The NDVI levels were obtained using the Terra Moderate Resolution Imaging Spectroradiometer Vegetation Indices (MOD13Q1) from the US Geological Survey.
%GA data for 922 cities and 47 greater cities were obtained from the European Urban Atlas 2012. For 39 cities and one greater city in the UK data were retrieved from the UK Land Cover Map 2015. For 17 cities and one greater city, data were retrieved from the Corine Land Cover 2012 inventory.
For both measures, green space data was estimated at the grid-cell level (250 m x 250 m resolution). To reflect the WHO recommendation of residential exposure to green spaces, the total averaged NDVI value was estimated by adding a 300 m buffer around each grid cell to indicate the proximity to greenness (ie, about 5 min walk along walkable pathways).