Research in context
Evidence before this study
No systematic review was done as part of the planning for this study. Evidence from prospective cohort studies suggests a causal role of ambient air pollution in the development of childhood asthma, but is less clear for the development of allergic rhinoconjunctivitis. Studies done so far differ with regard to exposure assessment, health outcome definition, and statistical analysis, which might explain part of the heterogeneity in study findings. In the past few years, cohort-specific cross-sectional analyses with subsequent meta-analyses of the association between air pollution and asthma prevalence, but not incidence, at specific ages were undertaken in five European cohorts with harmonised exposure and health data. Age-specific associations with air pollution have been studied in single cohorts, but statistical power is low, particularly for onset of disease.
Added value of this study
For the first time, we were able to pool data from four large prospective birth cohort studies. We included data from repeated follow-up assessments up to age 14–16 years and made full use of these data by undertaking longitudinal rather than cross-sectional analyses. We studied associations with incidence in addition to prevalence and estimated both overall and age-specific effects.
Implications of all the available evidence
We recorded adverse effects of exposure to air pollution early in life on asthma incidence and prevalence, in particular after the age of 4 years when asthma can be more reliably diagnosed. Our results strengthen the evidence for the contribution of ambient air pollution to the development of asthma in children and adolescents, and suggest that reductions in levels of air pollution could help to prevent the development of asthma in children.