Academic journal article Environmental Health Perspectives

Early Exposure to Traffic-Related Air Pollution, Respiratory Symptoms at 4 Years of Age, and Potential Effect Modification by Parental Allergy, Stressful Family Events, and Sex: A Prospective Follow-Up Study of the Paris Birth Cohort

Academic journal article Environmental Health Perspectives

Early Exposure to Traffic-Related Air Pollution, Respiratory Symptoms at 4 Years of Age, and Potential Effect Modification by Parental Allergy, Stressful Family Events, and Sex: A Prospective Follow-Up Study of the Paris Birth Cohort

Article excerpt

Introduction

The prevalence of respiratory and allergic diseases in early childhood has been rising globally, which is unlikely to be attributable to genetic changes only. These multifactorial diseases are associated with both individual and environmental factors. Recent decades have seen a change in air pollution profile in western urban areas, with motor vehicle traffic emissions now as a major source of air pollution (Mayer 1999). Traffic-related air pollution (TRAP) is known to worsen existing respiratory disease (Weinmayr et al. 2010). However, despite substantial literature on the relation between TRAP and the development of asthma and allergy in preschool years, results are still heterogeneous and some uncertainties persist (Braback and Forsberg 2009). For instance, although a meta-analysis of 19 published studies showed evidence for a relationship between TRAP exposure and wheezing in preschool children (Gasana et al. 2012), pooled analyses of five European birth cohorts within the European Study of Cohorts for Air Pollution Effects (ESCAPE) project revealed no significant association of TRAP exposure in early years of life with asthma prevalence at 4-5 years (Molter et al. 2015) or sensitization to inhalant or food allergens at 4 years (Gruzieva et al. 2014). These inconsistencies in findings may be attributable to methodological issues such as variability in the assessment of TRAP exposure (Brauer 2010) and the definition of health outcomes, and to the possible existence of susceptible subgroups.

In birth cohort studies, TRAP exposure of preschool children has been studied using various indicators: distance to traffic, land-use regression (LUR) models, and, less often, air dispersion models (Bowatte et al. 2015). There are also differences in TRAP pollutants that authors considered [e.g. nitrogen dioxide (N[O.sub.2]), nitrogen oxides (N[O.sub.x]), particulate matter with an aerodynamic diameter [less than or equal to] 10 [micro]m ([PM.sub.10]) or [less than or equal to] 2.5 [micro]m ([PM.sub.2.5]), soot, black carbon], as well as in place and timing of exposure, with most studies only considering the home address at birth, and a few studies considering residential mobility and/or other locations where infants spend time such as day care center. Furthermore, asthma may be difficult to reliably diagnose at preschool age when the clinical symptoms of asthma are variable and nonspecific, and asthma-like or allergy-like symptoms other than wheeze have not been extensively explored.

Besides methodological considerations, another explanation may be related to differences in vulnerability to TRAP. Even if early childhood is a critical period of vulnerability for everyone because of continued development and maturation of the lung and immune system, certain children may be at increased risk for adverse health effects from TRAP (Sacks et al. 2011). In particular, atopy may play a role as an effect modifier, but results from the literature are not entirely consistent. Stronger associations between TRAP exposure and asthma were observed in atopic children in some studies (Dell et al. 2014; Janssen et al. 2003; Schultz et al. 2012) and in nonatopic children in other studies (Gruzieva et al. 2013; McConnell et al. 2006; Nordling et al. 2008). Moreover, emerging research indicates that stress may play a role in increasing the deleterious effect of TRAP on school-age children's respiratory health (Chen et al. 2008; Clougherty et al. 2007; Islam et al. 2011; Shankardass et al. 2009); but, to our knowledge, no such studies have been conducted in preschool children. Further, whether the susceptibility to the effects of TRAP differs between preschool boys and girls remains unclear. Some authors reported evidence of stronger effects in boys (Gehring et al. 2002) or in girls (Nordling et al. 2008), whereas others did not find any evidence for an effect measure modification by sex (Gruzieva et al. 2014). Last, gene-environment interactions may also partially explain observed heterogeneity in associations between TRAP exposure and the incidence of asthma and allergic outcomes, as suggested by findings from the Traffic, Asthma, and Genetics (TAG) study (Fuertes et al. …

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