Trees function in the sound reduction process by modifying humidity and climate, by absorbing sound and by deflection and refraction. McCaul, John. 1990. Poole, William. 1993. The Case for Urban Open Space. Draft report prepared for Trust for Public Land, San Francisco, California.
Human activity place a stress on urban environments that green spaces help relieve.
Trees and other vegetation absorb ozone, carbon dioxide, sulfur dioxide, nitrogen dioxide and other noxious air pollutants. Trees remove dust and particles from the air, including dangerous heavy metals. During a one growing season, a single sugar maple 12-nches in diameter was found to have removed 60 milligrams and 140 milligrams respectively of the heavy metals cadmium and lead. The transpiration of water by plants helps control and regulate humidity and temperature. A single tree can remove as much heat from the air as five average-sized air conditioners.