Residential Exposure to Urban Trees and Medication Sales for Mood Disorders and Cardiovascular Disease in Brussels, Belgium: An Ecological Study

Abstract

BACKGROUND: The available evidence for positive associations between urban trees and human health is mixed, partly because the assessment of exposure to trees is often imprecise because of, for instance, exclusion of trees in private areas and the lack of three-dimensional (3D) exposure indicators (e.g., crown volume).OBJECTIVES: We aimed to quantify all trees and relevant 3D structural traits in Brussels (Belgium) and to investigate associations between the number of trees, tree traits, and sales of medication commonly prescribed for mood disorders and cardiovascular disease.METHODS: We developed a workflow to automatically isolate all individual trees from airborne light detection and ranging (LiDAR) data collected in 2012. Trait data were subsequently extracted for 309,757 trees in 604 census tracts. We used the average annual age-standardized rate of medication sales in Brussels for the period 2006 to 2014, calculated from reimbursement information on medication prescribed to adults (19-64 years of age). The medication sales data were provided by sex at the census tract level. Generalized log-linear models were used to investigate associations between the number of trees, the crown volume, tree structural variation, and medication sales. Models were run separately for mood disorder and cardiovascular medication and for men and women. All models were adjusted for indicators of area-level socioeconomic status.RESULTS: Single-factor models showed that higher stem densities and higher crown volumes are both associated with lower medication sales, but opposing associations emerged in multifactor models. Higher crown volume [an increase by one interquartile range (IQR) of 1.4 x 10(4) m(3)/ha] was associated with 34% lower mood disorder medication sales [women, beta = - 0.341 (95% CI: -0.379, -0.303); men, beta = - 0.340 (95% CI: -0.378, - 0.303)] and with 21-25% lower cardiovascular medication sales [women, beta = - 0.214 (95% CI: -0.246, -0.182); men, beta = - 0.252 (95% CI: - 0.285, -0.219)]. Conversely, a higher stem density (an increase by one IQR of 21.8 trees/ha) was associated with 28-32% higher mood disorder medication sales [women, beta = 0.322 (95% CI: 0.284, 0.361); men, beta = 0.281 (95% CI: 0.243, 0.319)] and with 20-24% higher cardiovascular medication sales [women, beta = 0.202 (95% CI: 0.169, 0.236); men, beta = 0.240 (95% CI: 0.206, 0.273)].DISCUSSION: We found a trade-off between the number of trees and the crown volumes of those trees for human health benefits in an urban environment. Our results demonstrate that conserving large trees in urban environments may not only support conservation of biodiversity but also human health.