The overall objective of this study is to examine the literature investigating the associations between traffic-related air pollution (TRAP)
IntroductionAir pollution is a commonly recognized external cost of the use of motor vehicles (Barabas, 2013). In many urban areas nowadays, road traffic is the principal source of outdoor air pollution which has been recently linked to around 4 million global preventable deaths per annum (Solvang Jensen 1999;Colvile et al., 2001;European Environment Agency 2007; Health Effects Institute 2010; World Health Organization, 2014a). Despite many air quality improvements in recent decades, air pollution continues to pose a significant threat to human health and wellbeing worldwide (World Health Organization, 2006;Lim et al., 2013;Vidal, 2014). In particular, traffic-related air pollution is a key component to the ambient air pollution mix in present urban areas, and has been linked to a wide spectrum of global disease (Health Effects Institute, 2010).In this study, the term traffic-related air pollution (TRAP) refers to the concentrations of primary and secondary air pollutants, elevated above background levels due to motor vehicles emissions. Traffic-related primary pollutants include carbon monoxide (CO), carbon dioxide (CO2), nitrogen oxides (NO x ), ammonia (NH 3 ), particulate matter (PM), and hydrocarbons (HC). Traffic-related secondary pollutants are those formed mainly by the various chemical reactions which take place in the atmosphere such as ozone (O 3 ), secondary particulates and secondary NO 2 .In particular, recent research provides evidence indicating that significant associations exist between residential trafficrelated pollutant levels and asthma occurrences (Brauer et al., 2007;Salam et al., 2008).With childhood asthma currently being a condition that has a profound impact on the quality of life for a large number of the population, especially in childhood; when individuals are most vulnerable, working towards establishing a clearer understanding of the interactions between a common exposure such as that to TRAP and the onset/ prevalence of the disease is clearly a priority. Such an understanding may be offering a partial explanation of the relatively rapid changes in prevalence, in times when traffic air pollution and certain fleets became more dominant. This study aims at examining the question of whether earlylife exposure to TRAP can cause asthma to develop in children and highlight the current limitations in the evidence base.
Traffic-related air pollution and childhood asthmaA sharp rise in the prevalence and incidence of asthma has been reported in many parts of the world, and particularly in the more developed and industrialized societies (