Does the presence of vegetation affect asthma hospitalizations among the elderly? A comparison between rural, suburban, and urban areas

Climate change and increasing urbanization have intensified scientific interest in understanding the impact of vegetation cover on human health. While the elderly population (persons 65+y.o.) continues to grow, environmental determinants for asthma in this group remain poorly understood. Using spatial and time series analysis we investigated the effect of vegetation cover as measured by the Normalized Difference Vegetation Index (NDVI) on asthma hospitalization rates (AHR)(ICD-9 codes 493.0-9) among Medicare recipients in seven northeastern United States. We considered potential confounders, including income, land cover category, seasonal influenza activity, and population density, classified as urban, suburban, and rural. Time-series analysis identified seasonal patterns in elderly asthma hospitalizations as counter-phase with NDVI: when vegetation vigor increases, AHR decreases (R 2 =0.59, p=0.001). Winter peaks of influenza correlated with an increase in AHR (R 2 =0.43, p=0.001). Spatial multivariate analysis found HRs increased significantly (RR=1.048, 95% CI:1.072-1.086, p=0.001) as population density increased. Regardless of location, living in poverty increased AHR (RR=1.014, 95% CI:1.026-1.018, p=0.001). After adjusting for income and percent elderly population, evergreen vegetation in urban areas demonstrated a small, yet protective effect (RR=0.998, 95% CI:0.993-0.997, p=0.02). These results suggest that urban evergreen vegetation cover could be an important factor for reducing the risk of elderly asthma hospitalizations.