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 . Ninety-two per cent of the world’s population is living in places where levels exceed recommended limits . 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 . Traffic-related air pollution (TRAP) is one of the main sources of urban air pollution     . 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 PM2.5 concentrations were found to consistently exceed WHO guidelines . 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 . The increase in number of obsolete vehicles , 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       and several systematic reviews have examined the relationship between exposure to traffic-related air pollution and respiratory health  - . 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.
This systematic review adhered to the recommendations outlined in the PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) statement .
2.1. Data Sources and Search Strategy
Searches were performed using a systematic literature search strategy with the removal of duplicates, identifying the available data on the topic. The following electronic scientific databases were primarily searched: MEDLINE (PubMed), Scientific Research Publishing: SCIRP (Open Journal of Air Pollution; Open Journal of Respiratory Diseases), Web of Science. Data sources such as the Google Scholar, search on internet for available grey literature, the table of contents of Environmental Health Perspectives and Atmospheric Environment were 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.
2.2. 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.
Table 1. Selection criteria.
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.
2.3. 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.
3.1. 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.
Figure 1. Preferred Reporting Item for Systematic Reviews and Meta-Analyses (PRISMA) flow diagram for article selection.
3.2. 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.
3.3. 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) , distance to major road (<200 m or ≥200 m) , distance from trunk road (0 - 49 m, ≥50 m) .
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 , number of cars per minute (0 - 1 cars/min; 2 - 5 cars/min; 6 - 10 cars/min and >10 cars/min)  or per 12 hours .
Seven articles used pollutants measure to appreciate the exposure to traffic-related air pollution. Several pollutants were measured such as PM10   ; PM2.5  ; NOx  , NO2      , NO , Benzene , BC   and SO2 .
3.4. Effects on Respiratory Health
Articles included showed effects of traffic-related air pollution on respiratory health.
Hu et al. found that compared to participants with a home road distance of >200 m, participants with a home road distance of <100 m or 100 - 200 m had a higher prevalence of chronic cough, with an odds ratio of 2.54 (95% CI: 1.57 - 4.10) and 1.97 (95% CI: 1.16 - 3.37), respectively . In the same vein, Bowate et al. also found that living < 200 m from a major road and NO2 exposure was associated with increased current asthma and current wheeze prevalence (adjusted OR 1.49; 95% CI: 1.09 - 2.05 and 1.61, 95% CI: 1.19 - 2.19, respectively) . Regarding the results reported by Mc Connell et al., living < 75 m was significantly associated with a higher prevalence of asthma (adjusted OR 1.29; 95% CI: 1.01 - 1.66), and a current wheeze (adjusted OR 1.40; 95% CI: 1.09 - 1.78)
Table 2. Studies about TRAP and respiratory health on population living/working along trunk road included in the review.
Definition of abbreviations: RS: Respiratory symptoms; RD: Respiratory diseases; BC: black carbon; NO: nitric oxide; NOx: total nitrogen oxides; NO2: nitrogen dioxide; PM2.5: particulate matter of aerodynamic diameter 2.5 µm or less; PM10: particulate matter of aerodynamic diameter 10 µm or less. COPD: Chronic Obstructive Pulmonary Disease; CBE: Chronic Bronchitis Emphysema; FEV: Forced expiratory volume.
. As for Schikowski et al., living < 100 m from major road was significantly associated with an increased risk of frequent cough (adjusted OR 1.24; 95% CI: 1.03 - 1.49) and COPD (adjusted OR 1.79; 95% CI: 1.06 - 3.02) .
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) . Schikowski et al. also found association between exposure to NO2 and chronic bronchitis
Table 3. Some significant associations between respiratory symptoms or diseases and distance to major road.
RS: Respiratory symptoms; RD: Respiratory diseases; OR: Odd ratio; COPD: Chronic Obstructive Pulmonary Disease.
(adjusted OR 1.25; 95% CI: 1.00 - 1.58), frequent cough (adjusted OR 1.13; 95% CI: 1.01 - 1.27) and COPD (adjusted OR 1.39; 95% CI: 1.20 - 1.63). Another result found by Schikowski et al. is the association between PM10 and COPD. He found that an increase of 7 of µg/m3 PM10 was associated with an increased risk of COPD (adjusted OR 1.33; 95% CI: 1.03 - 1.72) .
Table 4. Some significant associations between respiratory symptoms or diseases and air pollution concentrations.
RS: Respiratory symptoms; RD: Respiratory diseases; OR: Odd ratio; COPD: Chronic Obstructive Pulmonary Disease.
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 NO2 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) . 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  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 . Lindgren et al. found that living close to traffic (within 100 m of a road with >10 cars/minute) and annual average NOx was associated with prevalence of asthma diagnosis and COPD diagnosis and symptoms of asthma and bronchitis . Concerning the children’s, Migliore et al. found that children living in zones with intense traffic are at higher risk for respiratory effects  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 . 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 . 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 .
The different effects reported in this review can be classified in respiratory symptoms and respiratory diseases. The main respiratory symptoms found were wheezing      , cough      . As respiratory diseases, the articles mentioned asthma      , bronchitis (   , chronic respiratory diseases   .
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.
The authors found only 13 studies that meet the criteria and no study in Africa. To characterize the exposure to traffic-related air pollution, studies used distance to road   -     , traffic density        and pollutants measure        . Several previous systematic reviews came to the same conclusion in their study   .
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 . Sylla et al. and Weinmayr et al. found in their review that asthma was one of key health respiratory disease outcome  .
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  . 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.
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.
Le financement de cette étude a été accordé par le Centre de Recherches pour le Développement International (CRDI) dans le cadre du projet “Chairepol” (projet CRDI 107347) de la Communauté de pratique en écosanté pour l’Afrique de l’Ouest et du Centre (CoPES-AOC)/Chaire ECOSANTE.
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