Mixture analysis identifies ozone as driving factor for child asthma symptoms

The natural distributions of ambient air pollutants are often correlated. Existing studies have found that exposures to various air pollutants are associated with elevated risks of asthma symptoms among children. However, most studies applied single-pollutant models, which cannot distinguish between causal effects and associations due to correlations with other measured or unmeasured pollutants.

Objective

Investigate air pollutant mixtures and child asthma symptoms and identify key risk factors.

Methods

The Children’s Health and Air Pollution Study (CHAPS) recruited 299 children in Fresno, California, 63 of whom had ever-diagnosed asthma. We assessed the children’s prior 12-month exposures to eight ambient air pollutants, namely, PM2.5, PM10, nitrogen oxides (NOx), nitrogen dioxide (NO2), ozone, carbon monoxide (CO), elemental carbon (EC), and polycyclic aromatic hydrocarbons (PAHs), and asthmatic symptoms (wheeze and cough) at two visits, at approximately 7 and 9 years of age. We conducted repeated measures analysis with mixture analysis methods, including principal component analysis (PCA) and quantile-based g-computation (q-gcomp).

Results

The eight air pollutants exhibited strong inter-correlation. In single-pollutant models, exposure to ozone was associated with higher risk of cough (Odds ratio = 1.39, 95% confidence interval: 1.06 - 1.82). Using PCA and q-gcomp, exposures to NOx, EC and ozone had relatively high contributions to cough and wheeze. The association between ozone and cough was consistently positive from single-pollutant models, double-pollutant models, PCA and q-gcomp with negative control.

Conclusion

Ozone stands out among the eight air pollutants and may be a driving risk factor for persistent cough among children with asthma.