title: Exposure to traffic-related air pollution and risk of development of childhood asthma: A systematic review and meta-analysis.
abstract: The question of whether children's exposure to traffic-related air pollution (TRAP) contributes to their development of asthma is unresolved. We conducted a systematic review and performed meta-analyses to analyze the association between TRAP and asthma development in childhood.
  
  
  
  We systematically reviewed epidemiological studies published until 8 September 2016 and available in the Embase, Ovid MEDLINE (R), and Transport databases.
  
  
  
  We included studies that examined the association between children's exposure to TRAP metrics and their risk of 'asthma' incidence or lifetime prevalence, from birth to age 18years old.
  
  
  
  We extracted key characteristics of each included study using a predefined data items template and these were tabulated. We used the Critical Appraisal Skills Programme checklists to assess the validity of each included study. Where four or more independent risk estimates were available for a continuous pollutant exposure, we conducted overall and age-specific meta-analyses, and four sensitivity analyses for each summary meta-analytic exposure-outcome association.
  
  
  
  Forty-one studies met our eligibility criteria. There was notable variability in asthma definitions, TRAP exposure assessment methods and confounder adjustment. The overall random-effects risk estimates (95% CI) were 1.08 (1.03, 1.14) per 0.5×10-5m-1black carbon (BC), 1.05 (1.02, 1.07) per 4μg/m3nitrogen dioxide (NO2), 1.48 (0.89, 2.45) per 30μg/m3nitrogen oxides (NOx), 1.03 (1.01, 1.05) per 1μg/m3Particulate Matter <2.5μm in diameter (PM2.5), and 1.05 (1.02, 1.08) per 2μg/m3Particulate Matter <10μm in diameter (PM10). Sensitivity analyses supported these findings. Across the main analysis and age-specific analysis, the least heterogeneity was seen for the BC estimates, some heterogeneity for the PM2.5and PM10estimates and the most heterogeneity for the NO2and NOxestimates.
  
  
  
  The overall risk estimates from the meta-analyses showed statistically significant associations for BC, NO2, PM2.5, PM10exposures and risk of asthma development. Our findings support the hypothesis that childhood exposure to TRAP contributes to their development of asthma. Future meta-analyses would benefit from greater standardization of study methods including exposure assessment harmonization, outcome harmonization, confounders' harmonization and the inclusion of all important confounders in individual studies.
  
  
  
  PROSPERO 2014: CRD42014015448.
authors.affiliation: Institute for Transport Studies, University of Leeds, Leeds, United Kingdom. Electronic address: ts12hrk@leeds.ac.uk.
authors.forename: Haneen
authors.initials: H
authors.lastname: Khreis
authors.affiliation: Institute for Transport Studies, University of Leeds, Leeds, United Kingdom; Leeds Institute of Health Sciences, University of Leeds, Leeds, United Kingdom.
authors.forename: Charlotte
authors.initials: C
authors.lastname: Kelly
authors.affiliation: Institute for Transport Studies, University of Leeds, Leeds, United Kingdom.
authors.forename: James
authors.initials: J
authors.lastname: Tate
authors.affiliation: Leeds Institute of Cardiovascular and Metabolic Medicine, University of Leeds, Leeds, United Kingdom.
authors.forename: Roger
authors.initials: R
authors.lastname: Parslow
authors.affiliation: Institute for Transport Studies, University of Leeds, Leeds, United Kingdom.
authors.forename: Karen
authors.initials: K
authors.lastname: Lucas
authors.affiliation: ISGlobal CREAL, C/Dr. Aiguader 88, 08003 Barcelona, Spain; Universitat Pompeu Fabra (UPF), C/Dr. Aiguader 88, 08003, Barcelona, Spain; CIBER Epidemiología y Salud Pública (CIBERESP), C/Monforte de Lemos 3-5, 28029 Madrid, Spain.
authors.forename: Mark
authors.initials: M
authors.lastname: Nieuwenhuijsen
mesh_terms: D000293:Adolescent; D000393:Air Pollutants / Q000032:analysis*; D000397:Air Pollution / Q000032:analysis*; D001249:Asthma / Q000139:chemically induced / Q000453:epidemiology*; D002648:Child; D002675:Child, Preschool; D004781:Environmental Exposure; D005260:Female; D006801:Humans; D015994:Incidence; D007223:Infant; D007231:Infant, Newborn; D008297:Male; D015995:Prevalence; D012307:Risk Factors; D001335:Vehicle Emissions / Q000032:analysis*
chemical_list: D000393:Air Pollutants; D001335:Vehicle Emissions
keywords: Asthma; Black carbon; Childhood; Meta-analysis; Traffic-related air pollution; Transport policy