Association of Changes in Air Quality With Incident Asthma in Children in California, 1993-2014

<h3>Importance</h3><p>Exposure to air pollutants is a well-established cause of asthma exacerbation in children; whether air pollutants play a role in the development of childhood asthma, however, remains uncertain.</p><h3>Objective</h3><p>To examine whether decreasing regional air pollutants were associated with reduced incidence of childhood asthma.</p><h3>Design, Setting, and Participants</h3><p>A multilevel longitudinal cohort drawn from 3 waves of the Southern California Children’s Health Study over a period of air pollution decline. Each cohort was followed up from 4th to 12th grade (8 years): 1993-2001, 1996-2004, and 2006-2014. Final follow-up for these data was June 2014. Population-based recruitment was from public elementary schools. A total of 4140 children with no history of asthma and residing in 1 of 9 Children’s Health Study communities at baseline were included.</p><h3>Exposures</h3><p>Annual mean community-level ozone, nitrogen dioxide, and particulate matter less than 10 μm (PM₁₀) and less than 2.5 μm (PM_2.5) in the baseline year for each of 3 cohorts.</p><h3>Main Outcomes and Measures</h3><p>Prospectively identified incident asthma, collected via questionnaires during follow-up.</p><h3>Results</h3><p>Among the 4140 children included in this study (mean [SD] age at baseline, 9.5 [0.6] years; 52.6% female [n = 2 179]; 58.6% white [n = 2273]; and 42.2% Hispanic [n = 1686]), 525 incident asthma cases were identified. For nitrogen dioxide, the incidence rate ratio (IRR) for asthma was 0.80 (95% CI, 0.71-0.90) for a median reduction of 4.3 parts per billion, with an absolute incidence rate decrease of 0.83 cases per 100 person-years. For PM_2.5, the IRR was 0.81 (95% CI, 0.67-0.98) for a median reduction of 8.1 μg/m³, with an absolute incidence rate decrease of 1.53 cases per 100 person-years. For ozone, the IRR for asthma was 0.85 (95% CI, 0.71-1.02) for a median reduction of 8.9 parts per billion, with an absolute incidence rate decrease of 0.78 cases per 100 person-years. For PM₁₀, the IRR was 0.93 (95% CI, 0.82-1.07) for a median reduction of 4.0 μg/m³, with an absolute incidence rate decrease of 0.46 cases per 100 person-years.</p><h3>Conclusions and Relevance</h3><p>Among children in Southern California, decreases in ambient nitrogen dioxide and PM_2.5between 1993 and 2014 were significantly associated with lower asthma incidence. There were no statistically significant associations for ozone or PM₁₀.</p>