Estimating Exposure to Traffic-Related Air Pollution and Its Consequences on Respiratory Health in Population Working or Living along the Trunk Road: A Systematic Review

Introduction: Urban Air pollution is increasingly becoming a major health and sustainable development issue. Several studies showed that Traf-fic-related air pollution (TRAP) is one of the main sources of urban air pollution and has serious consequences on respiratory health. As no systematic review focused on the traffic-related air pollution and respiratory health in the target population of individuals working in a shop or in an office or individuals living along the trunk road, the authors conducted the current study to try to fill this gap. Methods: A systematic review search was conducted using MEDLINE (PubMed), Scientific Research Publishing: SCIRP, Web of Science, Google scholar. Studies were included if they meet the following selection criteria: 1) focus on population working or living along a major/trunk road; 2) studies had reported clearly at least on the exposure variables related to TRAP; 3) the association between TRAP and development of respiratory symptoms or respiratory diseases was established. Results: 13 articles were selected on the 192 articles that were retrieved in the initial research. Exposure to traffic-related air pollution was determined by using distance to road, traffic intensity and pollutants measured. The main respiratory health problems found were cough, wheeze, asthma and bronchitis.


Introduction
Urban Air pollution is increasingly becoming a major health and sustainable development issue. Compared to all other forms of environmental pollution, air pollution causes the largest number of environmental pollution-related deaths [1]. Ninety-two per cent of the world's population is living in places where levels exceed recommended limits [2]. The World Health Organization (WHO) estimates that ambient (outdoor) air pollution was estimated to cause 4.2 million premature deaths worldwide per year in 2016 [3]. Traffic-related air pollution (TRAP) is one of the main sources of urban air pollution [4] [5] [6] [7] [8]. In major African cities, the air quality has deteriorated significantly. In a study conducted from 2015 to 2017 in Benin and Côte d'Ivoire, 24-hour PM 2.5 concentrations were found to consistently exceed WHO guidelines [9]. In Ethiopia, Kenya and Uganda, PM pollution levels were estimated to have increased by 182%, 162% and 62% respectively since the 1970s to the current period [10]. The increase in number of obsolete vehicles [11], the urbanization plan that does not keep up with the rapid population growth and the rural exodus are among the main reasons for this situation.
The relationship between TRAP exposure and respiratory morbidity has been studied worldwide [8] [12] [13] [14] [15] [16] and several systematic reviews have examined the relationship between exposure to traffic-related air pollution and respiratory health [17]- [23]. But to the author's knowledge, there were not enough systematic reviews that focused on the target population of individuals working or living along the trunk road. The current review is an attempt to fill this gap. The first step will be to take stock of the different approaches used to estimate the exposure of this target group to traffic-related air pollution and to identify the different pollutants that have been studied. Secondly, the different respiratory health problems are identified.
The aim of this review is to examine if the different approaches used to estimate exposure to traffic-related air pollution and its consequences on respiratory health fully respond to the context of West Africa's cities such as Cotonou.

Methodology
This systematic review adhered to the recommendations outlined in the PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) statement [24]. removal of duplicates, identifying the available data on the topic. searched. The authors also hand-searched references list from included studies in order to found any relevant titles. Medical search terms (MeSH) and keywords such as "traffic" AND ("Air pollution" OR "TRAP") AND ("respiratory symptoms" OR "respiratory diseases") AND ("major road" OR "trunk road" OR "main road" OR "busy road" OR "main street" OR "major street") were used as combinations of text or thesaurus terms to conduct search in the various databases, with the aim of identifying eligible articles for inclusion in the review.

Studies Selection
All original studies, published in English or French including observational or epidemiological studies (cross-sectional, cohort, case-control studies) published from inception January 2000 to December 2019, which discussed about traffic-related air pollution and respiratory symptoms in population working or living along major or trunk roads were eligible for this review. The authors have only included those studies that mentioned clearly the exposure variable addressed. Another inclusion criterion was that the article should address the various respiratory symptoms or respiratory diseases that were associated with traffic-related air pollution.
These different selection criteria concerning specifically the type of study and all expected characteristics of the results are clearly summarized and detailed in Table 1. The selection process of the articles was done iteratively in three stages, based on all the inclusion criteria previously defined. In the first stage, one author screened all titles and removing the titles of publications that are not relevant, and then the others authors screened the remaining publications based on abstract. Thus, after having screening the publications based on titles and abstracts, the last step will be to evaluate the eligibility of the full articles remaining.

Data Extraction, Management and Synthesis
For all included studies, according to the inclusion criteria on consensus of two reviewers (HL & VA), data have been extracted and synthesized in a table. Data extracted included authors, year of publication, the location, the study design, the type of exposure variable and the different respiratory symptoms or respiratory diseases found in these studies as health outcomes.

Search Results and Flow Chart
The initial research produced a total of 192 publications from the various databases and other sources. After removing 18 duplicates, 136 publications were excluded after an evaluation, based on a title analysis, and abstract analysis. This left a final total of 38 studies whose full text articles were reviewed for eligibility. At the end of the review process, 13 publications met the eligibility criteria and were included in the review. The PRISMA flow diagram below ( Figure 1) summarizes the study selection process, including the number of publications found, the review process, and the reasons for exclusion of publications.

Study Design, Site and Population Studied
On the 13 articles included (see Table 2), 6 were a cohort study and 6 were a cross-sectional study; one article used a case-control study and a cross-sectional study. The studies were implemented in Europe (Switzerland, UK, Netherlands, Sweden, Italy, Germany), in USA, in Asia (China, Japan) and in Oceania (Australia). Different populations were studied such as residents or children in class.

Exposure Variables Measured in Studies Included in the Review
Three variables were used to estimate the exposure to traffic-related air pollution: distance or proximity from a major road, traffic intensity and pollutants measured. As shown in Table 2, most of articles (11 on 13) based the characterization of the exposure to traffic-related air pollution on the distance by using different approaches. Among them were distance to closet main street per 100 m, distance to nearest main road (≤30 m; 60 m; 90 m; 120 m; 150 m) [26], distance to major road (<200 m or ≥200 m) [28], distance from trunk road (0 -49 m, ≥50 m) [35].
Six articles used traffic intensity to characterize the exposure to traffic-related air pollution. Traffic intensity were defined by Truck traffic density, car traffic density, <10,000 vehicles per day versus ≥10,000 vehicles per days [12], number of cars per minute (0 -1 cars/min; 2 -5 cars/min; 6 -10 cars/min and >10 cars/min) [31] or per 12 hours [35].
Seven articles used pollutants measure to appreciate the exposure to traffic-related air pollution. Several pollutants were measured such as PM 10

Effects on Respiratory Health
Articles included showed effects of traffic-related air pollution on respiratory health.
These effects could be mostly classified in two groups: effects found based on the distance to road (Table 3) and effects found based on the concentrations of pollutants (    [34]. Based on pollutants concentrations, several authors found associations between fine particles or nitrogen oxide and respiratory symptoms or diseases. Kim et al. found that exposure to NO was significantly associated to development of bronchitis (adjusted OR 1.79; 95% CI: 1.06 -3.02) [30]. Schikowski et al. also found association between exposure to NO 2 and chronic bronchitis Open Journal of Air Pollution Table 3. Some significant associations between respiratory symptoms or diseases and distance to major road.  In the rest of this paragraph, we presented some results that combined the effects of distance to major road, the concentrations of pollutants or the intensity of traffic on respiratory health. Bowate et al. also found that living < 200 m from a major road and NO 2 exposure was associated with increased current asthma prevalence (adjusted OR 1.49; 95% CI: 1.09 -2.05 and aOR 1.10, 95% CI: 0.96 -1.27, respectively) [28]. In the same vein, Garshick et al. reported that exposure to vehicular emissions by living near busy roadways contribute to symptoms of chronic respiratory diseases in adults [12] and Hu et al. reported that long-term exposure to traffic-related air pollution in people living near major roads in Beijing is associated with lower lung function, airway acidification and a higher prevalence of chronic cough [27]. Lindgren et al. found that living close to traffic (within 100 m of a road with >10 cars/minute) and annual average NO x was associated with prevalence of asthma diagnosis and COPD diagnosis and symptoms of asthma and bronchitis [31]. Concerning the children's, Migliore et al.
found that children living in zones with intense traffic are at higher risk for respiratory effects [33] and Kim et al. also reported an observation of a difference concentrations between schools nearby versus more distant (or upwind) from major roads and association between respiratory symptoms and traffic related pollutants [30]. Shima et al., findings confirmed that traffic-related air pollution may be of a particular importance in the development of asthma among children living near major trunk roads with heavy traffic [35]. Hoek et al. goes much deeper by establishing that long-term exposure to traffic-related air pollution may shorten life expectancy due to fact that cardiopulmonary mortality was associated with living near a major road [32].

Discussion
This systematic review focused on different methods to estimate the exposure to traffic-related air pollution and its consequences on respiratory health in population working or living along or near the trunk road. So, what we were going to discuss in this paper was not the degree of association between exposure to traffic-related and respiratory health but we would like to highlight the different approaches used for these relationships. Several previous systematic reviews came to the same conclusion in their study [20] [37] [38].
Based on these three characterization approaches, the authors of different papers have estimated the health outcomes.
The main health outcomes found were wheeze and cough for respiratory symptoms, asthma and bronchitis for respiratory diseases. This is consistent with some results in literature. Indeed Weinmayr et al., found that cough was one of key health respiratory symptom outcome [39]. Sylla et al. and Weinmayr et al. found in their review that asthma was one of key health respiratory disease outcome [20] [39].
Whether to characterize exposure to air pollution related to road traffic or to determine the effects of this exposure on respiratory health, the different articles included did not specifically address the particularity of roundabouts. Certainly, because these studies did not take place in the context of major West African cities. In fact, in these cities, urbanization is not always well carried out and the populations are enthusiastic to settle along the roads but also around the roundabouts with heavy traffic for their commercial activities [40] [41]. But living or working closet to high-traffic may have an additional risk depending on the location in relation to roundabouts and this situation has not been considered in different studies included.

Conclusion
In the present study, the authors found that distance to road, traffic density and pollutants measure are the usual methods to characterize the exposure to traffic-related air and his consequences on respiratory health. No study mentioned the situation about specifically populations around the roundabouts. But the situation in Africa's cities is different regarding the urbanization and the enthusiasm of population to settle along high traffic road and roundabouts. The authors suggest that it would be very useful if research focused on the populations living around the roundabouts in order to determine their possible overexposure to air pollution linked to traffic.