A study* by a team at Barcelona Institute for Global Health (ISGlobal) published in The Lancet has found that a 30% tree canopy cover can reduce premature deaths due to urban heat island impacts by 1/3rd.
*See study summary in The Lancet 
Cooling cities through urban green infrastructure: a health impact assessment of European cities
See report by the Guardian Planting more trees in cities could cut deaths from summer heat, says study
