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 PM2.5, 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 PM2.5 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.