Oxidative Potential of Airborne Particulate Matter: A Bibliometric Analysis of Current Research, Challenges, and Future Perspectives.

doi:10.21203/rs.3.rs-3318278/v1

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Systematic Review

Oxidative Potential of Airborne Particulate Matter: A Bibliometric Analysis of Current Research, Challenges, and Future Perspectives.

https://doi.org/10.21203/rs.3.rs-3318278/v1

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The present comprehensive study provides a detailed bibliometric analysis of the oxidative potential (OP) of particulate matter (PM). This area of research is rapidly expanding and has significant implications for human health and the environment. We carefully screened 569 articles. 368 studies were selected for further bibliometric analysis. Our bibliometric results indicate strong international collaboration in this field. In particular, the growing importance of OP-PM research is highlighted by a remarkable increase in the number of publications in the 2010s, with a 24-fold increase compared to the early 2000s. The dissemination of research is concentrated in a select group of eight journals. The remaining content is spread across 75 different journals. Furthermore, our study reveals a concentration of research in a limited number of institutions, mainly in the academic and university sector. There is a notable absence of contributions from the private sector. From the analysis of the research field, we have identified the main global research hotspots for OP-PM. These hotspots highlight the importance of capacity building, innovation, cross-disciplinary collaboration, and improved understanding of emission sources, atmospheric processes, and the impacts of PM and its OP. Furthermore, our study argues in favour of better understanding of personal exposure in different environments and the impact of emerging contaminants, including nanoparticles. Our findings also highlight the need to consider climate change effects on OP-PM and the regulatory context for PM research. Pursuing these research directions will improve our understanding of PM and its effects. It will also support more effective air quality management and public health protection strategies.

Environmental Chemistry

airborne

particulate matter

oxidative potential

oxidative stress

paradigm

The modern epoch is marked by a profound paradigm shift in atmospheric chemistry and toxicology research brought about by significant changes in atmospheric composition (Crutzen and Lelieveld, 2001; NASEM, 2016). These changes include not only the accumulation of greenhouse gases but also an increase in the global presence of potentially hazardous chemical pollutants (Crutzen, 2006). These pollutants have serious consequences for human health and biodiversity (Alan Pounds et al., 2006; Islam et al., 2020). The present epoch, known commonly as the Anthropocene, is characterized by large-scale human disruption of natural systems (Lewis and Maslin, 2015). Air pollution is a key concern that comes with this period (Chen and Kan, 2008; Lelieveld, 2017).

Toxicologically, changes in atmospheric constituents have broad and profound implications (Englert, 2004; Offer et al., 2022). Airborne particles, which include fine and ultrafine particles and their constituents, as well as organic compounds, heavy metals, nitrogen and sulfur oxides, and ozone, among others, have potentially deleterious effects on human health (Belleudi et al., 2010; Yang et al., 2019). These effects on health are subsequently manifested in an increase in morbidity and mortality rates (Díaz-Robles et al., 2014; Jerrett et al., 2013; Sommar et al., 2021). In addition, many pollutants can accumulate in ecosystems and food chains, with potential wide-ranging adverse effects on both biodiversity and human health (Ali et al., 2019; Greaver et al., 2012). The World Health Organization (WHO) has highlighted air pollution as one of the most pressing global public health problems facing us today. In 2019, 99% of the global population was estimated to live in regions that do not meet WHO air quality guidelines, resulting in an estimated 6.7 million premature deaths per year (WHO, 2019).

During the past few decades, a growing body of epidemiologic evidence has established an incontrovertible link between exposure to particulate matter (PM) and adverse health outcomes (Brook et al., 2010). Thus, the WHO has classified air pollution as a class 1 carcinogen, indicating a strong association, particularly between PM exposure and human carcinogenesis (IARC, 2016). This correlation has led to a particular focus on studying PM, a complex mixture of solid and liquid particles suspended in the air, mainly due to its high impact on human health (Morakinyo et al., 2016). Robust correlations were identified between lung cancer progression and exposure to various PM components, including copper (Cu), zinc (Zn), sulfur (S), nickel (Ni) and potassium (K) (Raaschou-Nielsen et al., 2016). Furthermore, increased mortality from various causes has been associated with exposure to compounds or elements such as nitrates, sulfate, and organic and elemental carbon (Cakmak et al., 2011; Wang et al., 2019). Exposure to PM-bound elements, such as zinc (Zn), copper (Cu), and iron (Fe), has also been associated with chronic respiratory diseases, such as obstructive lung disease (Gehring et al., 2015; Zhang et al., 2021). Furthermore, the incidence of preterm birth and low birth weight may increase with exposure to PM, which contains polycyclic aromatic hydrocarbons (PAHs), organic carbon (OC), elemental carbon (EC), benzene, chromium (Cr), iron (Fe), nickel (Ni), sulfur (S), aluminum (Al), and titanium (Ti), from motor vehicle emissions and biomass burning (Balidemaj et al., 2022; Pedersen et al., 2016; Wilhelm et al., 2011). However, it is important to recognize that efforts to directly link the chemical composition of PM with the underlying toxic mechanisms and health effects have had only partial success, in part because of the extraordinarily complex mixture of PM, which consists of hundreds or even thousands of compounds. This complexity makes it difficult, if not impossible, to fully identify and quantify PM, much less to determine and assess its health effects (Kelly and Fussell, 2012; Li et al., 2022). Furthermore, PM components can have synergistic and antagonistic interactions that cannot be adequately assessed from individual mass concentrations (Kastury et al., 2017). Recognizing these obstacles and limitations to the assessment of the effects of PM on human health, a novel approach has emerged. This approach is based on the fundamental premise that oxidative stress is a key driver in the physiological pathway that orchestrates both health and disease outcomes induced by exposure to PM (Delfino et al., 2013; Ghio et al., 2012).

The harmful effects of PM toxicity are mainly attributed to the formation of free radicals, specifically reactive oxygen species (ROS) and reactive nitrogen species (RNS), induced by its components (Tuet et al., 2016). These radicals initiate a powerful chain reaction that can compromise the body's antioxidant defenses, culminating in a state known as oxidative stress. Consequently, oxidative stress damages cellular macromolecules, potentially leading to apoptosis (Montiel-Dávalos et al., 2010). Two main mechanisms are thought to underlie PM exposure-induced oxidative stress: The first involves redox reactions triggered by redox-active PM constituents, including soluble transition metals, quinones, and polycyclic aromatic hydrocarbons (PAHs), which can deplete dissolved oxygen (Ghio et al., 2012); the second involves a biological reaction associated with cellular oxidative phosphorylation via the sequential addition of electrons to dissolved oxygen — essentially, the biological generation of ROS in response to PM exposure (Soberanes et al., 2009). Within this conceptual framework, the notion of oxidative potential (OP), or the redox activity of PM, emerges as a descriptor and metric of the relationship between the chemical composition of PM, its physical properties, and the potential toxicological effects it may exert on human health upon exposure. Thus, OP is closely related to the biological mechanisms by which the adverse health effects of PM constituents can manifest themselves (Yang et al., 2014).

OP, a quantifiable measure of PM capability to oxidize a target molecule, is intrinsically linked to free radical generation and the resultant oxidative stress, key aspects that can be evaluated using a variety of OP assays, both cellular and acellular (Bates et al., 2019; Murphy et al., 2022). There has been a noticeable shift toward the use of acellular assays for OP quantification because of their relative simplicity in terms of analytical chemistry procedures, in contrast to those employing biological systems. This makes them advantageous for early, efficient, and direct environmental monitoring, potentially allowing for earlier assessment of PM impact on human health (Ayres et al., 2008). The most widely used acellular methods for OP measurement include antioxidant depletion (such as ascorbic acid, uric acid, and glutathione), dichlorofluorescein, dithiothreitol, salicylic acid/benzoates and electron paramagnetic resonance spectroscopy (Bates et al., 2019).

OP PM is an important aspect to consider not only in the field of environmental (bio)technology, it is also crucial in the disciplines of atmospheric chemistry and toxicology. Considerable progress has been made in OP measurement in recent years. This progress is an important step toward establishing OP as an innovative and promising measure. Remarkable progress has been made in the design and standardization of toxicity assays, as methodologies have been refined (Charrier and Anastasio, 2012; Eiguren-Fernandez et al., 2017). As a result, our ability to quantify and interpret the effects of air pollutants has been greatly enhanced by the increased precision and reliability of studies that focus on the assessment of OP, which has also provided the basis for conducting both short- and long-term epidemiological assessments to characterize the toxicity of PM and its constituents (Bates et al., 2015; Fang et al., 2015). Significant advances are being made in understanding how various components of particulate matter, including transition metals and organic compounds, may contribute to their oxidative potential and hence their ability to cause oxidative damage in human tissues (Gao et al., 2020). These discoveries, along with advances in the standardization of toxicity assays, have provided a solid framework for future research in this rapidly evolving field. Finally, we must emphasize that this progress also allows for the development of effective strategies to mitigate the harmful effects of air pollution on the health of the population.

The main objective of the present research is to identify the main points of interest in the study of OP-PM. This will be achieved through a careful bibliometric analysis of current knowledge. As demonstrated in the academic literature, such bibliometric studies facilitate a clear delineation of scientific research trajectories and meticulously examine the current state of a given discipline or research area (de Oliveira et al., 2019). The chosen methodology, supported by an extensive bibliographic resource anthology, will facilitate the formation of a holistic view that combines both qualitative and quantitative aspects. The assessment includes aspects such as scientific productivity by country, corresponding authors' countries, international collaboration, analysis of core journals and journal distribution, most local and global cited documents, and characteristics of institutional contribution. It also deciphers the current state of the art while identifying potential avenues for future research.

The Preferred Reporting Items for Systematic Reviews and Meta-Analyses approach (PRISMA) was used for the selection of reports from scientific databases with clear inclusion and exclusion criteria and quality assessment (Page et al., 2021). This approach consists of well-established steps followed in the present work, which are as follows (see Figure S1), a PRISMA 2020 statement that addresses the different sections of a systematic review report, in the Supplementary Material): i) field definition, ii) search strategy and database selection, iii) refinement of search and eligibility criteria, iv) quantitative and qualitative analysis, and v) visualization of results. A brief methodological description of each of these steps follows.

Step 1: Definition of the field of study. The focus of the research is the OP-PM, which determines the search strategy and database to be used.

Step 2: Database Selection and Search Strategy. The selected database in the present study is the Web of Science (WoS) Core Collection. This database, which is often used in such studies, integrates several databases, including the Science Citation Index Expanded (SCI-Expanded), the Social Sciences Citation Index (SSCI), the Arts & Humanities Citation Index (AHCI), and the Emerging Sources Citation Index (ESCI). The search strategy within the WoS was implemented through a series of seven meticulous search queries, each using unique groups of keywords that were agreed upon by consensus by all authors (see Table 1).

Searches #1, #2, and #3 correspond to independent queries that, for each set of keywords separately, include the fields tags title (TI), abstract (AB), and author keywords (AK), linked by the Boolean operator 'OR'. These searches are performed across all available issues and publication years (1975-present). In search #4, searches #1 and #2 are merged by using the Boolean operator “OR”. Next, in query #5, queries #4 and #5 are combined using the “AND” Boolean. Finally, in queries #6 and #7, the results are filtered by adding "articles" only and excluding articles published after 2022, respectively. At the end of the process, a database report was obtained.

Table 1

Keyword sets and search strategy used in the study.
Keyword Set #1
("pollut" AND air) OR (atmosp AND pollut) OR ("bad air quality"); ("Ambient air pollution") OR ("quali" NEAR air)
Keyword Set #2
("partic* matter" OR "fine particulate" OR particulate OR "ultrafine partic" OR "ultrafine partic" OR "partic* pollut" OR aeroso OR Size segreg* OR Ozone OR Smog OR ("photochemical" NEAR smog) OR "nitrogen oxide" OR "sulfur oxide" OR "carbon monoxide" OR "heavy metal" OR "volatile organic gases" OR "organic gases" OR dioxi OR fura* OR "polycyclic aromatic hydrocarbon" OR "Polychlorinated biphenyl" OR "persistent organic pollutant" OR "volatile organic compound" OR "quinone" OR AAdepletion OR GSH*depletion OR “Congo Red” OR “DCF” OR “DCFH-DA” OR “2-deoxyribose” OR DHE OR DTT OR ESR OR EPR OR Luminol OR CRAT OR “2′,7′-dichlorofluorescin” OR “dithiothreitol” OR “acid ascorbic” OR “Dihydroethidium” OR “glutathione” OR “electron spin resonance” OR “electron paramagnetic resonance”)
Keyword Set #3
(oxidative NEAR/5 potential)
Search N°	Search query
1	TITLE, ABSTRACT AND AUTHOR KEYWORDS: TI= (Keyword Set #1) OR AB= (Keyword Set #1) OR AK= (Keyword Set #1) with the options Search in: Web of Science Core Collection; Edition: All; Publication date: All years (1975-present).
2	TITLE, ABSTRACT AND AUTHOR KEYWORDS: TI= (Keyword Set #2) OR AB= (Keyword Set #2) OR AK= (Keyword Set #2) with the options Search in: Web of Science Core Collection; Edition: All; Publication date: All years (1975-present).
3	TITLE, ABSTRACT AND AUTHOR KEYWORDS: TI= (Keyword Set #3) OR AB= (Keyword Set #3) OR AK= (Keyword Set #3) with the options Search in: Web of Science Core Collection; Edition: All; Publication date: All years (1975-present).
4	#1 OR #2 (Merge Search N°1 and Search N°2)
5	#3 AND #4 (Combine Search N°3 and Search N°4)
6	[Refine] by Document Types: Article
7	[Exclude] by Publication Years: 2022-present

Step 3

Refinement of the search and/or eligibility criteria. Duplicate verification using an automated tool and evaluation by three reviewers (LFS, FC, and LV) was used to refine the report database. These reviewers independently determine the eligibility of a report by examining the title, abstract, keywords of the author, and/or the full report. To be considered eligible for a report, all three reviewers must agree that the report is eligible. In case of disagreement, the report is given a "conflict" status and must be reviewed by a fourth reviewer (MALG) and discussed in a panel. The final decision on eligibility is made by a majority of reviewers. In this process, only reports directly related to OP-PM were included, i.e., reports that involve measurements of OP in PM samples or evaluate and/or develop methods for measuring OP. Finally, reports that where reviews or were not in the search scope were excluded.

At the same time, during the review process, a questionnaire was utilized to gather additional information, with which homogenized keywords were assigned to each report (details of these keywords are provided in Supplementary Material, Table S1). These keywords take into account aspects such as author affiliation, study characteristics, study objectives, type of OP assay, complementary assessment method to OP, pollutants or variables other than OP, spatiotemporal scales, spatial scale, emission source contributing to OP, and potential implications of the study. In order to minimize the risk of bias and increase confidence in the results, the homogenized keywords were discussed and agreed in a panel by all reviewers. Categorical variables were designed to represent a range of values and extract numerical data, such as spatiotemporal scales. Furthermore, an “other” option was included next to a text box in each question of the questionnaire to allow comments regarding information not initially considered. All reviewers on the panel reviewed these comments. If the level of occurrence warranted it, new homogenized keywords were assigned and the corresponding correction was applied to the reports reviewed previously. Comments with weighted occurrences of less than 1% were categorized under the "other" option. This method enables the acquisition of more precise information, thus minimizing potential heterogeneity in the results of individual reports.

This process was facilitated by a Web-enabled collaborative systematic review application, SWIFT ActiveScreener, provided by SCIME (Howard et al., 2020). This application allows for a double assessment of each report and a review by a panel of experts. In addition, it includes a deduplication function that verifies the presence of duplicates when uploading the report database based on the title, journal, and/or year of publication.

Step 4

Quantitative and qualitative analysis. This analysis takes into account aspects related to authors, coauthors, journal sources, keywords, author affiliations, and countries or regions of authorship. The findings are systematized with a PRISMA flow diagram (Boers, 2018; Mayo-Wilson et al., 2018; Stovold et al., 2014).

VOSviewer version 1.6.19 (van Eck and Waltman, 2010) and the R-based web interface Bibliometrix version 3.2.1 (Aria and Cuccurullo, 2017) were used to construct and visualize bibliometric networks for quantitative and qualitative bibliometric analyses. Furthermore, MS Excel Mac version 16.59 (MS-Excel, 2022) was used to tabulate and process the data. Within the R environment (Posit team, 2022), the Ggplot 2 package R (Wickham, 2009) was used to generate specific plots. This comprehensive approach integrates different tools and methods to elucidate patterns and networks and provides insightful representations of bibliographic data. When bibliographic data was incomplete or unclear, the missing fields (such as DOI, Author keywords, Journals, etc) were complete by extracting information directly from the available articles. The implementation of this control measure enhances the level of certainty and confidence in the information contained in the database.

Step 5

Results analysis. Finally, Step 5 consists of analyzing and discussing the results. This is presented in the next section.

3.1. PRISMA and descriptive analysis of the data

An initial collection of 569 articles was identified during the examination of the OP-PM using the PRISMA approach. Subsequently, a sample of 368 articles (hereafter referred to as the "article collection") was obtained, as shown in the flow chart (Fig. 1a), with 368 final articles obtained by exporting to an Excel bibliometrics file using Biblioshiny ( BibOP-PM-Bibliometrix.xlsx is available online in: https://t.ly/uZjdd or upon request). Descriptive statistics can be used to make an initial exploration of the results. Figure 1b shows the main results of the article collection to be analyzed. A total of 1724 authors contributed to the article collection, with an average coauthorship rate per document of 8.11 and a coauthorship rate involving authors from different countries of approximately 41%. This reflects a strong degree of international collaboration on the subject. The average age of the article collection is 5.53 years, with an average citation rate of 28.93. These results demonstrate first the recent development of this field of research and second the great interest in the topic. Publications in the 2010s represent 65.5% of the total number of publications. This represents an increase of more than 24 times compared to the first decade of the 2000s. The number of publications showed an exponential growth trend of 26.1% per year (R2 = 0.95, see Fig. 1c).

3.2. Scientific Productivity by Country, Corresponding Authors’ Countries, and International Collaboration

The geographical distribution of the articles according to the country (ISO code) of affiliation of the authors and coauthors is presented in Fig. 2a as a cartogram-type map and as a histogram of the total number of authors affiliated with an institution in a given country. Assuming a similar contribution of authors, the cartogram-type map shows a projection of each country in proportion to the number of authors and coauthors affiliated with that country. The results show the participation of authors affiliated with institutions from 46 countries. These countries are located on five continents. However, there are countries with zero productivity, including almost all of Africa (with the exception of NGA) and countries in the Middle East, Central Asia, and Southeast Asia. More than 62% are contributed by the top five countries. The authors from the USA contributed the most (24.9%). This was followed by CHN (17.9%), ITA (7.6%), NLD (6.0%) and GBR (5.8%). Figure 2b also shows the cartogram and histogram of the most productive countries according to the country of origin of the corresponding authors and the percentage of publications of multiple countries (MCP) and publications of a single country (SCP). These results show that the most prominent countries according to the participation of authors from one country (Fig. 3a) and according to the corresponding author are basically the same. This indicates that the countries with the highest production also had the highest number of articles published as countries of the corresponding authors. The five countries with the highest number of corresponding authors represent 66.4% of the contribution and correspond to the USA (27.4%), followed by CHN (19.3%), ITA (8.7%), NLD (5.7%) and CAN (5.2%); the first four coincide with the four most productive countries. NLD and CAN are the countries with the highest proportions of multicountry publications (MCP: 76.2% and 73.7%, respectively), indicating strong international collaboration. The lowest proportions are found in the USA and ITA (MCP: approximately 22%), suggesting that authors from these countries collaborate less with researchers from other countries. Figure 2c on the origin-destination, which shows the collaboration of corresponding authors from one country with respect to other countries, shows that the sources of the articles are concentrated in North America (USA and CAN), Europe (NLD, ITA, GRB, FRA, and CHE) and East Asia (CHN).

3.3. Analysis of Core Journal and Journal Distribution

The frequency of articles per journal shows how attractive it is for researchers to publish their research results in certain sources. The histogram of the distribution of articles published by journals is shown in Fig. 3a, embedded figure. The results show that a total of 75 different journals have published articles in the field. Most of the publications were concentrated in a few sources. Overall, 45 journals published one to two papers, and only eight journals published more than ten papers.

On the other hand, Bradford's law was used to identify core sources (Brookes, 1985). This law describes a quantitative relationship between the journals and the articles contained in a collection on a given subject and states that the production of articles in journals is established by a highly uneven distribution, where most of the articles are concentrated in a small population of journals, while a small proportion of articles are spread over a large number of journals, following a distribution proportional to a ratio of 1 (zone 1): n (zone 2): n² (zone 3). where zone 1 is the number of core journals preferred by researchers and therefore most specific to a domain of knowledge. The results obtained by applying Bradford's law, Fig. 3a, to the article collection show that the "core zone" is composed of 3 sources with a total of 134 articles published, corresponding to 36.4% of the total number of articles; the second and third zones are composed of 11 and 64 sources, respectively.

The top 10 journals preferred by the authors were ranked according to the number of articles published (NP) in each journal (Fig. 3b), followed by the total local citations (TC) of the journals (Fig. 3c), the impact factor (IF, Fig. 3d) and the H-index (Fig. 3e) associated with the article collection. This group of journals includes, as expected, the core group that predicts Bradford's law (Fig. 3a). The ATMOS ENVIRON journal stands out with a total of 15.2% of published articles (NP: 56), making it the journal chosen by the majority of authors in this field. It is followed by ENVIRON SCI TECHNOL (NP: 40), SCI TOTAL ENVIRON (NP: 38) and ENVIRON POLLUT (NP: 23). In terms of NP, the top 10 journals present an average of 22 published articles (CI95%: 12 to 32) and an average of 14, their contribution representing almost 60% of the articles in the collection (Fig. 3b). The TC of this group of journals also stands out with an average of 707 citations (CI95%: 348 to 1066, Fig. 3b). It should be noted that an important aspect in the choice of the journal to be published is its IF. The top 10 journals have an average IF of 9 (CI95%: 7 to 11) and a mean of 10 (Fig. 3d). Regarding the H-index, the top 10 journals have an average of 13 (CI95%: 8 to 17) and a mean of 10 (Fig. 3e). The journal with the highest H-index is ATMOS ENVIRON (H-index: 25). The indexing categorization of the top 10 journals is mainly in the fields of "Environmental Sciences" and "Meteorology and Atmospheric Sciences", and all of them are in the top five categories. In general, given their scope, the journals have published a large number of articles on atmospheric pollution, which, together with their high citation counts and IF and H indices, makes them attractive to researchers in this field and makes them the reference sources in this area of research.

3.4. Most relevant authors and cocitation network

In this section, we present the authors who have contributed the most to the knowledge in the field. First, the productivity of the authors is assessed using Lotka's law of scientific productivity in Fig. 4a. Lotka's law considers that, regardless of the field analyzed, most of the production is concentrated in a limited number of authors (López-Fernández et al., 2016) and is described as the number of articles = C x (Nº Authors)^-2, where C is an arbitrary constant. In the present study, the data adjusted for Lotka's law show a good fit (Chung et al., 1992). Therefore, we conclude that the results follow the inverse of Lotka's quadratic law (R2 = 0.9995). In our case, production is diversified, with many authors having only one publication (1243 out of 1724), representing 72.1% of total authors. The top 10 authors represent 0.58% of the total number of authors. This implies that they contribute at least 16 articles on the topic.

The 10 top authors by productivity are shown in Figs. 4b-d. They are presented in descending order according to the number of published articles (NP, Fig. 4b) and the number of local citations (LC, Fig. 4c), which measures how often an author included in this collection has been cited by articles also included in the collection. Finally, the H-index is presented (Fig. 4d). The results show that, considering the NP, Schauer JJ is the most prolific with a total of 36 published articles, followed by Sioutas C (NP: 30) and Kelly FJ (NP: 30). However, the predominance of authors is not uniform across the different indicators. Considering the publication impact in terms of citations (LC), Verma V is the most important author with 505 citations, followed by Schauer JJ (LC: 419), Weber RJ (LC: 364) and Cassee FR (LC: 357). In terms of the H-index, Schauer JJ is the top author (H-index: 25), followed by Sioutas C (H-index: 22) and Kelly FJ (H-index: 21).

Figure 4e shows the coauthorship network among the authors who have published on the topic studied. In coauthorship networks, the authors of the network are connected as nodes by coauthored scientific publications. The colors represent the working groups as clusters. Authors coauthoring more than one paper are grouped in the same cluster. In this case, there are clusters of 4 to 7 authors with a high level of coauthorship. For example, Verma V, Fang T, Gao D, and Weber RJ have a strong collaborative relationship, as shown by the orange cluster. Verma V seems to be the central author of this community of researchers. The topology of the collaborative network is as follows. There are groups with high collaboration (yellow, blue, and red clusters) and others that are more dispersed. This is an indication that there is not much collaborative activity among the other clusters or working groups, so there is a need for an increase in collaborative activity to advance research in this area.

3.5. Characteristics of the Institutions Contribution

The analysis of the distribution of institutions can aid in understanding the installed capacity for research in this field. In total, 532 institutions are associated with the authors in the collection. One or more authors are affiliated with an academic institution in 100% of the articles in the collection. However, 14.7% of the articles in the collection have almost one author affiliated with governmental organizations. In the article collections, there are virtually no authors from private institutions (1.6%) (see Figure S2 in supplementary materials). Figure 5a shows the 10 most productive institutions in terms of the number of publications. Approximately 18% of the publications were contributed by these top 10 institutions. They are mainly located in the USA with 4 institutions, followed by GRB, AUS, NLD, CAN, GER and CHN with 1 institution each. UNIV WISCONSIN leads the ranking. It is followed by KINGS COLL LONDON and QUEENSLAND UNIV TECHNOL. The results clearly show the strong contribution of academic institutions (universities) in this area of research. There is no contribution from private sector institutions. The affiliation of the most productive authors is closely linked to these institutions. Finally, a stronger collaboration between institutions in the United States and between the United States and institutions in European countries can be seen in the collaboration network of the 50 most productive institutions (Fig. 5b).

3.6. Most Local and Global Cited Documents

Table 2 shows the 10 most highly cited publications at the local level (LC), together with the number of global citations (GC). In this case, the LC takes into account the number of times each publication was cited by other publications within the collection of articles, while the GC measures the number of times each of the publications was cited in the entire WoS database; thus, it could take into account the relevance of a publication to other fields of study. The article by Charrier, JG, 2012 turned out to be the most significant both locally (highest LC) and globally for other fields (highest GC) (Charrier and Anastasio, 2012). Charrier JG and Fang T stand out on the list, with two of their articles in the top 10 of the most relevant literature in the field. In general, these papers are of interest as they deal with aspects of OP methodologies and effects of PM components in the different assays. On the other hand, the LC/GC ratio indicates that these articles not only have a local impact on the research topic but are also of interest in other research areas. This could be explained by the fact that OP is a metric that directly relates to the potential impact of PM on human health. Therefore, it is not surprising that these articles are of interest, for example, in the fields of toxicology and environmental epidemiology.

Table 2

Top 10 relevant publications in research on OP-PM based on the number of local citations (LC). In each case, the number of global citations (GC) and the percentage of the LC/GC ratio are indicated.
Ranking (Nº)	Most Local Cited Documents (Reference)	Journal Abbreviations	Digital Object Identifier DOI	LC (Nº)	GC (Nº)	LC/GC (%)
1	(Charrier and Anastasio, 2012)	ATMOS CHEM PHYS	10.5194/acp-12-9321-2012	137	273	50.18
2	(Fang et al., 2016)	ATMOS CHEM PHYS	10.5194/acp-16-3865-2016	84	143	58.74
3	(Verma et al., 2012)	ENVIRON SCI TECHNOL	10.1021/es302484r	73	193	37.82
4	(Yang et al., 2014)	ATMOS ENVIRON	10.1016/j.atmosenv.2013.10.049	62	107	57.94
5	(Delfino et al., 2013)	J EXPO SCI ENV EPID	10.1038/jes.2013.25	60	130	46.15
6	(Fang et al., 2015)	ATMOS MEAS TECH	10.5194/amt-8-471-2015	57	98	58.16
7	(Steenhof et al., 2011)	PART FIBRE TOXICOL	10.1186/1743-8977-8-26	52	215	24.19
8	(Charrier et al., 2015)	ATMOS CHEM PHYS	10.5194/acp-15-2327-2015	48	67	71.64
9	(Saffari et al., 2014)	J ENVIRON SCI HEAL A	10.1080/10934529.2014.854677	46	68	67.65
10	(Biswas et al., 2009)	ENVIRON SCI TECHNOL	10.1021/es9000592	41	127	32.28

3.7. Research hotspot analysis

Two types of analysis were performed for the global evaluation of OP-PM research hotspots. The first was based on the cooccurrence method between thematic WoS categories (Fig. 6a) and author keywords (Fig. 6b) of the article collection, with at least 3 interactions between them, grouped by the leading eigenvalue algorithm. The second analysis consisted of a detailed examination of each article in the collection, using a questionnaire to obtain and classify the information explicitly mentioned by the authors in relation to the main characteristics of the scope of the study, the objectives, the methods used, and the potential impact of the results obtained. The results are expressed in terms of the percentage of occurrence of each response. These percentages are weighted by the total number of articles in the collection. Both analyses allow us to understand the main characteristics of the research that has been conducted worldwide in relation to OP-PM and to identify possible research questions that have not yet been answered.

The results of analyzing the WoS subject categories indicate that the five main subject categories in which the articles in the collection were published correspond to the following (Fig. 6a): "Environmental Sciences", "Meteorology & Atmospheric Sciences", "Engineering, Environmental", "Public, Environmental & Occupational Health", and "Toxicology". The categories have been grouped into 4 clusters. These clusters can be characterized by the following topics: Cluster 1—Biological, chemical and health sciences (red cluster in Fig. 6a), this is the group with the largest number of items and includes mainly categories related to chemical and biological health sciences such as 'biochemistry & molecular biology', 'chemistry, multidisciplinary', 'nanoscience & nanotechnology', 'chemistry, medical', pharmacology & pharmacy, 'toxicology' and 'public, environmental & occupational health'; Cluster 2—Engineering (green cluster in Fig. 6a), this group includes categories such as 'Engineering, Chemical, 'Engineering, Mechanical' and 'Energy and Fuel'; Cluster 3—Environmental Science and Chemical Analysis (blue cluster in Fig. 6a), with categories such as Environmental Science, Meteorology and Atmospheric Science and Chemistry, Analytical. Finally, Cluster 4—Sustainable infrastructure (yellow color cluster in Fig. 6a) includes categories such as 'Construction & Building Technology', 'Engineering, Civil', and building technology, engineering, civil engineering, and engineering, environmental engineering.

However, the keyword analysis of the author shows that the five main keywords in the collection are particulate matter, oxidative potential, dithiothreitol assay, reactive oxygen species, and air pollution (Fig. 6b). Keywords have been grouped into 7 clusters. These clusters can be characterized as Cluster 1: Fuels and toxicological health effects (red color cluster in Fig. 6b), this group represents the cluster with the highest number of elements and includes keywords related to the emission of pollutants from the use of fuels such as "PM emission", "gasoline direct injection" or "diesel/biodiesel" and health effects such as "cytotoxicity", "genotoxicity", "toxicity", "inflammation", "free radicals" and "oxidative stress"; Cluster 2—Oxidative potential assays and spatial variability (green color cluster in Fig. 6b), correspond to the second largest group and includes keywords mainly related to assays to measure oxidative potential such as "ascorbic acid assay", "dithiothreitol assay", "esr" (electron spin resonance) or "epr" (electron paramagnetic resonance) and spatial distribution variation models such as "land use regression model" or "spatial variation"; Cluster 3—Biomass burning and its impacts (blue color cluster in Fig. 6b), includes keywords mainly related to biomass burning and its emissions such as "biomass burning", "source apportionment", "black carbon", and "PAH" (polycyclic aromatic hydrocarbons), and its impacts such as "air quality" and "health"; Cluster 4—The physical characterization of particulate matter (yellow color cluster in Fig. 6b), includes keywords related to the characterization of particulate matter, such as "chemical composition" and "physicochemical properties"; Cluster 5—Air pollution and respiratory health (purple color cluster in Fig. 6b), this group includes keywords such as "air pollution" and "asthma", on the other hand, it includes "antioxidants" and "metals", which are keywords that can be related through the mechanism of oxidative stress; Cluster 6—Oxidative potential due to exposure to particulate matter (cyan color cluster in Fig. 6b), including keywords such as "exposure", "particulate matter" and "oxidative potential". Finally, Cluster 7—Indoor pollution (orange color cluster in Fig. 6b) includes keywords related mainly to indoor or workplace pollution, such as "indoor air quality" and "office building".

The main findings from the application of the questionnaire to each of the articles collected are described below, as are the results of our analysis. Figures supporting this analysis and a brief description of the categories used are provided in the supplementary material (see Tables S1 – S12 and Figures S2 – S14):

Main characteristics of the scope of the study (see Table S3 and Figure S3 in the supplementary materials). The primary emphasis of the research encapsulated in the article collection centers on evaluating the OP by utilizing environmental PM samples, which constitute a significant 70.65% of the entire dataset, referred to here as "Ambient". Succeeding this, artificially created samples under regulated conditions, labeled "laboratory-generated samples", account for a substantial 27.72% of the research. Last, the analysis includes samples procured from enclosed residential or occupational settings, denoted "Indoor", and this forms 10.33% of the overall investigation. Together, these three aspects provide the structural foundation for field research. Recognizing the different proportions of these data sources is important, as it underscores the distinct importance of each in this field. The focus of the study on unraveling the OP in real-world situations is highlighted by the heavy reliance on environmental samples. In addition, laboratory and indoor study samples provide valuable controlled data and contextual information, respectively.

Objectives of the study (see Table S4 and Figure S4 in the supplementary materials). Regarding the objectives of the studies, the first and most common is to "analyze the causes of air pollution and/or oxidizing potential (OP)" (70.92%). It involves the evaluation and characterization of zones where air pollution, air quality, or OP is affected or driven by comparable parameters (e.g., emission sources, dispersion conditions, deposition, or atmospheric chemistry that explain a spatial or temporal variability of OP in a specific area) or emission sources. Delimitation of areas where air pollution/air quality or OP is influenced/triggered by similar parameters or emission sources (e.g., sources with similar temporal variations, meteorological factors). The second most common focus is on "exposure assessment" (38.32%), which often involves determining the amount, duration, and frequency of human exposure by employing both mass-based exposure estimates and various OP metrics. Finally, a significant number of studies focus on the "evaluation of OP measurement methods" (26.36%). This could involve comparing the reliability and accuracy of different measurement techniques or establishing new methodologies to improve the accuracy of OP measurement.

Methods used, OP assays and chemical characterization (see Tables S5 – S8 and Figures S5 – S8 in the supplementary materials). In terms of the type of OP assay used, acellular assays have emerged as the predominant method used by researchers in article collections, accounting for 86.14% of all tests, significantly outweighing the use of cellular assays, which accounts for only 21.47%. The five most commonly used OP assays, listed in descending order of prevalence, include the dithiothreitol assay (DTT: 57.07%), the ascorbic acid assay (AA: 24.73%), the dichlorofluorescein assay (DFCH: 21.20%), the reduced glutathione assay (GSH: 14.40%), and the electron paramagnetic/spin resonance assay (EPR/ESR: 11.14%). This preference for acellular assays is not surprising given their distinct advantages over cellular assays. They are more efficient and cost-effective because they provide rapid results, require less stringent environmental conditions, and lend themselves to automation. However, despite the current dominance of acellular assays, research is beginning to shift toward the use of in vivo models. This transition is primarily driven by an increased understanding of the biological mechanisms related to PM. By authentically replicating complex biological interactions, in vivo models provide a more holistic and accurate assessment of the effects of PM.

All studies quantify the concentration of various air pollutants, such as PM or others (with 100% "concentration"), and a significant proportion evaluate their chemical composition (85.05%) as a complementary assessment method to OP. Toxicity is of tertiary concern, with a total toxicity of 38.86%. In general, these studies are aimed at correlating OP with chemical or physical parameters of the concentration of PM and/or gases, meteorological variables, analysis of emission sources, and others. However, of the large number of pollutants or variables that have been prominently featured in OP studies, five stand out due to their relative prevalence. These include PM, which tops the list with an overwhelming 89.67% representation. Next, in descending order, are "Elemental Composition" with 61.14% and "Carbon Compounds" with a substantial frequency of 51.36%. Further down the list, "Anions and Cations" appear with 30.16%. Finally, the "Biological Endpoint" completes our list with 25.27%. Each of these elements shows a significant presence within this environmental context. This underscores its relevance in the overall analytical framework and in understanding the drivers of OP.

PM samples Geographical origin and spatiotemporal scales of the studies (see Tables S9 – S11 and Figures S9 – S13 in supplementary materials). According to the results described in Section 3.2, the three main continents are Europe with 35.05% of the samples, Asia with 32.61% and North America with 31.79% of the samples, regarding the geographical origin of the PM samples. Among them, the United States, China, and Italy stand out, representing 24.73%, 21.20% and 11.14% of the total sample, respectively. This suggests that there is a growing interest in this area of research, especially in developed countries, compared to other geographical areas. In addition, an analysis of the emission sources that contribute to the oxidative potential (OP) emerges, and five major contributors emerge. First and foremost, "traffic" emerges as the most significant, accounting for 61.41% of OP. This is followed by "domestic heating - wood burning" with 26.09%. This shows the influence of domestic activities on the OP. The category "industrial and commercial emissions" accounts for 22.28%. This underlines the role of the industrial emission source in the OP. Interestingly, "unspecified biomass burning" accounts for 21.47% of the OP. This suggests a significant influence of uncharacterized biomass burning. Finally, indicating the significant role of the environment itself in PM emissions and impacts, "natural sources" contribute 18.75%.

Regarding the spatiotemporal variation of the studies, we categorize the temporal scales from most extended to least. We find that 13.59% of the studies cover "More than a year – 1 year", 33.42% cover "Year - Season", 16.03% are conducted over "3 months − 1 month", and 11.41% range from "three weeks to a few hours". For the spatial scale, ordered from the most extensive to the least extensive, we find that 0.82% of the studies are conducted at a "global" scale, 5.43% at a "regional" scale, a substantial 59.51% at a "local" scale, and 34.78% at a "micro" scale. By systematically categorizing temporal and spatial scales, this observation provides a clear picture of how studies are distributed across different spatiotemporal scales.

The geographic origin and spatiotemporal expansion of PM sample studies is important in understanding the relationship between composition, temporal variability, and oxidative potential (OP), which varies significantly from region to region. These scales also suggest that the impact and implications of OP can differ markedly depending on both the time frame and the spatial context, reinforcing the need for multidimensional research approaches. Therefore, insightful information on pollutant sources can be obtained by understanding the geographical origin and providing the ability to assess the long- and short-term effects of the source and composition of PM in the OP.

Potential impact of the study (see Table S12 and Figure S14 in supplementary materials): The study primarily reveals valuable, application-oriented perspectives in the health sector, a component here referred to as "human health," which accounts for an estimated 75.27% of the study's impacts. These results underscore a substantial correlation of OP with health outcomes on the usefulness of the exposure metric, over mass concentration. Subsequently, the regulatory and legal sphere, referred to as "regulatory and legal", gains substantial impact, comprising approximately 25.27% of the potential ramifications. This facet relates to the usefulness of OP as a surrogate measure for PM exposure in the formulation and implementation of mitigation strategies for the reduction in PM pollution. It also promotes the establishment of air quality standards that are more health-based, as opposed to being based solely on physicochemical parameters such as mass concentration. Finally, we address the elucidation of the intrinsic mechanisms associated with the OP metric, an area referred to as "understanding the OP mechanism", which accounts for approximately 14.67% of the research implications. These insights are related to understanding the driving factors of OP in relation to the chemical-physical characteristics of PM, including emission sources, chemical composition and size, as well as meteorological and orographic variables related to the dynamics and transport of pollutants.

The results of these studies deeply emphasize the potential implications of OP in a variety of sectors. The breadth of the scope of OPs and their implications in these areas imply a complex interweaving of diverse disciplines. This underscores the essential role of interdisciplinary collaboration and understanding in the pursuit of a deeper understanding and application of the results of OP research in the real world.

The field of atmospheric science, in particular the study of PM and its OP, is of paramount importance as a result of its impact on human health and environmental sustainability. One of the key biological mechanisms involved in the development of adverse health effects of PM is the ability of PM to induce the generation of cellular ROS at the expense of antioxidant defenses, resulting in responses to oxidative stress and various health outcomes (such as chronic and acute systemic inflammation). In this regard, current information suggests that OP may be a more biologically relevant parameter for addressing human exposure to PM than the mass concentration of PM in bulk. Despite the significant progress that has been made in this area, there are still many gaps and areas of controversy that warrant further investigation. In the following, we provide some key outlines of potential directions for future research in this area. Although there may be others, we attempt to open debate and discussion on these future research directions. Each of these addresses a unique aspect of the complex interaction between PM, its OP, and the broader social, environmental, and health contexts.

Capacity Building and Innovation: A critical need in the field is the development of accurate, sensitive, and rapid measurement capabilities for OP. Metrological advances, including uncertainty calculation and traceability, require urgent attention to ensure comparability of results within and between assays. In addition, considerable emphasis should be given to the development of next-generation instrument platforms tailored to measure OP and facilitate near real-time monitoring capability. Such innovative instrumentation has the potential to enable longitudinal assessment of OP. This will provide invaluable data for both air quality management and scientific research efforts. Most importantly, advancing capacity building in this area will involve the dissemination of expertise and knowledge, thereby enhancing the competence of organizations to protect public health and the environment. By cultivating collective skills and sharing knowledge, the effectiveness of air pollution control measures can be greatly enhanced. Therefore, concerted efforts in capacity building and innovation in oxidation potential research are essential to achieve sustainable and improved air quality management practices.
Characterization of the impact of emission sources. An understanding of the sources of PM and their effects on OP is critical for air quality management and public health protection. Future research could focus on identifying sources of oxidative stress induced by ambient PM, and determining the role of meteorology on emissions and atmospheric processes of atmospheric species will help to understand how sources and atmospheric processes drive OP. This would require a detailed chemical and physical characterization of PM from key sources, as well as the development of state-of-the-art statistical methods to analyze these data.
Personal exposure to different environments and activities. Understanding the health effects associated with OP PM, personal exposure to different environments and activities plays a critical role. For effective health risk assessment, a mechanistic understanding of the role of atmospheric trace species in influencing human health effects is imperative. In addition, further research should be directed toward the quantification of the spatial and temporal variability of atmospheric constituents that influence OP, as well as the investigation of the measurement of personal exposure to OP PM in different environmental and activity contexts. These research efforts should include both epidemiologic and toxicological studies, coupled with the development of accurate and reliable measurement techniques to assess both external and internal exposure scenarios. The complex relationship between personal exposure, personal daily activity, atmospheric factors, and oxidative potential can be better understood, and strategies for protecting public health can be developed through such comprehensive investigations.
Emerging pollutants. Emerging contaminants are a new category of environmental pollutants. They have recently received considerable attention. Due to their unique properties and potential adverse effects on human health and the environment, nanoparticles stand out among them. The inclusion of these pollutants in oxidation potential research has introduced a new dimension of complexity. Specialized studies are needed to address the challenges they pose. The focus on nanoparticles is intended to pave the way for informed decisions in environmental management and public health policy by understanding their physicochemical properties that influence toxicity. The development of comprehensive research methods and risk assessment models from the perspective of the oxidative stress paradigm and the characterization of their oxidative potential will help us develop effective strategies to mitigate the effects of emerging pollutants and ensure the well-being of humans and ecosystems.
Climate Change and Mitigation Actions. To understand the multifaceted dynamics of climate change and its impacts, it is essential to study the interactions between extreme weather events and the oxidative potential of particulate matter (OP-PM). Understanding how these events affect OP-PM and the potential health and environmental impacts associated with OP-PM exposure under different climatic conditions is critical, as climate change is expected to increase the frequency and severity of extreme weather events. Special focus should be placed on quantifying emissions and elucidating the intricate atmospheric processes that control OP-PM within our dynamically changing Earth system to pave the way for future research efforts. This will provide essential insights into the complex relationships between global change and societal choices regarding emissions and atmospheric removals. A multifaceted approach that not only involves understanding atmospheric processes but also considers the broader context of societal and political influences is warranted.
Societal, Legal and Policy Frameworks. The complexity of addressing air pollution necessitates a multidimensional approach that spans societal, legal, and policy domains. By integrating knowledge from atmospheric science and social sciences, policy-makers can design comprehensive strategies to combat PM pollution effectively. It is crucial to prioritize equitable reduction of pollution while also protecting vulnerable communities and leveraging advanced metrics such as OP for better regulatory standards. Through collaborative efforts and evidence-based decision-making, we can safeguard public health and the environment from the adverse impacts of air pollution.
Embrace interdisciplinary research. The study of PM and its OP is a complex and diverse area, and there are many potential directions for future research. Future research can contribute to a deeper understanding of the environmental and health impacts of PM and inform effective strategies for air quality management and public health protection by addressing the gaps and areas of controversy outlined in the present study. An integrated and multidisciplinary approach is needed to study PM and its OP. Conducting well-coordinated studies that address multiple aspects of the source-exposure-dose‒response paradigm for OP will be a focus of future research. By integrating insights from diverse disciplines, such as chemistry, biology, public health, socioeconomic studies, and legal frameworks, we can enhance future OP PM research and inform effective mitigation strategies.

OP PM research has been key to understanding the impacts of PM on human health and environmental sustainability. In this sense, the present study has emphasized the importance of the linkages between emissions, atmospheric fate and transport, human exposure, health and environmental effects, risk assessment, and control. It has also facilitated interdisciplinary collaboration in this research area. By integrating research across the exposure-risk assessment-risk management continuum, it is possible to contribute to the protection of public health and the environment through the integration of the OP metric, which is a more reliable proxy for the assessment of exposure to PM.

Bibliometric analysis, which provides a quantitative approach to evaluating the scientific literature in a given field, is a powerful research tool. Bibliometric analysis offers several strengths in the context of studying OP-PM. It can map the hotspots of research and the evolution of the field over time, as well as identify key trends, influential authors, institutions, and countries. This type of analysis can also highlight the most cited articles. This indicates studies that have had a significant impact in the field.

However, it is important to recognize the inherent limitations of bibliometric analysis. These limitations can lead to biased interpretations or potentially erroneous conclusions. These limitations often stem from the completeness or lack thereof of the databases used. The scope of the analysis is limited to articles indexed in the selected databases. This may exclude relevant studies published in other bibliographic databases. Additionally, databases evolve over time, which may lead to variations, usually minor, by including or excluding articles in the source database for the same time period. Therefore, the results of the bibliographic analysis provide a snapshot of the time the search was performed and do not necessarily reflect the current search criteria.

The methodology used to obtain the collection of articles also plays a fundamental role. A search strategy based on specific keywords and Boolean strings may inadvertently exclude or include relevant articles. As a result, the full breadth of the research area may not be captured. Inconsistencies between the target keywords and the actual content of the articles may occur if these keywords are biased or insufficient or if Boolean strings are not selected appropriately, which would bias the results.

One of the strengths of this study is the methodology used. It ensures the reliability and validity of the conclusions reached. A panel was used to evaluate the articles, in addition to using specialized software tools to analyze the articles. Each selection of criteria, both for the search and for the visualization of the results, was carefully considered throughout the research. Thus, in all phases of the study, from the collection of articles to the analysis and interpretation of the results, as well as in the visualization of the results, the proposed methodological work followed rigorous deliberation processes, redundant steps, and successive verification. The aim of this approach was to identify the best methodological and judgmental options to obtain the most accurate results.

The present study aims at a comprehensive analysis of current research on OP-PM. It uses a bibliometric analysis and systematic review approach. The study aims to provide a strategic framework for the advancement of the field. It also addresses perspectives on emerging trends. By analyzing 569 articles on OP-PM using the PRISMA methodology, 368 articles were selected for analysis, revealing robust international collaboration with an average coauthorship rate of 8.11 per document and a cross-country coauthorship rate of approximately 41%. The results highlight the recent evolution of the field, as evidenced by the average age of the article collection of 5.53 years and a significantly elevated citation rate of 28.93. This indicates substantial interest in the topic. In particular, the number of publications in the 2010s has increased 24 times compared to the early 2000s, representing an exponential annual growth rate of 26.1%, underscoring the growing importance of PM-OP research.

Analysis of the articles collected revealed that a small group of eight journals represented more than ten published articles. The remaining content was spread over 75 different sources. The top ten journals, each of which contributed an average of 22 articles, account for approximately 60% of the total collection. With 15.2% of publications, the ATMOS ENVIRON journal is the most frequently chosen journal. Environmental Sciences and Meteorology and Atmospheric Sciences were the main disciplines of these journals. Furthermore, this study identified the top 10 contributors, representing 0.58% of the authors and producing at least 16 articles on the topic. In particular, Schauer, JJ, Sioutas, C, and Kelly, FJ emerged as the most productive authors. This was reflected in their local citations and H-indices. The analysis of the coauthor network revealed different levels of collaboration among the authors. These key contributors play a critical role in shaping the field while highlighting the importance of increased collaboration for further progress.

The study also found that 18% of the publications came from the top 10 institutions. This underscores the concentration of research in a limited number of institutions with a notable focus on academia and universities. However, the absence of private sector contributions is notable. In addition, the global dimension of this scientific endeavor was underscored by a significant trend toward collaboration between institutions in the United States and European countries. The importance of research goes beyond local boundaries, as evidenced by the high LC/GC ratio. It has relevance to broader disciplines such as toxicology and environmental epidemiology. The direct impact of OP metrics on human health may be the reason for this. In particular, authors Charrier JG and Fang T stood out with two contributions to the most influential literature in the field.

Key global research hotspots on OP-PM were identified through a dual-method approach combining cooccurrence analysis between Web of Science subject categories and author keywords and detailed article review. Environmental Sciences, Meteorology & Atmospheric Sciences, Environmental Engineering, Public, Environmental & Occupational Health, and Toxicology are the predominant thematic areas. The research was also grouped into four key clusters: biological, chemical and health sciences; engineering, environmental, and chemical analysis; and sustainable infrastructure. Keyword analysis resulted in seven clusters. These clusters highlighted the main research directions. The focus of OP-PM research and the gaps for future investigation were efficiently identified using this dual-method analysis.

Several prominent research areas, including environmental science, atmospheric science, environmental engineering, public health, and toxicology, emerged from the co-occurrence analysis of global OP-PM research hotspots. Evaluation of OP and understanding of the causes of air pollution using environmental PM samples are the main focus. Due to their cost-effectiveness and efficiency, acellular assays, particularly the DTT assay, have been used. There is a bias toward research in developed countries, an indication of the high level of interest. The impact is particularly observed in the health sector. The correlation between OP and health outcomes is emphasized. This supports OP as a potential metric for the development of more health-centric air quality standards and PM mitigation strategies. Future research directions are suggested, given the significant impact of PM and its OP on human health and environmental sustainability. These include strengthening capacity building and innovation, such as the development of advanced OP measurement instrumentation, and fostering interdisciplinary collaboration. A detailed investigation of the emission sources, atmospheric processes, and effects of PM and its OP is recommended. A better understanding of personal exposures in different environments and the impacts of emerging pollutants such as nanoparticles is essential. Considering the impact of climate change on OP-PM and the regulatory context for PM research is also critical. Pursuing these research directions will deepen our understanding of PM and its effects. This will support more effective air quality management and public health protection strategies.

A significant advance in atmospheric science is the study of OP-PM. OP-PM is being positioned as a more appropriate metric and/or proxy for PM exposure (from a conceptual and real-world perspective) to assess and understand the underlying mechanisms of exposure action in terms of the relationship between PM mass concentration induced on health and environmental effects. Continued efforts in this direction will contribute to a more comprehensive understanding of the risks associated with PM and will support the development of targeted mitigation strategies to protect public health and environmental sustainability.

CRediT author statement

Luis Felipe Sánchez P: Conceptualization, Data Curation, Formal analysis, Visualization, Methodology, Investigation, Writing, Francisco Catalan: Data Curation, Formal analysis, Visualization, Loreto Villacura: Data Curation, Formal analysis, Visualization, Richard Toro A.: Conceptualization, Editing, Manuel Leiva G.: Founding, Conceptualization, Methodology, Investigation, Writing - Reviewing and Editing
Acknowledgments: The authors acknowledge and are grateful for the contribution of Sciome (https://www.sciome.com) for providing free of charge the SWIFT-Active Screener software, which allowed a fast and efficient systematic review of the articles included in this study.This research was partial funded by Vicerrectoría de Investigación y Desarrollo (VID), Universidad de Chile, Programa de Apoyo a Proyectos de Enlace con Concurso Fondecyt Regular VID 2020 and 2023, grant nº ENL17/20 and ENL21/23, and Chilean National Fund for Scientific and Technological Development (FONDECYT) ANID FONDECYT Regular Grant No. 1220948 LV acknowledges support from ANID: 2021 National Doctoral degree Scholarship Program, no. Nº 21212276. LFS acknowledges support from ANID, CONICYT: 2019 National Doctoral degree Scholarship Program, no. 21191906.

Declaration of competing interest: All authors declare that they have no conflicts of interest.

Data Availability Statement: Data supporting the findings presented in this study are available in the Supplementary Materials online in: https://t.ly/uZjdd or by request to the corresponding author.

Disclaimer of Generative AI and AI-assisted technologies in the writing process: During the preparation of this paper, the author(s) used CHatGPT to edit and proofread English writing. After using this tool/service, the author(s) reviewed and edited the content as needed. The author(s) assumes full responsibility for the content of the publication.

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"article collection" by exporting to an Excel bibliometrics file using Biblioshiny

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Oxidative Potential of Airborne Particulate Matter: A Bibliometric Analysis of Current Research, Challenges, and Future Perspectives.

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Abstract

Figures

1. Introduction

2. Materials and Methods

3. Results and Discussion

3.1. PRISMA and descriptive analysis of the data

3.2. Scientific Productivity by Country, Corresponding Authors’ Countries, and International Collaboration

3.3. Analysis of Core Journal and Journal Distribution

3.4. Most relevant authors and cocitation network

3.5. Characteristics of the Institutions Contribution

3.6. Most Local and Global Cited Documents

3.7. Research hotspot analysis

4. Future research directions and remarks

5. Study limitations: Strengths and weaknesses

6. Conclusion

Declarations

References

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