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

Abstract

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 (PM2.5) 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 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 relevance: Among 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.

Figure 1.. Annual Mean Air Pollutant Concentration During the Follow-up Period in 9 Communities of the Southern California Children’s Health Study, 1993-2011

Black, dark gray, and light gray horizontal bars represent follow-up periods for the 1993-2001, 1996-2004, and 2006-2014 cohorts, respectively. Follow-up for the 2006-2014 cohort is truncated on the graph at 2011, the last year with air pollution data. ppb indicates parts per billion.

Figure 2.. Asthma Incidence Rates and Air Pollutant Concentrations in 9 Communities During the 1993-2001, 1996-2004, and 2006-2014 Cohorts of the Southern California Children’s Health Study, 1993-2014

Symbol colors indicate community and the size—small, medium, and large—indicates the data are from the 1993-2001, 1996-2004, and 2006-2014 cohorts, respectively. Simple linear regression models based on asthma incidence and air pollution concentration used to generate regression lines separately for each community. ppb indicates parts per billion.