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

Question  What is the association between reductions in regional air pollutant concentration and incidence of childhood asthma?

Findings  In this longitudinal study that included 4140 children, each 4.3–parts-per-billion decrease in nitrogen dioxide was associated with a reduction of 0.83 cases per 100 person-years in asthma incidence; each 8.1-μg/m³ decrease in particulate matter less than 2.5 μm was associated with a reduction of 1.53 cases per 100 person-years in asthma incidence. There were no statistically significant associations with change in ozone and particulate matter less than 10 μm.

Meaning  Declines in nitrogen dioxide and particulate matter less than 2.5 μm may be associated with decreased childhood asthma incidence.

Importance  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.

Objective  To examine whether decreasing regional air pollutants were associated with reduced incidence of childhood asthma.

Design, Setting, and Participants  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.

Exposures  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.

Main Outcomes and Measures  Prospectively identified incident asthma, collected via questionnaires during follow-up.

Results  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.

Conclusions and Relevance  Among children in Southern California, decreases in ambient nitrogen dioxide and PM_2.5 between 1993 and 2014 were significantly associated with lower asthma incidence. There were no statistically significant associations for ozone or PM₁₀.