BACKGROUND: Traffic-related air pollution (TRAP) has significant public health implications and a wide range of adverse health effects, including cardiovascular, respiratory, pulmonary, and other health problems. This study aimed to determine the public health impacts of traffic-related air pollution across the world that can be used as an input for protecting human health.
METHODS: This study considered studies conducted across the world and full-text articles written in English. The articles were searched using a combination of Boolean logic operators (AND, OR, and NOT), MeSH, and keywords from the included electronic databases (SCOPUS, PubMed, EMBASE, Web of Science, CINAHL, and Google Scholars). The quality assessment of the articles was done using JBI tools to determine the relevance of each included article to the study.
RESULTS: In this study, 1 282 032 participants ranging from 19 to 452 735 were included in 30 articles published from 2010 to 2022. About 4 (13.3%), 9 (30.0%), 12 (40.0%), 8 (26.7%), 2 (6.7%), 15 (50.0%), 3 (10.0%), 3 (10.0%) 1 (3.3%), and 3 (10.0%) of articles reported the association between human health and exposure to CO, PM10, PM2.5, NOx, NO, NO2, black carbon, O3, PAH, and SO2, respectively. Respiratory diseases, cancer, cognitive function problems, preterm birth, blood pressure and hypertension, diabetes, allergies and sensitization, coronary heart disease, dementia incidence, and hemorrhagic stroke were associated with exposure to TRAP.
CONCLUSIONS: Exposure to nitrogen dioxide, nitrogen oxide, sulfur dioxide, and fine particulate matter was associated with various health effects. This revealed that there is a need for the concerned organizations to respond appropriately.
Introduction
Urban areas are hot spots for human exposure to air pollution, mainly originating from road traffic. Understanding efforts to curb traffic related air pollution (TRAP) in urban areas is particularly critical, as the world is currently witnessing its largest urban growth in human history. According to the United Nations and the Department of Economic and Social Affairs Population Division, about two-thirds of the world’s population is estimated to reside in urban areas by 20501 meaning more people will be at risk of exposure to TRAPS.
Traffic-related air pollution is a complex mixture of particulate matter (PM2.5 and PM10) derived from combustion and non-combustion sources such as road dust, tire wear, and brake wear, as well as primary gaseous emissions, including nitrogen oxides. These primary emissions lead to the generation of secondary pollutants such as ozone, nitrates, and organic aerosols that can cause various health problems, including asthma and other health conditions.2
The prominence of traffic emissions and TRAPS has great implications for human exposure and its wide range of adverse health effects. Traffic emissions disperse into the ambient air that humans are exposed to and cause health impacts that result from direct exhaust emissions or non-exhaust emissions.3,4 Exposure to air pollution increases health risks, including adverse cardiovascular, respiratory, pulmonary, and other health-related outcomes. Particularly, low-income countries suffer the highest burden of disease and premature death attributable to environmental pollution.5
According to some findings, the health impacts associated with TRAPS have proven costly, including the cost of death from ambient air pollution (over $496 000 000 in the United States, $201 000 000 in Japan, $148 000 000 in Germany, $102 000 000 in Italy, and $85 000 000 in the United Kingdom). Despite the growing awareness of the links between traffic, air pollution exposure, and associated adverse health impacts, many cities across the globe struggle to meet the air quality guidelines set to protect public health.6
Various review articles are conducted on the impacts of traffic-related air pollution on specific health conditions, such as lung function and other respiratory illnesses.7,8 In addition, the previous studies addressed some air pollutants such as NO2, elemental carbon, and PM2.57,9 and were conducted on a specific group of the population, particularly among students7 and children.8 However, the current study addressed the health impacts of various traffic-related air pollutants in addition to those addressed by previous studies, such as CO, NOx, NO, O3, PAH, and SO2, and multiple health conditions, including respiratory diseases, cancer, cognitive function problems, preterm birth, coronary heart disease, allergic diseases, dementia, and hemorrhagic stroke.
Materials and Methods
The Preferred Reporting Items for Systematic Reviews and Meta-Analysis (PRISMA) guideline was used to perform this systematic review.10
Eligibility criteria
Articles that met the following predetermined inclusion criteria were included in the systematic review.
i. Location: This study included traffic-related air pollutants across the world, regardless of their location and their health impacts.
ii. Study design: There was no restriction based on the study design used in the study.
iii. Types of pollutants: carbon monoxide (CO), nitrogen monoxide (NO), nitrogen dioxide (NO2), nitrogen oxide (NOx), sulfur dioxide (SO2), ozone (O3), and particulate matter (PM2.5 and PM10) were included in the current study.
iv. Outcome: Studies reported quantitative outcomes (magnitude, frequency, rate, or prevalence).
v. Language: Studies written in English
Sources of information and Search Strategies
The searches of the literature were performed by the authors using keywords from search strategies such as the databases SCOPUS, PubMed, Embase, Web of Science, CINAHL, and Google Scholars from June 1, 2023, to December 30, 2023. Articles were searched using a combination of Boolean logic operators (“AND, OR, and NOT”), medical subject headings (MeSH), and keywords. The authors used the following main keywords to search articles from the included electronic databases: public health OR health impacts, OR health consequences, OR asthma (related terms), OR respiratory disease, OR respiratory illness OR cancer, AND traffic-related air pollution AND air pollution AND particulate matter AND gases AND pollutants AND mobile sources of air pollution, etc.
For example, the following are the search strategies used by all authors in the initial search of PubMed: “health” OR “public health” OR “population” OR “community” OR “rrespiratory disease” OR “asthma” OR “pulmonary” OR “disease OR illness” OR etc. AND “Impact” OR “implication” OR “risk” OR “hazards” AND “Air pollutants” OR “air pollution” OR “traffic related” OR “transport related” OR “ambient air pollution” OR “urban air pollution” AND “Developing countries” OR “worldwide” OR “global” OR “low in countries” OR “developed countries” OR “high income countries” OR “low and middle income countries” etc.
The combination of the above terms was used based on the search protocols used for each database. Additionally, manual searching of the articles was done to cover those articles that were difficult to locate and missed from the included electronic databases. Finally, references within eligible articles were further screened for additional articles.
Study selection
The study selection process was performed using the PRISMA flow chart, showing the number of articles included in the current study as well as those excluded from the study and the reasons for their exclusion. Following the search for articles, duplicate articles were removed using the Endnote software version X5 (Thomson Reuters, USA). After duplicated articles were removed, the authors independently screened the articles based on their titles and abstracts to determine their eligibility for this study by applying the inclusion criteria. Any disagreements made with respect to the inclusion of studies were resolved by consensus. Finally, articles that met the inclusion criteria were included in this study.
Data extraction
The data were extracted using Microsoft Excel 2016 form, which was developed by authors under the following headings: author(s), year of publication, sample size, study region or country, and primary outcomes: carbon monoxide (CO), nitrogen monoxide (NO), nitrogen dioxide (NO2), nitrogen oxide (NOx), sulfur dioxide (SO2), ozone (O3), and particulate matter (PM2.5 and PM10).
Quality assessment
Then the selected articles were subjected to quality assessment using the Joanna Briggs Institute (JBI) Critical Evaluation,11 to determine the quality and relevance of the articles for the current study. The evaluation tools have 9 evaluation criteria: appropriate sampling frame, proper sampling technique, adequate sample size, description of the study subject and setting description, sufficient data analysis, use of valid methods for identifying conditions, valid measurement for all participants, use of appropriate statistical analysis, and adequate response rate. Then, failure to satisfy each parameter was scored as 0, and if it met the criteria, it was scored as 1. The score was then given for each study and graded as high (7/9 and above), moderate (5/9%-6/9% score), or low (if it scored less than 5/9) quality. The disagreement over what was to be extracted was solved by discussion after repeating the same procedures.
Results
A total of 1211 articles were searched through the selected electronic databases, such as Scopus, PubMed, EMBASE, Web of Science, Google Scholar, and Science Direct. After searching for articles, 452 duplicate articles were excluded from the study. About 750 articles were excluded after initial screening, and 41 articles were excluded after full-text articles were assessed for eligibility. Finally, 29 articles were included in the systematic review (Figure 1).
Characteristics of included articles
In the current study, 1 282 032 participants, ranging from 19 to 452 735, were included in 30 articles published from 2010 to 2022. Regarding the countries where the studies were conducted, 4 articles were conducted in the USA, 4 in China, 2 each in Spain, Germany, England, and the Netherlands, and 1 article from Taiwan, Japan, Sweden, France, Australia, Korea, Malaysia, Columbia, Brazil, India, Denmark, and Nigeria.
Among the included articles, about 4 (13.3%), 9 (30.0%), 12 (40.0%), 8 (26.7%), 2 (6.7%), 15 (50.0%), 3 (10.0%), 3 (10.0%) 1 (3.3%), and 3 (10.0%) reported the association between human health and exposure to CO, PM10, PM2.5, NOx, NO, NO2, black carbon, O3, PAH, and SO2, respectively. However, in 2 articles, the types of traffic-related pollutants were not reported.
Furthermore, regarding the health outcome of exposure to traffic-related air pollution, various health problems or impacts associated with exposure to traffic-related air pollution, such as respiratory diseases, cancer, cognitive function problems, preterm birth, blood pressure and hypertension, diabetes, allergies and sensitization, coronary heart disease, pediatric allergic diseases, dementia incidence, hemorrhagic stroke, and lung cancer, were identified in the current study (Table 1).
Table 1.
Characteristics of the included articles used to determine the public health impacts of traffic-related air pollution, 2023.
Abbreviations: CO, carbon monoxide; EC, elemental carbon; HR, hazard ratio; NO2, nitrogen dioxide; NOx, nitrogen compounds; O3, Ozone; PM2.5, fine particulate matter; SBP, systolic blood pressure; SO2, sulfur dioxide; TRAP, traffic related air pollution.
Discussion
The current study aimed to determine the public health impacts of traffic-related air pollution. This study identified various health impacts related to different types of traffic-related air pollutants across the world. The results of this systematic review indicate that exposure to higher levels of traffic-related air pollutants such as nitrogen dioxide, nitrous oxide, carbon monoxide, particulate matter, and sulfur dioxide can increase the risk of various health conditions.
According to the current finding, traffic-related air pollutants could cause respiratory disease, particularly among children12,18,21,22,26,28-30,32,34,36,37 and elderly people.16,24,25,27,35,38-40 For example, according to the study conducted in Nigeria,12 respiratory illness (phlegm and wheeze) among children with ages ranging from 7 to 14 years was about 1.38 times higher among those exposed to CO than those not exposed. It was in line with the findings of another study that reported the same outcome.42
Similarly, another study conducted in the US reported that an increase in near-roadway NOx of 17.9 ppb was associated with deficits of 1.6% in forced vital capacity among children aged 5 to 7 years old.18 A study conducted in the USA reported that an increase in exposure to traffic-related pollution was associated with acute lymphoblastic leukemia [OR: 1.05; 95%CI: 1.01, 1.10] and germ cell tumors [OR: 1.16; 95%CI: 1.04, 1.29].21
Most studies reported a significant association between traffic air pollutants such as PM10, NO2, PM2.5, and O3 and respiratory disease, illness, or function, including lung function and bronchitis.12,15-18,24,29,34 The current finding was in line with the findings of another systematic review and meta-analysis, which reported a positive association between asthma and exposure to vehicle air pollution such as nitrogen dioxide, nitrous oxide, and carbon monoxide, which were associated with a higher prevalence of childhood asthma.42
According to the current finding, exposure to traffic-related air pollutants such as PM10, NO2, PM2.5, and O3 can increase diastolic blood pressure,20,23,31 and pediatric allergic diseases.32 Another health problem reported in the included articles is cancer, including cervical cancer,14 and lung cancer.38 This study was in line with the findings of another study that reported a statistically significant association between traffic-related air pollutants such as nitrogen oxide, sulfur dioxide, fine particulate matter, and lung cancer, that was supported by the current evidence.43
Cognitive development37 and cognitive function problems19 is another health consequences related to traffic air pollution exposure. The study reported that children from highly polluted environments had a smaller growth in cognitive development than children from the paired lowly polluted37 and particle metrics (PM10 and PM2.5) were associated with lower scores in reasoning and memory. For example, higher PM2.5 was associated with a 5-year decline in standardized memory score.19 Furthermore, Exposure to a high concentration of traffic-related air pollutants, higher than the maximum recommended level, can be toxic to different organs. Some experimental evidence showed a toxic effect of traffic-related air pollutants, including inflammation and changes in lung tissue.44 Furthermore, TRAP, such as Particulate matters may cause neurotoxicity, such as neurodevelopmental and neurodegenerative disorders.45
In general, the current study found that there was a statistically significant association between various traffic-related air pollution caused by different air pollutants, including CO, NOx, NO2, PM2.5, and PM10, and human health. Despite current progress in different countries adopting vehicle emission standards, transportation emissions remain a major contributor to ambient air pollution and are associated with major health impacts.46 This indicates a need to implement control strategies to reduce traffic-related air pollution and its public health burden by having a TRAP management plan46 and policy.47 Furthermore, using alternative transportation methods or technology, and strict regulations by the concerned organizations across the world can play a major role in reducing TRAP.47-49 International cooperation on pollution, including research, development, developing policy, monitoring, and politics, is vital for effective air pollution control.47
Furthermore, the authors recommend future researchers to focus on identifying an effective traffic related air pollution control interventions and role of national and international entities, particularly in controlling health burden of traffic related air pollution.
Conclusion
In general, the current study found that exposure to nitrogen dioxide; nitrogen oxide, sulfur dioxide, and fine particulate matter was associated with various health conditions such as respiratory diseases, cancer, cognitive function problems, preterm birth, blood pressure and hypertension, diabetes, allergies and sensitization, coronary heart disease, pediatric allergic diseases, dementia incidence, hemorrhagic stroke, and lung cancer. This revealed that there is a need to take appropriate action, including using alternative transportation methods or technology, reducing exposure to air pollutants, and enforcing regulations.48,49
Limitations
Exposure was assessed differently using different methods. The publication is not evenly distributed across various countries. Even though there is a limited number of articles conducted on the public health, impacts of traffic-related air pollutants or pollution, particularly in developing countries as a result of poor databases for recording pollutants, various health outcomes have been reported in this study based on the previous findings.
Abbreviations
BC: Black Carbon; CO: Carbon Monoxide; EC: Elemental Carbon; HR: Hazard Ratio; NOx: Nitrogen compounds; PAH: Poly-Aromatic Hydrocarbon; PM: Particulate Maters; PRISMA: Preferred Reporting Items for Systematic Reviews and Meta-Analysis; SBP: Systolic Blood Pressure; TRAP: Traffic Related Air Pollution; WHO: World Health Organization.
Acknowledgements
The authors extend their deepest thanks to Haramaya University, School of Environmental Health staff, for providing their support.
© The Author(s) 2024
This article is distributed under the terms of the Creative Commons Attribution-NonCommercial 4.0 License ( https://creativecommons.org/licenses/by-nc/4.0/) which permits non-commercial use, reproduction and distribution of the work without further permission provided the original work is attributed as specified on the SAGE and Open Access pages ( https://us.sagepub.com/en-us/nam/open-access-at-sage).
Author Contributions
DD conceived the idea and had a major role in the review, extraction, and analysis of data, as well as the as well as the writing, drafting, and editing of the manuscript. DD, BM, DAM, WD, and AA have contributed to data extraction. DD, BM, DAM, WD, and AA contributed to the quality assessment, drafting, and editing of the manuscript. Finally, all authors read and approved the final version of the manuscript to be published and agreed on all aspects of this work.
Ethics Approval and Consent to Participate
Not applicable.
Consent for Publication
Not applicable.
Data Availability
Almost all data are included in this study. However, additional data can be available from the corresponding authors on the reasonable request.
