Declines in ambient nitrogen dioxide and particulate matter <2.5 µm (PM2.5) between 1993 and 2014 may be associated with a decreased incidence of childhood asthma, according to the results of a multilevel, longitudinal cohort study conducted in Southern California. Findings from the study were published in JAMA.

Recognizing that exposure to air pollutants has been well established as a cause of asthma exacerbation in children, investigators sought to examine whether a reduction in regional air pollutants was related to a reduced occurrence of childhood asthma. The longitudinal cohort evaluated was obtained from 3 waves of the Southern California Children’s Health Study during a period when a decline in outdoor air pollution was observed. Each of the 3 cohorts was followed from fourth grade to twelfth grade in the following periods: 1993 to 2001, 1996 to 2004, and 2006 to 2014. Population-based recruitment involved in this analysis occurred in public elementary schools.

A total of 4140 children who had no history of asthma and who resided in 1 of 9 Children’s Health Study communities at baseline were included in the study. The main outcome measure was prospectively identified incident asthma, which was obtained via questionnaires during follow-up. The mean age of the participants at baseline was 9.5±0.6 years. Overall, 52.6% of the participants were girls, 58.6% were white, and 42.2% were Hispanic. A total of 525 incident cases of asthma were identified.

Regarding 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 and an absolute incidence rate decline of 0.83 cases per 100 person-years. With respect to PM2.5, the IRR for asthma was 0.81 (95% CI, 0.67-0.98), for a median reduction of 8.1 µg/m3 and an absolute incidence rate decline 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 and an absolute incidence rate decline of 0.78 cases per 100 person-years. Regarding PM10, the IRR for asthma was 0.93 (95% CI, 0.82-1.07), for a median reduction of 4.0 µg/m3 and an absolute incidence rate decline of 0.46 cases per 100-person-years.

Major limitations of the study included using baseline, rather than time-varying, community-level annual average concentration of pollutants as the exposure in the main models that were analyzed. Moreover, the modeling framework, which controls for spatial confounding according to a fixed effect for community, remained susceptible to possible temporal confounding.

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The investigators concluded that in a population of pediatric participants from Southern California, reductions in ambient nitrogen dioxide and PM2.5 between 1993 and 2014 were significantly associated with a lower incidence of asthma. No statistically significant associations were observed for ozone or PM10.


Garcia E, Berhane KT, Islam T, et al. Association of changes in air quality with incident asthma in children in California, 1993-2014.JAMA. 2019;321(19):1906-1915.