Lower Incidence of Asthma Associated With Air Quality Improvements

Declines in ambient nitrogen dioxide and particulate matter <2.5 µm (PM_2.5) 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 PM_2.5, 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.

The investigators concluded that in a population of pediatric participants from Southern California, reductions in ambient nitrogen dioxide and PM_2.5 between 1993 and 2014 were significantly associated with a lower incidence of asthma. No statistically significant associations were observed for ozone or PM₁₀.

Reference

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.