Growing Urban Forests: The Secret to Cleaner, Cooler, City Air

Nov 21, 2012 by

by American Society of Landscape Architects | 1:52 pm October 6th, hospital 2011

From our sustainable landscape series, malady check out how urban forests cool and clean the air, and store excess carbon, and improve the health of residents. Learn more at on our website.

Poor air quality has led to an explosion of asthma cases and other health problems among vulnerable populations including children, the elderly, and low-income residents. Each year bad air causes two million deaths worldwide. Also, in the U.S., there have been 8,000 premature deaths from excessive heat over the past 25 years. Urban heat islands, which are caused, in part, by sunlight being absorbed by paved surfaces and roofs, lead to higher surface temperatures, up to 90 degrees. Atmospheric air temperatures are also higher: in the day by up to 6 degrees, and at night, by up to 22 degrees. Vulnerable populations also face greater risks of heat exhaustion. (Source: World Health Organization (WHO) and Heat Island Impacts, U.S. Environmental Protection Agency (E.P.A.) )

Increasing the tree canopy in cities is one way to fight both poor air quality and urban heat islands. Research shows significant short-term improvements in air quality in urban areas with 100 percent tree cover. There, trees can reduce hourly ozone by up to 15 percent, sulfur dioxide by 14 percent, and particulate matter by 13 percent. U.S. trees remove some 784,000 tons of pollution annually, providing $3.8 billion in value. Furthermore, a single large healthy tree can remove greater than 300 pounds of carbon dioxide from the atmosphere every year. In fact, New York City’s urban forest alone removes 154,000 tons of CO2 annually. Through their leaves, trees also provide evaporative cooling, which increases air humidity. Shaded surfaces may be 20-45 degrees cooler, and evapotranspiration can reduce peak summer temperatures by 2-9 degrees. (Source:“Heat Island Mitigation: Trees and Vegetation, U.S. Environmental Protection Agency (E.P.A.) and “Sustaining America’s Trees and Forests,” David J. Nowak, Susan M. Stein, Paula B. Randler, Eric J. Greenfield, Sara J. Comas, Mary A. Carr, and Ralph J. Alig, U.S. Forest Service. )

Some other benefits: Urban forests reduce energy use by providing shade in the summer and wind breaks in the winter, reduce stormwater runoff, remediate soils, and provide animal and plant habitat. Trees have economic benefits: they increase property value. Lastly, trees have positive cognitive effects and may even help improve moods. (Source: “Sustaining America’s Trees and Forests,” David J. Nowak, Susan M. Stein, Paula B. Randler, Eric J. Greenfield, Sara J. Comas, Mary A. Carr, and Ralph J. Alig, U.S. Forest Service; “Does Looking at Nature Make People Nicer?” The Dirt blog and “The Restorative Effects of Nature in Cities,” The Dirt blog)

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