Improved air quality and asthma incidence from school-age to young adulthood: a population-based prospective cohort study

<bold>Background:</bold> Few studies have evaluated the benefits of improved air quality on asthma. <bold>Aims:</bold> To investigate associations of changes in ambient air pollution with incident asthma from school-age until young adulthood. <bold>Methods:</bold> Participants in the BAMSE birth cohort from Stockholm without asthma before the 8-year follow-up were included (N=2371). We estimated the association of change in individual-level air pollutant exposure (particulate matter with diameter ⩽2.5 μm (PM_2.5) and, ⩽10 μm (PM₁₀), black carbon (BC) and nitrogen oxides (NO_x)) from the first year of life to the 8-year follow-up with asthma incidence from the 8-year until the 24-year follow-up. Multi-pollutant trajectories were identified using Group-Based Multivariate Trajectory model. We used parametric g-computation to quantify the asthma incidence under hypothetical interventions regarding air pollution levels. <bold>Results:</bold> A total of 395 incident asthma cases were identified. The odds ratio for asthma was 0.89 (95%CI: 0.80, 0.99) for each interquartile range reduction in PM_2.5 (equal to 8.1% reduction). Five multi-pollutant trajectories were identified, where the largest reduction trajectory displayed the lowest odds of asthma (OR=0.55, 95%CI: 0.31, 0.98) compared with the least reduction trajectory. If the PM2.5 exposure had not declined up to the 8-year follow-up, the hypothetical asthma incidence was estimated to have been 10.9% higher (95%CI: 0.8%, 20.8%). <bold>Conclusions:</bold> Decrease in PM_2.5 levels during childhood was associated with lower risk of incident asthma from school-age to young adulthood in an area with relatively low air pollution levels.