This week in our Why Trees Matter series we discuss another crucial benefit that trees provide: air pollution reduction. One aptly worded description for the role that trees play is that trees are the earth’s lungs. Trees filter the air of contaminates (such as nitrogen oxides, ammonia, sulfur dioxide, and ozone) by ingesting gaseous air pollution through tiny leaf openings called stomata (pictured right). Trees breathe in pollutants and then breathe out oxygen. One mature tree provides a day’s worth of oxygen for 4 people. The main sources of air pollution come from transportation (mobile), industry (stationary), agriculture (area), and nature (natural). Refer to the picture below.
In 2014, US Forest Service scientists and fellow researchers published a report detailing the largest study of air pollution removal by trees. The report concluded that trees save over 850 human lives a year and prevent 670,000 cases of acute respiratory symptoms--statistics based on air quality improvement of a mere 1 percent. In 2005, approximately 134,700 deaths in the US were attributed to air pollution. In a related study published in the journal Environmental Pollution, researchers estimated the value of human health benefits from air pollution reduction by trees to be almost $7 billion a year. The report noted that although most of the air pollution reduction occurred in rural areas (which have a higher concentration of trees), most of the health benefits were experienced in urban areas. One of the researchers concluded that because over 80 percent of Americans live in urban areas, the study reveals the crucial role that urban forestry plays in public health.
Trees emit gases themselves, called biogenic volatile organic compounds (BVOCs). Urban planners, however, can select low-emitting BVOCs trees since different species emit different amounts. Additionally, the amount of BVOCs is dependent on temperature—less emission occurs at lower temperatures—and since trees tend to lower temperatures, increasing tree coverage can reduce BVOCs.
So far we have seen how planting trees, particularly in highly populated urban areas, saves energy, reduces carbon in the atmosphere, and improves air quality. Next week we will continue learning why trees matter.
http://education-portal.com/academy/lesson/stomata-of-plants-function-definition-structure.html (picture 1)
http://www.nature.nps.gov/air/AQBasics/sources.cfm (picture 2)
“Tree and Forest Effects on Air Quality and Human Health in the United States” report: http://www.fs.fed.us/nrs/pubs/jrnl/2014/nrs_2014_nowak_001.pdf
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