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Environ Anal Health Toxicol > Volume 36:2021 > Article
Lee, Lim, and Kim: Publication trends in research on particulate matter and health impact over a 10-year period: 2009–2018

Abstract

Exposure to ambient particulate matter is a major health risk factor for numerous diseases, including those of the cardiovascular and respiratory varieties. The aim of this study was to estimate the latest global research activities regarding particulate matter and health impact. We performed a bibliometric analysis of this field’s scientific publication trends over a decade (2009–2018). Publications were retrieved from the Scopus and Web of Science databases using the search terms “particulate matter,” “fine particulate matter,” “health impact,” and their synonyms. The literature on health impact in the research fields of particulate matter (PM10) and fine particulate matter (PM2.5) trended to significantly increase over the decade in consideration. It appears to have been led by researchers of the United States and China. Worldwide research on particulate matter and health effects has focused primarily on respiratory and cardiovascular diseases. The precursors to and components of particulate matter (such as nitrogen dioxide, polycyclic aromatic hydrocarbon, sulfur dioxide, and black carbon) were also popular research topics in this field. Research on children, older adults, and pregnant women, who are most vulnerable to the health effects of air pollution, has increased dramatically over the past 10 years. Our findings provide the information necessary to predict unmet research topics and future research needs.

Introduction

Particulate matter in which 50% of the particles have an aerodynamic diameter lower than 10 μm—PM10—is a major outdoor air pollutant [1]. Its detrimental effects include causing or worsening various diseases, including those of the cardiovascular and respiratory varieties [2]. Fine particulate matter with particulate size less than 2.5 μm (PM2.5) is more harmful to health than PM10 [3]. As per the Global Burden of Diseases Study 2015, long-term exposure to ambient PM2.5 was the fifth leading worldwide cause of mortality, responsible for 4.2 million deaths (95% uncertainty interval [UI] 3.7–4.8 million) [4].
Children, older adults, and pregnant women are especially vulnerable to the health effects of air pollution [57]. PM10 [8,9] and PM2.5 [8,10,11] have been shown to be associated with the development and worsening of asthma in about 10% of children worldwide. In the context of older adults, studies have reported a relationship between PM2.5 exposure and increased premature mortality rates [12,13]. Further, PM2.5 exposure during pregnancy is said to be associated with low birth weight and preterm birth [14], as well as the development of asthma at six years of age in boys [15].
Bibliometric analysis is a statistical method for analyzing research trends in a given domain, useful for comparing the contributions of countries, institutions, and journals [16]. While bibliometric studies on PM2.5 [17] and air pollution and human health [18] have been reported, particulate matter and its health impact has not yet been the subject of such analysis. Therefore, this study aims to analyze research trends and keywords regarding the particulate matter and associated health impacts over a 10-year period (2009–2018).

Methods

Data sources and search strategy

The literature searches in Scopus and Web of Science (WoS) were performed on August 27 and September 4, 2019, respectively. The WoS database included the Science Citation Index Expanded and Emerging Sources Citation Index.
The search terms “particulate matter,” “fine particulate matter,” “health impact,” and their synonyms were sought in the titles, abstracts, or keywords of studies available in the abovementioned databases. This study focuses only on journal articles; therefore, other document types (conference papers, reviews, notes, book chapters, conference reviews, errata, letters, short surveys, editorials, data papers, business articles, abstract reports, books, proceeding papers, meeting abstracts, editorial material, early access papers, corrections, news items, reprints, and retracted publications) were excluded.
The search process was as follows: publications on PM10 and PM2.5 published from 2009 to 2018 were first retrieved from Scopus and WoS. Then, health impact-associated publications were extracted from the retrieved PM10 and PM2.5-related publications. Subsequently, publications related to children, older adults, and pregnant women were retrieved from within the research field on PM2.5 and health impacts on December 8, 2020. Data regarding retrieved articles, including subject area (research area), source title (journal), country or territory (location), and affiliation (organization) were downloaded from the databases. The data on research area, journal, and organization of the publications downloaded from WoS were analyzed using InCites, an online bibliometric analysis tool (http://incites.clarivate.com).
The key phrase analysis for particulate matter and health impact-related publications retrieved from Scopus was performed using SciVal, another online bibliometric analysis tool (https://www.scival.com) on December 8–9, 2020. It was possible to analyze the literature from only 2010 to 2018 using SciVal. Therefore, the research direction of the last 10 years was estimated using this data. Search terms and their synonyms were excluded in the top 15 keywords.

Statistical analysis

Linear regression analysis to assess publication trends in the time period of interest was conducted using SPSS version 18.0 (IBM Inc, Chicago, IL, USA). Statistical significance was set at p < 0.05.

Publication trends over 10 years

Using Scopus, we identified a total of 48,267 PM10-related articles published from 2009 to 2018, of which 10,520 (21.8% of the total PM10-related articles) concerned health impact (Figure 1). A total of 33,048 PM10-related publications were retrieved from WoS, of which 9,738 (29.5%) were health impact-related. The overall number of health impact-oriented studies retrieved from Scopus (linear regression, correlation coefficient (r) = 0.983; p < 0.001) and WoS (r = 0.965; p < 0.001) increased at an annual average of 12.5% and 14.6%, respectively.
Of the 20,462 PM2.5-related articles published between 2009 and 2018 retrieved from Scopus, 4,708 (23.0% of the total PM2.5-related articles) were health impact-related. In WoS, of the 21,526 PM2.5-related publications identified, 5,711 (26.5%) were health impact-related. The overall numbers of health impact-oriented studies retrieved from Scopus (r = 0.955; p < 0.001) and WoS (r = 0.957; p < 0.001) demonstrated an increasing annual trend over 10 years, with average rates of 16.7% and 19.0%, respectively. Among the number of publications on particulate matter (both PM10 and PM2.5), the proportion of publications on health impact trended to increase every year.

Country, organization-stratified, and journal analyses

Furthermore, we analyzed the data on countries, institutions, and journals that have directed research on the health effects of particulate matter, focusing on the period during 2014–2018 to observe the recent trends. Researchers from more than 100 countries or territories (Scopus = 132; WoS = 109) published articles on PM10 and health impact between 2014 and 2018 (Figure 2). By country, the maximum volume was associated with the United States (Scopus = 2,362, 35.0% of all publications on PM10 and health impact; WoS = 2,409, 37.1%) and China (Scopus = 1,728, 25.6%; WoS = 1,747, 26.9%). Health impact-related publications from the United States and China alone accounted for more than 30% (Scopus = 32.1%; WOS = 38.7%) and 18% (Scopus = 18.7%; WoS = 27.9%) of all PM10-related articles, respectively.
Approximately 100 countries or territories (Scopus = 103; WoS = 94) contributed to research related to PM2.5 and its health impact between 2014 and 2018. By country, the maximum volume was associated with the United States (Scopus = 1,281, 38.2% of all publications on PM2.5 and health impact; WoS = 1,654, 40.1%) and China (Scopus = 1,082, 32.3%; WoS = 1,311, 31.8%). Health impact-related publications from the United States and China alone represented about 40% (Scopus = 39.2%; WoS = 41.3%) and over 19% (Scopus = 19.1%; WoS = 25.1%) of all PM2.5-related articles, respectively.
We also investigated the 10 organizations that published the most articles on PM10 and health impact between 2014 and 2018 (Table 1a). In Scopus, the Chinese Academy of Sciences had the highest number of articles (278, 4.1% of all publications on PM10 and health impact), followed by the Harvard School of Public Health (235, 3.5%) and Peking University (194, 2.9%). In WoS, Harvard University had the highest number of articles (389, 6.0%), followed by the University of California system (303, 4.7%) and Chinese Academy of Sciences (297, 4.6%).
The 10 journals that published the most articles on particulate matter (PM10 and PM2.5) and health impact between 2014 and 2018 are presented in Table 1a. More than 30% (Scopus = 2,280, 33.8%; WoS = 2,235, 34.4%) of the articles on PM10 and health impact were published in the top 10 journals. As per Scopus data, Science of the Total Environment (impact factor = 5.589, 2018) was the most preferred journal (376, 5.6% of all publications on PM10 and health impact), followed by Atmospheric Environment (impact factor = 4.012, 2018) with 281 articles (4.2%), Environmental Pollution (impact factor = 5.714, 2018) with 249 articles (3.7%), and Environmental Research (impact factor = 5.026, 2018) with 249 articles. In WoS, Science of the Total Environment was the most common journal (341, 5.3%), followed by Atmospheric Environment with 284 publications (4.4%) and International Journal of Environmental Research and Public Health (impact factor = 2.468, 2018) with 249 publications (3.8%).
More than 35% (Scopus = 1,208, 36.1%; WoS = 1,569, 38.0%) of articles on PM2.5 and health impact were published in the top journals (Table 1b). In both Scopus and WoS, Science of the Total Environment was the most preferred journal (Scopus = 173, 5.2% of all publications on PM2.5 and health impact; WoS = 205, 5.0%), followed by Atmospheric Environment with 156 (4.7%) and 201 articles (4.9%), and International Journal of Environmental Research and Public Health with 142 (4.2%) and 184 articles (4.5%), respectively.

Keyword analyses

In order to identify directions and key themes in the research on particulate matter (PM10 and PM2.5) and health impact, we analyzed the top 15 keywords used in publications published between 2010 and 2018 available in Scopus. In PM10–related articles, the most frequently used keyword was “mortality” (3,327, 33.2% of all publications on PM10 and health impact) (Table 2 and Supplemental Figure 1). Disease-related terms such as “cardiovascular disease” (1,865, 18.6%), “asthma” (1,727, 17.2%), and “respiratory tract infection” (1,709, 17.0%) were ranked in the top 15. Air pollutant-related terms (such as “nitrogen dioxide”, “polycyclic aromatic hydrocarbon”, and “sulfur dioxide”) were also included in the top 15.
In case of PM2.5–related articles, the most frequently used keywords in these articles were “ozone” (895, 18.7% of all publications on PM2.5 and health impact), followed by “asthma” (883, 18.4%) and “environmental exposure” (872, 18.2%) (Table 3 and Supplemental Figure 2). The terms “respiratory tract infection” (827, 17.3%), “chronic obstructive lung disease” (419, 8.7%), and “lung neoplasm” (354, 7.4%) were ranked 4th, 11th, and 15th, respectively. Air pollutant-related terms (such as “nitrogen dioxide”, “polycyclic aromatic hydrocarbon”, “exhaust gas” and “black carbon”) and “indoor air pollution” were also included in the top 15.
PM2.5 is more harmful to health than PM10 [3]. Thus, the keyword analysis was conducted to identify key research topics especially in the United States and China, as they appear to have been leading the global research on the health effects of PM2.5. Keywords used in PM2.5 and health–related publications from the United States and China were compared. “Mortality” (438, 23.0%; 222, 16.7%), “respiratory tract infection” (193, 10.1%; 173, 13.0%), and “indoor air pollution” (174, 9.1%; 72, 5.4%) were the main keywords in publications from both countries. “Cardiovascular disease” (327, 17.2%), “asthma” (203, 10.7%), “ozone” (286, 15.0%), “nitrogen dioxide” (163, 8.6%), and “black carbon” (148, 7.8%) were the high-frequency keywords in publications from the United States but not China. “Polycyclic aromatic hydrocarbon” (111, 8.4%), “heavy metal” (91, 6.9%), “lung neoplasm” (91, 6.9%), and “chronic obstructive lung disease” (67, 5.0%) were highly frequent only in Chinese publications.

Subject area

We analyzed the subject areas (Figure 3) of publications on PM2.5 and health impact published between 2014 and 2018. According to the distribution of subject categories, environmental science (2,046, 61.1% of all publications on PM2.5 and health impact) and medicine (1,135, 33.9%) were the most common as per Scopus. In case of WoS, environmental science (2,489, 60.3%) and public, environmental, and occupational health (1,054, 25.5%) were the most common categories. The respiratory system (130, 3.1%) and cardiac and cardiovascular systems (76, 1.8%) stood at the seventh and ninth spots, respectively.

Publication trends regarding vulnerable population groups

As mentioned previously, children, older adults, and pregnant women can be more vulnerable to the health impact of particulate matter than other populations. Thus, we examined how much research attention the health impact of PM2.5 in these groups has received over 10 years. Articles on children, older adults, and pregnant women were extracted from studies on PM2.5 and health impact published between 2009 and 2018 (Figure 4). In Scopus, the number of publications on children, older adults, and pregnant women were 732 (15.5% of total publications on PM2.5 and health impact), 406 (8.6%), and 196 (4.2%) and increased 5.0, 5.1, and 7.2 times in 10 years, respectively. In WoS, the number of publications on children, older adults, and pregnant women were 721 (12.6%), 263 (4.6%), and 178 (3.1%) and increased 4.1, 3.4, and 6.3 times in 10 years, respectively. Thus, the increase in publications on pregnant women over the past 10 years is greater than the growth in the literature on PM2.5 and health effects; publications on PM2.5 and health effects increased 3.5 and 4.6 times in Scopus and WoS data, respectively.

Keyword analyses of publications on vulnerable population groups

The most frequently used keywords in articles on children, older adults, and pregnant women, which were published between 2010 and 2018, were “asthma” (209, 29.7% of children-related articles), “mortality” (139, 35.8% of the articles on older adults), and “exposure” (169, 88.9% of pregnant women-related articles), respectively (Table 4 and Supplemental Figure 3). “Asthma”, “respiratory tract infection” (158, 22.5%), “cardiovascular disease” (65, 9.2%), and “respiratory tract disease” (61, 8.7%) were the main keywords in children-related publications. Air pollutant-related terms such as “nitrogen dioxide” (112, 15.9%) and “polycyclic aromatic hydrocarbon” (54, 7.7%) also ranked in the top 15. Six disease-associated terms including “cardiovascular diseases” (126, 32.5%), “respiratory tract infection” (64, 16.5%), “chronic obstructive lung disease” (43, 11.1%), “stroke” (34, 8.8%), “asthma” (31, 8.0%), and “respiratory tract disease” (28, 7.2%) were among the top 15 keywords in the publications on older adults. Fetal growth-related terms such as “birth weight” (40, 21.1%), “low birth weight infant” (30, 15.8%), and “low birth weight” (18, 9.5%) were among the main keywords in publications on pregnant women.

Discussion and conclusion

In this study, we analyzed the trends in scientific publications on the health impact of PM10 and PM2.5 over 10 years (2009–2018). The number of studies on health impacts in both the PM10 and PM2.5 research fields increased rapidly over the period under consideration.
This increase appears to be spearheaded by researchers from the United States and China. China contributed approximately half of the number of studies published by researchers from the United States. According to the 2015 Global Burden of Disease study, China ranked highest with regard to the global disease burden attributable to ambient PM2.5 exposure [4]; PM2.5 caused 1.1 million (95% UI 1.0 million–1.8 million) deaths in China, accounting for more than 26% of the 4.2 million (95% UI 3.7 million–4.8 million) deaths worldwide. In the United States, the number of PM2.5 deaths was 88,400 (95% UI 66,800–115,000), approximately one-twelfth of the deaths in China. It can, thus, be assumed that awareness of the seriousness of the health effects of particulate matter is higher in the United States than in China.
The top 10 organizations that contributed to articles on the health impact of PM10 and PM2.5 from 2014 to 2018 were mostly from the United States and China, demonstrating that world-class research institutions play an important role in improving national research performance.
Science of the Total Environment and Atmospheric Environment published the most articles (about 10%) on health effects of PM10 and PM2.5 from 2014 to 2018. The top 10 journals have contributed to more than 30% of the literature on health effects in both the PM10 and PM2.5 research fields and are expected to continue to lead the way with regard to the study of particulate matter and its health effects. 106
To identify the major topics in research on particulate matter and health effects, we performed a keyword analysis. Air pollutants such as “nitrogen dioxide”, “polycyclic aromatic hydrocarbon”, “sulfur dioxide”, and “black carbon” were high-frequency keywords in global publications on particulate matter (PM10 or PM2.5) and health effects. This finding indicates that the precursors to and components of particulate matter have been frequently studied in this field. Respiratory diseases seem to have been key topics in the field of research on the health impact of PM10 and PM2.5. While “mortality” was most often used in publications related to PM10 and health effects, it was not included in the top 15 in PM2.5 and health effects. Since PM2.5 is smaller than PM10, it is able to reach small airways and pulmonary alveoli [19] and move to other organs through the circulatory system [2], thereby posing a greater threat [3]. In global publications, the terms “asthma”, “respiratory tract infection”, “chronic obstructive lung disease”, and “lung neoplasm” were frequently used as keywords in the literature on PM2.5 and health impact. “Cardiovascular disease” was the high-frequency keyword in publications from the United States. Many researchers have reported that ambient PM2.5 is an important risk factor for cardiovascular disease [4,20] and respiratory diseases including lung cancer [4,21], chronic obstructive pulmonary disease [4,22], and asthma [23,24]. In addition, PM2.5 can play a role in depressive symptoms [25,26], stroke [20,2629], dementia [29], Alzheimer’s disease [29], diabetes mellitus [30], and ocular diseases [31,32], but there is a lack of research in this regard. To better understand the health effects of PM2.5, research on various diseases should be actively conducted. We also identified that articles published in the United States and China had different keyword patterns; for example, “cardiovascular disease” and “asthma” were frequently used only in American publications, while “lung neoplasm” and “chronic obstructive lung disease” were highly frequent in only Chinese publications.
The number of publications related to the susceptible groups of children, older adults, and pregnant women in the field of PM2.5 and health effects has increased significantly over 10 years but remains small, indicating the possibility of expansion of such research. As per keyword analysis, the major research topics for these three populations differed. In studies on children, respiratory diseases such as asthma were a popular topic. Air pollutants such as nitrogen dioxide and polycyclic aromatic hydrocarbon, of which diesel engines are the main source, also seem to have attracted scholarly attention. A previous study has reported that PM2.5 and nitrogen dioxide are involved in the exacerbation of childhood asthma [8]. In studies on older adults, mortality was the most popular research topic, and the names of diseases (“cardiovascular diseases”, “respiratory tract infection”, “chronic obstructive lung disease”, “stroke”, “asthma”, and “respiratory tract disease”) were highly frequent. Studies on pregnant women seemed to focus on fetal health rather than the health of the women.
Despite the fact that it revealed some important trends, our bibliometric approach had certain limitations. First, all scientific publications in the fields of particulate matter (both PM10 and PM2.5) and health impact were not necessarily included. Second, the weight of importance of each article was not reflected. Third, bibliometric databases might contain erroneous data [18]. Fourth, the literature extracted may differ slightly depending on the time of search. Nevertheless, the bibliometric method is an objective and comprehensive way to analyze publication trends [33]. Moreover, we used two bibliometric tools, thereby increasing the credibility of the data.
We have demonstrated that, worldwide, there has been an impressive increase in the number of scientific publications in the fields of particulate matter and health impact over 10 years. This increase appears to be led by world-class research institutes in the United States and China. A large amount of work on particulate matter and health effects has focused on respiratory and cardiovascular diseases. The precursors to and components of particulate matter (such as nitrogen dioxide, polycyclic aromatic hydrocarbon, sulfur dioxide, and black carbon) were also core research topics. Studies on children, older adults, and pregnant women in the field of PM2.5 and health impact are expected to expand greatly in the future.

Acknowledgement

This research was supported by funds (2019-NI-102-00, 2019-NI-098-00) from the Korea Centers for Disease Control and Prevention.

Conflict of interest

Conflict of interest
The authors have no conflicts of interest associated with the material presented in this paper.

Notes

CRediT author statement
JEL: Conceptualization, Methodology, Writing - Original, Writing - Reviewing and Editing, Visualization. HJL: Writing - Reviewing and Editing, Supervision, Project administration, Funding acquisition. YYK: Supervision, Project administration, Funding acquisition.

Notes

Supplementary Material
Add short descriptions of supplementary material. This material is available online at www.eaht.org.

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Figure 1
Publication trends of research on particulate matter and health impact. (A, C): Scopus data; (B, D): Web of Science data.
eaht-36-1-e2021005f1.jpg
Figure 2
Top 10 countries regarding publication contribution. (A, C): Scopus data; (B, D): Web of Science data.
eaht-36-1-e2021005f2.jpg
Figure 3
Top 10 subject categories of research on particulate matter and health impact. (A) Subject areas of publications on PM10 and health impact; (B) Subject areas of publications on PM2.5 and health impact.
eaht-36-1-e2021005f3.jpg
Figure 4
Trends of publications on PM2.5 and health impact in children, older adults, and pregnant women. (A) Scopus data; (B) Web of Science data.
eaht-36-1-e2021005f4.jpg
Table 1a
Top 10 organizations and journals regarding research on PM10 and health impact (2014–2018).
Scopus database Web of Science database


Organization Journal Organization Journal


Total number of publications : 6,751 Total number of publications : 6,490
Chinese Academy of Sciences 278 Science of the Total Environment 376 Harvard University 389 Science of the Total Environment 341
Harvard School of Public Health 235 Atmospheric Environment 281 University of California system 303 Atmospheric Environment 284
Peking University 194 Environmental Pollution 249 Chinese Academy of Sciences 297 International Journal of Environmental Research and Public Health 249
Ministry of Education China 173 Environmental Research 249 Harvard T.H. Chan School of Public Health 294 Environmental Pollution 245
United States Environmental Protection Agency 153 International Journal of Environmental Research and Public Health 244 Peking University 215 Environmental Research 235
Fudan University 148 Environment International 239 United States Environmental Protection Agency 189 Environment International 222
University of Washington, Seattle 131 Environmental Science and Pollution Research 189 Fudan University 154 Environmental Science and Pollution Research 203
Utrecht University 115 Environmental Health Perspectives 161 University of Washington Seattle 144 Environmental Health Perspectives 170
Tsinghua University 114 Environmental Science and Technology 158 University of Washington 144 Environmental Science & Technology 146
Swiss Tropical and Public Health Institute Swiss TPH 107 PLOS ONE 134 University of London 127 PLOS ONE 140
Table 1b
Top 10 Organizations and journals regarding research on PM2.5 and health impact (2014–2018).
Scopus database Web of Science database


Organization Journal Organization Journal


Chinese Academy of Sciences 179 Science of the Total Environment 173 Harvard University 254 Science of the Total Environment 205
Harvard School of Public Health 166 Atmospheric Environment 156 Chinese Academy of Sciences 223 Atmospheric Environment 201
Peking University 118 International Journal of Environmental Research And Public Health 142 University of California system 209 International Journal of Environmental Research and Public Health 184
Ministry of Education China 111 Environmental Research 141 Peking University 154 Environment International 179
Fudan University 94 Environment International 139 Harvard T.H. Chan School of Public Health 143 Environmental Pollution 179
United States Environmental Protection Agency 86 Environmental Pollution 139 United States Environmental Protection Agency 118 Environmental Research 161
University of Washington, Seattle 83 Environmental Health Perspectives 93 University of Washington Seattle 113 Environmental Health Perspectives 133
Tsinghua University 81 Environmental Science and Pollution Research 84 University of Washington 113 Environmental Science and Pollution Research 123
Emory University 72 Environmental Science And Technology 78 Fudan University 110 Environmental Science & Technology 110
Harvard Medical School 67 PLOS ONE 63 Tsinghua University 102 Aerosol and Air Quality Research 94
Table 2
Keyword analysis of articles on PM10 and health impact (2000–2018).
Keywords Count %
Mortality 3,327 33.2
Cardiovascular Disease 1,865 18.6
Environmental Exposure 1,809 18.0
Ozone 1,789 17.8
Asthma 1,727 17.2
Respiratory Tract Infection 1,709 17.0
Exhaust Gas 1,535 15.3
Nitrogen Dioxide 1,463 14.6
Polycyclic Aromatic Hydrocarbon 1,274 12.7
Indoor Air Pollution 1,141 11.4
Sulfur Dioxide 1,095 10.9
Smoke 897 8.9
Time Series Analysis 876 8.7
Chronic Exposure 820 8.2
Beijing 722 7.2
Table 3
Keyword analysis of articles on PM2.5 and health impact (2000–2018).
Rank All countries Count % USA Count % China Count %
1 Ozone 895 18.7 Mortality 438 23.0 China 769 58.0
2 Asthma 883 18.4 Environmental Exposure 435 22.8 City 308 23.2
3 Environmental Exposure 872 18.2 Cardiovascular Disease 327 17.2 Beijing 241 18.2
4 Respiratory Tract Infection 827 17.3 Ozone 286 15.0 Mortality 222 16.7
5 Nitrogen Dioxide 644 13.4 Chronic Exposure 211 11.1 Respiratory Tract Infection 173 13.0
6 Indoor Air Pollution 634 13.2 Asthma 203 10.7 Environmental Exposure 163 12.3
7 Polycyclic Aromatic Hydrocarbon 588 12.3 Respiratory Tract Infection 193 10.1 Polycyclic Aromatic Hydrocarbon 111 8.4
8 Chronic Exposure 531 11.1 Exhaust Gas 191 10.0 Time Series Analysis 93 7.0
9 Exhaust Gas 508 10.6 Indoor Air Pollution 174 9.1 Heavy Metal 91 6.9
Lung Neoplasm 91 6.9
10 Beijing 507 10.6 Nitrogen Dioxide 163 8.6 Haze 88 6.6
11 Chronic Obstructive Lung Disease 419 8.7 Black Carbon 148 7.8 Exhaust Gas 85 6.4
12 Smoke 412 8.6 Premature Mortality 142 7.5 Chronic Exposure 75 5.7
13 Black Carbon 399 8.3 California 135 7.1 Indoor Air Pollution 72 5.4
14 Hospital Admission 366 7.6 Smoke 133 7.0 Air Pollution Control 71 5.4
15 Lung Neoplasm 354 7.4 Cooking 112 5.9 Chronic Obstructive Lung Disease 67 5.0
Table 4
Keyword analysis of articles on PM2.5 and health impact in children, older adults, and pregnant women (2000–2018).
Rank Child count % Older adults count % Pregnant women count %
1 Asthma 209 29.7 Mortality 139 35.8 Exposure 169 88.9
2 Child Health 202 28.7 Environmental Exposure 138 35.6 Maternal Exposure 91 47.9
3 Environmental Exposure 178 25.3 Cardiovascular Disease 126 32.5 Mother 82 43.2
4 Respiratory Tract Infection 158 22.5 Chronic Exposure 78 20.1 Prenatal Exposure 59 31.1
5 Ozone 118 16.8 Nitrogen Dioxide 65 16.8 Pregnancy Outcome 54 28.4
Ozone 65 16.8
6 Nitrogen Dioxide 112 15.9 Respiratory Tract Infection 64 16.5 Cohort Study 49 25.8
7 Indoor Air Pollution 110 15.6 Hospitalization 61 15.7 Environmental Exposure 41 21.6
75 10.7 Time Series Analysis 61 15.7
8 School Hospital Admission 58 14.9 Birth Weight 40 21.1
9 School child 69 9.8 Chronic Obstructive Lung Disease 43 11.1 Premature Labor 32 16.8
10 Cardiovascular Disease 65 9.2 Crossover Procedure 36 9.3 Birth Cohort 30 15.8
Low Birth Weight Infant 30 15.8
11 Exhaust Gas 64 9.1 Stroke 34 8.8 Nitrogen Dioxide 28 14.7
12 Lung Function 63 9.0 Sulfur Dioxide 32 8.2 Child Health 27 14.2
Time Series Analysis 63 9.0
13 Hospital Emergency Service 61 8.7 Asthma 31 8.0 Land Use 25 13.2
Respiratory Tract Disease 61 8.7 Maternal Welfare 25 13.2
14 Hospital Admission 58 8.3 Respiratory Tract Disease 28 7.2 Early Life 19 10.0
15 Polycyclic Aromatic Hydrocarbon 54 7.7 Mortality Risk 25 6.4 Low Birth Weight 18 9.5
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