Is local air pollution concentration a moderator or mediator of the association between residential greenspace and pediatric asthma exacerbations? A longitudinal study of pediatric patients in Philadelphia, Pennsylvania

Abstract

Background

Despite mixed evidence supporting the link between greenspace and asthma exacerbations, several studies suggest a negative association. The mechanisms underlying this relationship are unclear, with air pollution concentrations potentially playing a key role. This study investigated whether air pollution concentrations modify or mediate the relationship between residential greenspace and pediatric asthma exacerbations.

Methods

Data were drawn from a pediatric asthma cohort at Children's Hospital of Philadelphia (2011–2016), including children aged <18 years. Participants were followed from their initial visit until their first asthma exacerbation. Greenspace (tree canopy, grass/shrub cover) near homes was assessed. Daily air pollution data, including PM_2.5, ozone, NO₂, and SO₂, were obtained from the US Environmental Protection Agency. Hazard ratios (HRs) and 95% confidence intervals (CIs) were estimated, adjusting for individual and neighborhood characteristics. Effect modification by air pollution concentrations was tested, and causal mediation analyses were conducted.

Results

On days with high SO₂ concentrations, children living near the highest quartile of tree canopy coverage had a 14% lower incidence of asthma exacerbations compared to those in the lowest quartile (HR = 0.86, 95% CI = 0.74, 0.98). Conversely, on days with low SO₂ concentrations, tree canopy coverage was associated with a nonsignificant 12% higher incidence (HR = 1.12, 95% CI = 0.97, 1.28). Similar patterns were observed for NO₂, but no interactions were found for PM_2.5 or ozone. Mediation analyses indicated no significant mediation by air pollution.

Conclusion

In urban areas with high SO₂ or NO₂ levels, greenspace near homes may support children with asthma by mitigating air pollution's impact, suggesting greenspace-based urban strategies.