Long-term exposure to air pollution and the incidence of asthma: meta-analysis of cohort studies

We quantified the association between long-term exposure to air pollution and the incidence of asthma by conducting a systematic review and meta-analysis of cohort studies. Incidence was defined as the incidence of diagnosed asthma or of new wheeze symptom between two assessments or, in birth cohorts followed up to 10 years of age, a lifetime prevalence estimate of asthma or wheeze symptom. We identified 17 cohorts (eight birth cohorts and nine child/adult cohorts) with a total of 99 population-based risk estimates. The studies were heterogeneous in their design and methods of measurement. Follow-up ranged from 3 to 23 years. Most studies were based on within-community exposure contrasts dominated by traffic pollution. Twelve of the cohorts reported at least one positive statistically significant association between air pollution and a measure of incidence. Of the total of 99 estimates, only a minority (29) were positive and statistically significant. Estimates for meta-analysis were chosen a priori using a protocol. For the 13 studies with estimates for nitrogen dioxide (NO2), the random effects odds ratio was 1.07 (95% CI 1.02 to 1.13) per 10 mu g/m(3). For five studies with estimates for particulate matter with aerodynamic diameter < 2.5 mu m (PM2.5), the random effects estimate was 1.16 (95% CI 0.98 to 1.37) per 10 mu g/m(3). These estimates were reduced in size and statistical significance by adjustment for publication bias but remained positive. The results are consistent with an effect of outdoor air pollution on asthma incidence. Future meta-analyses would benefit from greater standardisation of cohort methods.