generate the greatest comprehensive influence in this field. In addition, the other two journals, namely, PLoS One (208 papers, 1.86% of the total) and the Cochrane Database of Systematic Reviews (152 papers, 1.36%), belong to the fields of general journals and evidence-based medicine, respectively. The main journals publishing papers in this field are concentrated in specialized areas that focus on respiratory diseases, and a large number of high-quality studies have relied on meta-analysis and systematic review methods. The trend in the number of publications in the top 5 journals over time is shown in Figure 3B. The earliest studies in this field were published in journals with high comprehensive influence, such as Chest, European Respiratory Journal, and Respiratory Medicine. It was not until the 2010s that large-scale studies in this field were published in International Journal of Chronic Obstructive Pulmonary Disease and PLoS One, and the number of publications in these two journals exceeded that in journals with high comprehensive influence over time. Furthermore, the sponsors of all the top 10 journals originate from developed countries, with five from the UK, four from the USA, and one from New Zealand.

Co-citations were first proposed by the information scientist Henry Small (30). The top 10 co-cited journals in the field of the effect of air pollution on COPD-related health outcomes are listed in Table S2. American Journal of Respiratory and Critical Care Medicine (24,568 citations, 6.75% of the total), European Respiratory Journal (20,355 citations, 5.59%), Chest (19,299 citations, 5.30%), Thorax (13,981 citations, 3.84%), and The New England Journal of Medicine (10,355 citations, 2.85%) are the top 5 co-cited journals. All the top 10 co-cited journals exhibit total co-citation counts exceeding 4,000. Of these co-cited journals, 50% are from the USA, 40% are from the UK, and 70% exhibit an impact factor greater than 10. A network diagram of the collaboration of co-cited journals in this field is shown in Figure 3C. The largest node represents the American Journal of Respiratory and Critical Care Medicine, which suggests that this journal exhibits the highest frequency of being cited by other journals. Among the 33,984 co-cited journals, 95 attain a total co-citation count no smaller than 600. As shown in Figure 3C, these 95 co-cited journals form 5 clusters. The red cluster comprises 27 journals, including The New England Journal of Medicine, Lancet, JAMA-Journal of the American Medical Association, Circulation, and BMJ. The green cluster comprises 24 journals, such as the European Respiratory Journal, Chest, Thorax, Respiratory Medicine, and International Journal of Chronic Obstructive Pulmonary Disease. The blue cluster comprises 24 journals, including Environmental Health Perspectives, PLoS One, American Journal of Epidemiology, Environmental Research, and Science of the Total Environment. The yellow cluster comprises 16 journals, such as American Journal of Respiratory and Critical Care Medicine, Journal of Allergy and Clinical Immunology, American Journal of Respiratory Cell and Molecular Biology, Journal of Applied Physiology, and American Journal of Physiology. The purple cluster comprises 4 journals, namely, Clinical Infectious Diseases, Critical Care, Critical Care Medicine, and Intensive Care Medicine.

The dual-map overlay function involves superimposing one CiteSpace map onto another, where the former is referred to as the overlay map and the latter as the base map. Essentially, a dual-map overlay reflects the connections between the citing journals and the cited journals. Through the use of an overlay map, the knowledge flow between disciplines at the journal level can be visualized. The dual-map overlay of journals is shown in Figure 3D. The left side of the figure shows the distribution clusters of major journals in the research field of the effect of air pollution on COPD-related health outcomes, and the right side shows the major clusters of cited journals. The labels represent the themes covered by the journals. Notably, research in this field is concentrated mainly in two major categories of journal clusters, namely, molecular, biological, and immunological journals and medical, health care, and clinical journals. The cited journals can be grouped primarily into two categories, i.e., molecular, biological, and genetic journals and health, nursing, and medical journals. This grouping is consistent with the research findings for the distributions of journals and co-cited journals. Within the citing domain on the left side of the figure, there are two outward citation paths in the medical, health care, and clinical journal cluster, rendering this cluster the most prominent citing cluster. Moreover, when the medical, health care, and clinical cluster serves as the citing journal group and the corresponding health, nursing, and medical cluster serves as the cited journal group, the highest Z value is reached, at 10.151.

Authors and co-cited authors

A total of 51,192 authors were included in bibliometric analysis of this field. The top 10 authors by number of publications are listed in Table 3. Celli BR (Harvard University, USA), Vestbo J (University of Manchester, UK), Criner GJ (Temple University, USA), and Martinez FJ (University of Michigan, USA) are the top four authors with the greatest number of publications, with more than 50 papers, namely, 67, 61, 59, and 55 papers, respectively. In terms of the total citation frequency, average citation frequency per paper, and H-index, Celli BR and Vestbo J ranked highest, indicating that their research is of high quality and widely recognized. The author collaboration network map of this field is shown in Figure 4A. In the map, each node represents one author. Moreover, the size of a node indicates the number of papers the author has published, the lines between nodes indicate collaborations, and the thicker the line is, the closer the collaboration between the corresponding authors (31). Clustering was conducted for 91 authors who had published no less than 15 papers. Among them, 87 authors formed 10 collaborative groups (four authors did not form a group). The largest collaborative group comprised 19 members; two groups contained 13 members each; there was one group with 12 members, one group with 11 members, one group with six members, one group with five members, and one group with four members; and two groups encompassed two members each. The collaboration among members within each group was relatively close, while collaboration between different groups also occurred.

Figure 4 Authors and co-cited authors in the field of the impact of air pollution on COPD health outcomes. (A) Author collaboration network map; (B) co-cited author collaboration network map in the field of the effect of air pollution on COPD-related health outcomes. COPD, chronic obstructive pulmonary disease.

Co-cited authors refer to two or more authors who form a co-citation relationship by being cited simultaneously in another one or more papers. The top 10 authors by cocitation count in this field are listed in Table S3. The top three authors by co-citation count all exhibit co-citation counts greater than 1,000, which are Celli BR (n=1,391), Jones PW (n=1,374), and Barnes PJ (n=1,078), indicating that the research findings of these scholars are widely recognized in this field. The co-cited author collaboration network map of this field is shown in Figure 4B. There are 105 authors with a co-citation count no less than 200, and these 105 authors form four groups. The largest group comprises 31 members, and there three groups with 27, 24, and 23 members each. The different co-cited authors form clusters, which indicates that these authors are often cited together in the literature.

Country distribution and collaboration relationships

A total of 136 countries have participated in relevant research in this field. The number of publications by country worldwide in this field is shown in Figure 5A. The top 10 countries by number of publications in this field are listed in Table 4. This research field has received widespread international attention, and the number of papers published by authors from the USA, China, and the UK far exceeds that published by authors from other countries. The number of publications of the top 10 countries accounts for 86.35% of that of all 136 participating countries, with each of these countries publishing more than 300 papers. Except China, all the other nine countries are developed countries, which indicates that the research field of the effect of air pollution on COPD-related health outcomes has received extensive attention from researchers in developed countries. Currently, the USA, China, and the UK are far ahead in terms of the number of publications, with 3,313, 1,507, and 1,353 papers, respectively, accounting for 29.67%, 13.50%, and 12.12%, respectively, of the total. Spain, Australia, and Germany have also made important contributions to this field. The USA and the UK are far ahead in terms of the total citation frequency, with 201,758 and 109,294 citations, respectively, demonstrating that these two countries greatly influence research in this field. Although China exhibits a large number of publications, its influence is relatively low. New Zealand demonstrates the highest average citation frequency per paper, at 103.56, which indicates that the country generates notable influence and has conducted high-quality research in this field. In terms of international collaboration and the H-index, the USA and the UK are ranked highest.

Figure 5 Country distribution and collaboration relationships in the field of the impact of air pollution on COPD health outcomes. (A) Number of publications by country worldwide; (B) intercountry collaboration network; (C) collaboration network map created based on 66 countries with no fewer than 10 published papers; (D) temporal variation trend of the number of publications by the top 10 countries in the research field of the effect of air pollution on COPD-related health outcomes. COPD, chronic obstructive pulmonary disease.

The intercountry collaboration network in this research field is shown in Figure 5B. The collaboration network map established based on 66 countries with no less than 10 published papers is shown in Figure 5C. The 66 countries form 4 clusters: the red cluster is mainly composed of 26 countries, with Italy, Spain, Germany, the Netherlands, and France as the core; the green cluster is mainly composed of 22 countries, including Canada, Türkiye, India, Brazil, and Iran as the core; the blue cluster is mainly composed of nine countries, with the USA, Australia, and China as the core. The yellow cluster is mainly composed of nine countries, including England, New Zealand, and others. Notably, countries that lead in terms of the number of publications have established close collaborative relationships, which is consistent with the results provided in Table 4.

An integrated analysis of publication years, number of publications, and countries can reveal the overall research level and development status of a country in this field across different years. The temporal variation trend of the number of publications for the top 10 countries in the research field is shown in Figure 5D. Notably, during the research period, the number of publications of the top 10 countries increased overall. The USA always occupied a dominant position in terms of the number of publications in this field; the number of publications of China remained low before 2015 but rapid increased after 2015. In 2018, China surpassed the UK in terms of the number of publications and has maintained a relatively high increase rate since then. This phenomenon may be associated with the effective air pollution control policies implemented by the Chinese government in approximately 2015: in September 2013, the Chinese government issued the Air Pollution Prevention and Control Action Plan (also referred to as the Ten Measures). In July 2018, the Chinese government issued the Three-Year Action Plan to Win the Battle for a Blue Sky (also referred to as the Blue Sky Defense Battle), and in December 2023, the Chinese government issued the Action Plan for the Continuous Improvement in Air Quality (also referred to as the Action Plan). The intensive rollout of these policies may explain the increased research attention in this field.

Institution distribution and collaboration relationships

An analysis of the number of publications by research institutions can reveal the research strength and influence of institutions in this field. A total of 11,302 institutions have participated in relevant research in this field. The top 10 research institutions by number of publications in this field are shown in Figure 6A, and the top 10 institutions by number of publications are summarized in Table 5. The number of publications of the top 10 research institutions accounts for 30.34% of that of all 11,302 participating research institutions, with each of these institutions publishing more than 100 papers. These 10 institutions are all from developed countries, among which four are from the USA. Notably, although the number of publications of China in this field reaches as high as 1,507 (ranking 2nd globally), none of the top 10 research institutions are from China. This finding indicates that the research strength of Chinese institutions in this field is relatively scattered. Harvard University (USA) occupies a dominant position in this field, with 1,119 publications (accounting for 10.02%), as well as the highest total citation frequency, average citation frequency per paper, and H-index. The University of California System (USA) ranks 2nd with 361 publications, followed closely by Imperial College London (UK) and the University of London (UK), with 327 and 320 publications, respectively. Although The University of British Columbia (Canada) does not occur in the top five in terms of the number of publications, its average citation frequency per paper reaches 160.04, which is second only to Harvard University.

Figure 6 Institution distribution and collaboration relationships in the field of the impact of air pollution on COPD health outcomes. (A) Top 10 research institutions by number of publications; (B) interinstitution collaboration network in the research field of the effect of air pollution on COPD-related health outcomes. COPD, chronic obstructive pulmonary disease.

The interinstitution collaboration network in this research field is shown in Figure 6B. A total of 70 institutions with no fewer than 50 published papers form six distinct collaborative groups: Group 1 (red cluster) comprises 21 institutions, including Johns Hopkins University, University of Washington, University of California, San Francisco, University of Michigan, and University of California, Los Angeles; Group 2 (green cluster) comprises 19 institutions, including University of Copenhagen, University of Groningen, University of Barcelona, and Imperial College London; Group 3 (dark blue cluster) comprises 17 institutions, including Peking University, Fudan University, The University of Sydney, Monash University (Australia), and Guangzhou Medical University; Group 4 (yellow cluster) comprises six institutions, including University of Toronto, University of British Columbia, and McGill University; Group 5 (purple cluster) comprises five institutions, including Brigham and Women’s Hospital, Harvard University, and Boston University; and Group 6 (light blue cluster) comprises two institutions, namely, Seoul National University and Ulsan University. Notably, the members of each group collaborate closely, while collaboration between different groups also occurs. The research institutions that lead in terms of the number of publications have established close collaborative relationships, which is consistent with the results provided in Table 5.

Cited literature, co-cited literature, and citation burst

The top 10 globally cited studies among those included in this study are listed in Table S4. The article published by Murray CJ, Vos T, Lozano R, et al. in The Lancet in 2012 attracted extensive attention from researchers worldwide, with a citation count of 6,156 (32). This study revealed that air pollution (especially particulate matter from biomass and coal fuels) constitutes a key environmental risk factor for the incidence of COPD.

Other globally highly cited literatures among those included in this study focus mainly on the following aspects: exposure-response mechanisms, disease burden, and variation trends of PM2.5 and ozone with COPD occurrence (33); the mechanisms of action of key air pollutants (e.g., particulate matter and ozone) for COPD and the effect of short- and long-term pollutant exposure on COPD (34); the disease burden of air pollution for COPD in China, along with policy interventions and their potential impacts (4); the association between air pollution and skeletal muscle dysfunction in COPD patients and the synergistic effects of ageing and air pollution (35); the association between short-term exposure to PM2.5 and the risk of hospitalization for cardiovascular and respiratory diseases among elderly individuals (36); the disease burden of air pollution for COPD in the USA (37); key air pollutants and their contribution ratios, as well as the mechanisms of action and effect of pollutants (38); the synergistic effects of air pollution and factors such as tobacco exposure and occupational dust on the burden of COPD (39); and indoor biomass fuel pollution as a major global risk factor for COPD (40). Among the top 10 globally cited studies included in this study, five are from The Lancet, one is from its subjournal The Lancet Respiratory Medicine, and two are from JAMA. These findings indicate that majority of the most globally cited literature in this field is published in top-tier general medical journals.

Co-cited references refer to references that are cited together within a set of references (30). The top 10 co-cited references in the field of the effect of air pollution on COPD-related health outcomes are shown in Figure 7A and Table 6. All of the top 10 co-cited references exhibit a co-citation count greater than 200, with one of them demonstrating a co-citation count larger than 500. The clusters of co-cited references in this field are shown in Figure 7B. Notably, the cluster modularity value (Q) is 0.7667 (>0.4), which indicates that the clustering result is valid. Moreover, the average silhouette value (S) is 0.8999 (>0.5), which suggests that the clustering results are reasonable. This field encompasses 10 main clusters, with the following labels: (#0) global burden, (#1) long-acting beta-agonist, (#2) air pollution, (#3) systemic inflammation, (#4) chronic obstructive pulmonary disease, (#5) pulmonary rehabilitation program, (#6) blood eosinophil, (#7) hospital admission, (#8) long-term exposure effect, (#9) COVID-19 patient, and (#11) cardiorespiratory disease. The emergence of systemic inflammation, blood eosinophils and other topics in cluster analysis indicates that numerous studies, while addressing the effect of air pollution on COPD-related health outcomes, have also aimed to explore the underlying mechanisms.

Figure 7 Cited literature, co-cited literature, and citation burst in the field of the impact of air pollution on COPD health outcomes. (A) Top 10 co-cited references; (B) cluster visualization of co-cited references; (C) top 25 references with citation bursts in the research field of the effect of air pollution on COPD-related health outcomes. COPD, chronic obstructive pulmonary disease.

To address the citation accumulation advantage of older, highly cited papers, this study introduced the percentile in subject area metric, which is a standardized citation indicator provided by the Web of Science. Its core function is to eliminate the time accumulation bias caused by publication year. This metric represents the percentage of publications by which the target paper excels in terms of citation performance among all global publications of the same year, subject category, and document type. The results revealed that the percentile in subject area values of the top 10 globally cited studies and the top 10 co-cited references were all above 99%, indicating that these top 10 studies outperformed more than 99% of the publications in the same year, subject category, and document type worldwide. Their high citation counts are not merely the result of longer publication histories. Instead, these papers already occurred within the top tier of their respective disciplines at the time of publication and are widely recognized as landmark studies in the field. These findings validate the robustness of the core node identification results of bibliometric analysis.

References with citation bursts are defined as those that are frequently cited within a certain period. Citation bursts can reflect the developmental dynamics within a specific field (41). The top 25 references with citation bursts are shown in Figure 7C. The periods when citation bursts are identified for these references are indicated by red lines, representing the first and last years of the burst duration (42). The first citation burst in this field emerged in 2002. The reference with the highest burst intensity is an article published by Vestbo J et al. The burst of this reference began in 2013 and ended in 2018. The most recent citation bursts correspond to the articles written by Adeloye et al. (43), Safiri et al. (44), Agustí et al. (45), and Christenson et al. (2) published in prestigious journals such as BMJ, European Respiratory Journal, and The Lancet. Their bursts have not yet ended, and they may exert a notable impact in the coming years.

Main keyword distribution, cluster analysis, and keyword burst analysis

Keyword co-occurrence analysis

To identify hotspots and frontiers of published literature from 1964 to 2025, a comprehensive analysis of keyword co-occurrences is essential. Cite Space 6.2. R3 was employed to create a knowledge graph of keyword co-occurrence (Figure 8A). The time slice was set to 1 year, resulting in 374 nodes connected by 2,179 edges, with a network density of 0.0067. To better understand these keywords, the top 10 keywords with the highest occurrence frequencies and their respective frequencies are provided in Table 7. The top 10 keywords are COPD (5,826 times), mortality (2,485 times), risk (1,288 times), air pollution (1,251 times), prevalence (869 times), quality of life (805 times), disease (779 times), lung function (759 times), management (753 times), and outcome (725 times). Among these keywords, air pollution exhibited the highest centrality of 0.36. These keywords provide insights into the mainstream research themes and focus areas within this research field. The highest-frequency keywords, such as quality of life and lung function, emerged because numerous studies provided extended analyses in this field.

Figure 8 Main keyword distribution, cluster analysis in the field of the impact of air pollution on COPD health outcomes. (A) Knowledge graph of keyword co-occurrences; (B) 10 main clusters.

Keyword cluster analysis

In cluster analysis, the cluster modularity value (Q) is 0.8422 (>0.4), indicating that the clustering result is valid. The average silhouette value (S) is 0.9586 (>0.5), which suggests that the clustering results are reasonable. The 10 main clusters are shown in Figure 8B, and information on these 10 keyword clusters is provided in Table 8.

Keyword burst analysis

Keyword burst refers to a significant increase in the occurrence frequency of a given keyword over a certain period, reflecting the research content focused on by researchers in this field during that period (46). The top 25 keyword burst maps for the effect of air pollution on COPD-related health outcomes are shown in Figure 9, which reveal the start and end times of keyword bursts. As shown in Figure 9, the keyword “quality of life”—which emerged in 1995 and disappeared in 2011, exhibits the highest burst intensity. The most recent keyword bursts are “China” and “climate change”, which emerged in 2021. These two keywords are still experiencing bursts as of May 31, 2025, indicating that these two directions constitute not only current research hotspots but also future development trends.

Figure 9 Top 25 keyword burst maps for the effect of air pollution on COPD-related health outcomes. COPD, chronic obstructive pulmonary disease.

Sensitivity analysis results

After the publication thresholds for countries, institutions, and authors and the co-citation frequency thresholds were adjusted in VOSviewer, the structures of the country/institution/author collaboration networks, core cluster divisions, and the lists of the top 10 countries, institutions, journals, and authors remained stable. No significant changes were observed in the rankings of the core nodes or collaborative relationships. After the threshold was set to the top 100 in CiteSpace and full burst detection was enabled, the core findings with respect to keyword clustering, co-cited reference clustering, cluster labels, and burst keywords/references remained substantively unchanged. The research hotspots and frontier directions remained highly consistent with the original analysis results. These findings demonstrate that the bibliometric conclusions of this study are robust, with reasonable threshold settings and reliable findings.

Discussion

This study, for the first time, systematically revealed the research status, hotspots, and development trends in the field of the effect of air pollution on COPD-related health outcomes from 1964 to 2025 using bibliometric methods based on the WOSCC. Through the analysis of 11,167 publications, several key findings emerge.

The field entered a period of rapid growth after 1990, when it reached a peak of 945 publications in 2021. In terms of the temporal trend in the national publication output, the USA has consistently dominated the field. Publications are highly concentrated in the USA, the UK, and China, and nearly all core research institutions are from developed countries. These findings suggest that research resources, data accumulation, and research systems are highly concentrated in Western countries. Even though LMICs face greater pollution exposure and disease burdens, their indigenous research capacity remains severely insufficient, making it difficult to develop research evidence tailored to local air pollution health risks. This pattern produces findings biased towards Western populations, which cannot be directly generalized to high-pollution and high-burden regions in Asia and Africa, thereby reducing the global generalizability of prevention and control policies. As a developing country, China maintained a low level of publications before 2015, followed by a rapid increase after 2015. This trend is highly synchronized with the intensive introduction of air pollution control policies in China, suggesting that policy-driven research investment may have played a catalytic role. China ranks second globally, with 1,507 publications, and has experienced explosive growth over the past decade. However, its average number of citations per paper is the smallest among the top 10 countries, at only 39.59. This comparison result suggests that the notable increase in research quantity driven by short-term policy stimulus still has room for improvement in terms of the efficiency of translation into high-quality outcomes.

High-impact journals such as the American Journal of Respiratory and Critical Care Medicine, the European Respiratory Journal, The Lancet, and JAMA dominate knowledge dissemination, which confirms that population-level studies, including cohort research, disease burden analysis, and public health interventions, remain mainstream. Co-citation networks form three major clusters, namely, respiratory medicine, environmental science, and basic medicine. While interdisciplinary integration is observed, with journals in environmental science and respiratory medicine forming strong co-citation links, integration remains superficial, is limited to the “pollution exposure → inflammatory injury” pathway, and lacks multidimensional perspectives such as socioeconomic factors, behavioural exposure, and health care accessibility. In recent years, many studies have focused on the molecular biological mechanisms through which pollutants such as PM2.5 regulate the development of COPD (47-51). This study also revealed that evidence-based medicine methods are becoming increasingly prevalent in this field. The high proportion of the Cochrane Database of Systematic Reviews (152 publications) among the top 10 journals highlights the authority of meta-analyses in pollution risk assessment. This finding also confirms that indoor air pollution constitutes a global risk factor for COPD, which is consistent with the World Health Organization (WHO) conclusion that indoor biomass fuel accounts for 35% of the total COPD cases in LMICs.

However, this study also revealed areas of research saturation, gaps, and limitations. The research on short-term exposure to single pollutants (e.g., PM2.5, PM10, and ozone) and their associations with COPD exacerbation and mortality, as well as disease burden estimates from Western cohorts and meta-analyses, is highly saturated. These topics are characterized by large publication volumes and homogeneous methodologies, leading to limited marginal innovation and a risk of low-level repetition. Moreover, critical gaps persist, namely, indigenous cohort and local pollution source studies in LMICs remain severely inadequate, whereas research on short-term exposure is abundant. In contrast, longitudinal evidence on long-term exposure (≥5–10 years) and the onset, progression, and prognosis of COPD is lacking. Research on air pollution and COPD under climate change is an emerging hotspot, yet studies on the mechanisms, population susceptibility, and regional disparities of heatwaves, extreme weather, and frequent haze events that synergistically exacerbate COPD remain at their early stages. Although indoor biomass exposure is confirmed as a global risk, few studies have aimed to evaluate the effectiveness and cost-effectiveness of rural clean fuel substitution and ventilation interventions. Additionally, public health intervention research on pollution control, early screening, and drug–pollution combined prevention is extremely limited, reflecting a research bias towards risk association rather than prevention and translation.

Future research can improve existing deficiencies and further explore relevant research directions. Priority areas include improving indigenous research in LMICs; conducting long-term cohort studies to analyse the exposure-response relationships of local pollution sources; advancing research on long-term exposure and climate change interactions; systematically investigating vulnerable populations by sex, age, and occupation; and implementing randomized controlled trials for synergistic interventions involving clean fuel substitution, community pollution prevention, and drug-pollution combined prevention.

Limitations

This study exhibits several limitations. First, all the data were retrieved exclusively from the WOSCC, without including other major databases such as Scopus and PubMed or non-English literature. This approach may introduce selection bias, thus limiting the full coverage of global research and potentially skewing geographical conclusions towards highly productive regions such as Europe, North America, and China. Second, bibliometric analysis relies heavily on citation metrics, which generally favour older, highly cited publications and may underrepresent emerging research topics. Finally, bibliometric methods provide only macrolevel trend analysis and can reveal associations but not establish clinical causal relationships between air pollution and COPD occurrence.

Conclusions

The results of this bibliometric analysis revealed that the relationship between air pollution and COPD-related health outcomes represents an active research area, with major themes including disease burden, mortality, hospitalization, household air pollution, oxidative stress, and climate change.

Acknowledgments

None.

Footnote

Reporting Checklist: The authors have completed the BIBLIO reporting checklist. Available at https://jtd.amegroups.com/article/view/10.21037/jtd-2026-0784/rc

Peer Review File: Available at https://jtd.amegroups.com/article/view/10.21037/jtd-2026-0784/prf

Funding: This study was supported by 2022 Gansu Provincial Key Talent Project (No. 2022RCXM025), Gansu Province Soft Science Special Project (No. 22JR11RA087), Lanzhou Science and Technology Planning Project (No. 2025-2-104), The Scientific and Technological Innovation of Health Industry in Gansu Province is Significant Item (No. GSWSZD2024-20), Lanzhou City Science and Technology Development Guidance Plan Project (No. 2024-9-31), and 2026 Gansu Provincial Key Talent Project (No. 2026RCXM042).

Conflicts of Interest: All authors have completed the ICMJE uniform disclosure form (available at https://jtd.amegroups.com/article/view/10.21037/jtd-2026-0784/coif). The authors have no conflicts of interest to declare.

Ethical Statement: The authors are accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved.

Open Access Statement: This is an Open Access article distributed in accordance with the Creative Commons Attribution-NonCommercial-NoDerivs 4.0 International License (CC BY-NC-ND 4.0), which permits the non-commercial replication and distribution of the article with the strict proviso that no changes or edits are made and the original work is properly cited (including links to both the formal publication through the relevant DOI and the license). See: https://creativecommons.org/licenses/by-nc-nd/4.0/.

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