Ozone exposure in early life increases asthma risk in kids

A new study finds that children exposed to higher ozone levels in their first two years are significantly more likely to develop asthma and wheezing by age six, highlighting the urgent need for early environmental intervention.

Study: Early-Life Ozone Exposure and Asthma and Wheeze in Children. Image Credit: Dennis MacDonald / Shutterstock.com

A recent JAMA Network Open study investigates the impact of early-life ozone (O₃) exposure on asthma and wheezing in children alone and in combination with other air pollutants.

Asthma in children

In 2021, approximately 6.5% of children living in the United States were diagnosed with asthma, the most common chronic disease affecting children throughout the world. Exposure to various environmental pollutants has been implicated in the development of asthma, some of which include fine particulate matter (PM_2.5), nitrogen dioxide (NO₂), and O₃.

To date, few studies have evaluated how long-term exposure to O₃ impacts asthma progression in children. Furthermore, existing studies have produced mixed results without considering how other circulating pollutants in the environment may also contribute to any observed asthma exacerbations.

Gaining additional insights into the relationship between O₃ exposure and chronic airway disease development is crucial to support progress in preventative strategies and future treatments. It is particularly important to understand whether early life exposure to O₃ impacts children’s health, as this period is critical for their immune and respiratory development.

About the study

The researchers hypothesized that greater ambient O₃ exposure, independently or in multipollutant models, would increase the likelihood of asthma and wheezing across all age groups.  

To this end, the association between O₃ exposure during the first two years of life and the risk of asthma and wheezing later in life was determined. The impact of O₃ exposure in a multipollutant mixture on asthma and wheezing between four to six years of age was also analyzed to predict disease trajectories up to nine years of age.

Study participants were recruited from three prospective pediatric cohorts of the Environmental Influences on Child Health Outcomes (ECHO) prenatal and Early Childhood Pathways to Health Consortium (ECHO-PATHWAYS) consortium. All eligible candidates had valid geocoded address histories between birth and two years of age, along with comprehensive data on their respiratory condition between four to six and eight to nine years of age. Preterm births were excluded from the cohort.

Early-life O₃ exposures between birth and two years of age were measured in parts per billion (ppb) using the point-based national spatiotemporal model. This model considered pollutant concentrations and hundreds of geographic covariates from regulatory agency monitors and research campaigns. Multipollutant models evaluated NO₂ (ppb) and PM_2.5 (μg/m³) exposure for same age groups.

Information on asthma onset and wheezing trajectories on specific ages were provided by caregivers in airway surveys.

Logistic regression analysis was performed with staged covariate modeling to calculate odds ratios (ORs) of asthma incidence due to early-life O₃ exposure. Multinomial regressions were used to compare wheezing phenotypes with non-wheezing children serving as the reference population.

Study findings

A total of 1,188 participants were selected from three cohorts with a mean age of 4.6 years at the four to six years age visit, 51.7% of whom were female and 81.9% had mothers with no history of asthma. At this time point, 12.3% of the children already had asthma and 15.8% had a current wheeze.

At the eight to nine year visit, the mean age of the participants was 8.9 years and 9.4% had developed strict asthma. Based on their airway assessments, these children were classified into different groups, with 59.5% of children categorized as never wheeze, 20.8% as early wheeze, 11.3% as late wheeze, and 8.3% as persistent wheeze.

For site-specific O₃ distributions, the mean ambient O₃ concentration between birth and age two was 26.1 ppb. Postnatal pollutants of 8.8 ppb of NO₂ and 9.3 μg/m³ of PM_2.5 were also recorded.

Primary model analysis revealed that a two ppb increase in early-life O₃ exposure was associated with an OR of 1.31 for current asthma and 1.30 for current wheeze at the age four to six year visit.

As compared to primary model results, secondary models with extended covariate adjustment revealed ORs of 1.26 for current asthma and 1.27 for current wheeze. Prenatal and co-pollutant adjustment for early-life O₃ and airway outcomes were not associated with any significant differences.

A combined O₃, NO₂, and PM_2.5 mixture was associated with an increased risk of asthma; however, this mixture did not have a significant impact on the development of current wheeze. Bivariate interactions identified consistent associations between O₃ and current asthma across all concentrations of NO₂.

The evidence for associations between O₃ and current asthma and wheezing were consistent for PM_2.5 at the 50^th or 90^th percentile but not for lower levels of PM_2.5.

Regulating and reducing exposure to ambient O₃ may help reduce the significant public health burden of asthma among U.S. children.”

Conclusions

Children exposed to O₃ during the first two years of their life are more likely to develop asthma and wheezing between four and six years of age, but not between ages eight and nine. In contrast to lower O₃ concentrations within mixture, higher concentrations increased the risks of asthma and wheezing in children.