• Erika Garcia, Kiros T Berhane, Talat Islam, Rob McConnell, Robert Urman, Zhanghua Chen, and Frank D Gilliland.
• Department of Preventive Medicine, University of Southern California, Los Angeles.
• JAMA. 2019 May 21; 321 (19): 1906-1915.

ImportanceExposure 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.ObjectiveTo examine whether decreasing regional air pollutants were associated with reduced incidence of childhood asthma.Design, Setting, And ParticipantsA 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.ExposuresAnnual mean community-level ozone, nitrogen dioxide, and particulate matter less than 10 μm (PM10) and less than 2.5 μm (PM2.5) in the baseline year for each of 3 cohorts.Main Outcomes And MeasuresProspectively identified incident asthma, collected via questionnaires during follow-up.ResultsAmong 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 PM2.5, 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.Conclusions And RelevanceAmong children in Southern California, decreases in ambient nitrogen dioxide and PM2.5 between 1993 and 2014 were significantly associated with lower asthma incidence. There were no statistically significant associations for ozone or PM10.

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