Review articleTraffic-related organic and inorganic air pollution and risk of development of childhood asthma: A meta-analysis
Introduction
Asthma is a common chronic inflammatory disease of the respiratory system in the world. About 235 million people worldwide suffered from asthma, and 383,000 people died of asthma in 2015 (World Health Organization, 2017). Previous epidemiological studies have shown a global increase of asthma prevalence in children and adults (Anandan et al., 2010). About 6–19% of childhood asthma can be extended to adulthood (Bateson and Schwartz, 2008). Asthma is a complex disease, whose occurrence and development are influenced by genetic and environmental factors. Recently air pollution has led to significant environmental changes, especially in some developing countries. Traffic-related emissions are considered the main source of urban air pollution.
In 2015, around 947 million passenger cars and 335 million commercial vehicles were in operation worldwide (Statista, 2018). Vehicle emissions usually mean traffic-related air pollution (TRAP), which mainly include ozone, particulate matter, carbon oxide, nitrogen oxide and hydrocarbon etc. Children are particularly vulnerable to pollution because their respiratory system and immune system are immature. Children have larger unit weight ventilation and more outdoor activities than adults (Bateson and Schwartz, 2008). Existing research (Strickland et al., 2014; Wendt et al., 2014; Winquist et al., 2014; Hua et al., 2014; Ghosh et al., 2016; Gleason et al., 2014) indicated that exposure to traffic-related air pollutants were closely related to childhood asthma. In Bradford, a city in the northern England, more than 18%–38% of the asthma cases among children could be attributed to the ambient air pollution (Khreis et al., 2018). Using environmental and health data collected at primary schools in London, Guerriero et al. (2016) concluded that reducing the concentration of NO2 can decrease an average of 82 asthma attacks per school per year. Moreover, Benzene and TVOC are toxic air pollutants produced by incomplete fuel combustion, and they are especially detrimental to the susceptible groups. VOC could damage the cardiorespiratory system (Tsai et al., 2010), and has a significant correlation with asthma, allergy, and cough in children (Zhu et al., 2017; Nicolai et al., 2003). Benzene exposure is positively associated with hospitalization rate of respiratory diseases (Oftedal et al., 2003; Hagen et al., 2000). An investigation conducted by WHO (2010) (World Health Organization, 2010) showed that when the exposure concentration of B[a]P (benzopyrene) exceeded 1ng/m3, DNA would be damaged.
More and more studies confirmed that TRAP was closely related to childhood asthma, but TRAP encompassed many different types of pollutants. Current research mainly focused on PM and NOx, and paid less attention to organic pollutants. In addition, due to the spatial-temporal differences in TRAP concentrations, there may be large differences in asthma risks across different regions. Five meta-analyses of TRAP and asthma development have been published (Khreis et al., 2017; Favarato et al., 2014; Anderson et al., 2013; Gasana et al., 2012), which provided synthesis of observed epidemiological studies that analyzed the association between TRAP exposures (mainly PM and NOx) and the subsequent development of asthma (especially childhood asthma). Therefore, the objective of our meta-analysis review was to clarify potential associations between TRAP and childhood asthma by integrating and synthesizing the results from previous research. Compared with those previous meta-analysis, our review expanded the range of TRAP (from PM2.5 and NO2 to TVOC and Benzene) and examined regional difference of TRAP exposure and childhood asthma development.
Section snippets
Data sources and search methods
We searched database of ELSEVIER, LISTA (EBSCO) and Web of Science for published studies evaluating the relationship between TRAP (PM2.5, NO2, VOCs, etc.) exposure and the prevalence or incidence of asthma, wheeze, hospitalization rate of respiratory diseases. Search period was from January 1st, 2000 to January 1st, 2019. All database searches were done on January 1st, 2019. The language was limited to English. We performed the searches using the following three sets of keyword combinations.
- 1.
Results
According to our inclusion criteria, a total of 748 articles were retrieved through web databases including Elsevier, LISTA (EBSCO) and Web of Science databases. 618 relevant articles were retained after deleting reviews, comments and duplicate articles. Afterwards, we reviewed the titles and abstracts of all articles, and excluded articles that were irrelevant to this study, leaving 59 articles for further review. Then, another 32 articles were excluded due to ineligible outcome, ineligible
Overview and strengths
HEI (2010) concluded that there was sufficient evidence to support a causal relationship between exposure to TRAP and asthma incidence. NO2 and PM2.5 are frequently used in TRAP studies, while Benzene and TVOC are increased by traffic emissions.
Traffic emission contributed to about 39% (29%) of total US (Europe) NOx emissions (Wild et al., 2017; EEA, 2018). Carlsten et al. (2011) proved that early exposure to traffic-related NO2 increased the risk of incident asthma in a well-characterized
Conclusion
In summary, this systematic review and meta-analysis synthesized 27 studies that published from January 1st, 2000 to January 1st, 2019, investigating the association between exposure to TRAP and the development of childhood asthma. Our meta-analysis provides evidence for a positive correlation between TRAP exposure and subsequent childhood asthma development. The results indicate that PM2.5, NO2, Benzene and TVOC will increase the risk of development of childhood asthma (meta-OR: 1.07, 95% CI:
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This study was funded by the National Key Research and Development Program of China (2017YFC0212100), National Natural Science Foundation of China (21477057), and International Clean Energy Talent Program 2017 of China Scholarship Council (201702660024).
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