Exposure to traffic-related air pollution and risk of development of childhood asthma: A systematic review and meta-analysis

Background and objective 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. Data sources We systematically reviewed epidemiological studies published until 8 September 2016 and available in the Embase, Ovid MEDLINE (R), and Transport databases. Study eligibility criteria, participants, and interventions 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 18 years old. Study appraisal and synthesis methods 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. Results 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− 5 m− 1black carbon (BC), 1.05 (1.02, 1.07) per 4 μg/m3nitrogen dioxide (NO2), 1.48 (0.89, 2.45) per 30 μg/m3nitrogen oxides (NOx), 1.03 (1.01, 1.05) per 1 μg/m3Particulate Matte