Background Epidemiological studies on the effect of urban air pollution on childhood asthma have shown conflicting results and so far no consistent association has emerged. However, a common limitation in previous studies has been exposure misclassification leading to uncertainties in risk estimates.The aim of this study was to analyse the effects of long-term exposure to particulate matter (PM₁₀) and nitrogen dioxide (NO₂) on the prevalence of asthma and wheeze within a population-based birth cohort—the Manchester Asthma and Allergy Study (MAAS).

Methods The prevalence of asthma and current wheeze within the cohort (N=1185) was determined through parental questionnaires at ages 3, 5, 8 and 11 years. The typical monthly PM₁₀ and NO₂ exposure of each child was estimated through a novel microenvironmental exposure model from birth to age 11. The association between exposure and asthma or wheeze was analysed using generalised estimating equations and multiple logistic regression.

Results The range of asthma prevalence was 15.2–23.3%, with the lowest prevalence at age 3 and the highest at age 5. The prevalence of current wheeze decreased from ages 3 to 8 (23.7–18%). The mean NO₂ exposure decreased from the 1st year of life (21.7 µg/m³) to the 11th year of life (16.0 µg/m³). The mean PM₁₀ exposure showed a smaller decrease (12.8 –10.7 µg/m³). The statistical analysis showed no significant association between the exposures and either outcome.

Conclusions No evidence of a significant association between long-term exposure to PM₁₀ and NO₂ and the prevalence of either asthma or wheeze was found.