Streets is basically accounting for secondary benefits of trees. The avoided category in the Streets Air Quality report is the amount of air pollution that is avoided because the trees are reducing energy consumption by shading in the summer and protecting from wind in the winter. Since the trees are reducing energy consumption in nearby buildings, fewer fossil fuels need to burned to create the energy and therefore some pollution is avoided. The numbers are based on the amount of reduction in energy consumption and the average types of energy use in your region. If for example most of the energy in your region is from hydroelectric sources the avoided pollution would be relatively low since hydroelectric sources do not produce air pollution.
Refer to your regional Community Tree Guide for more details. The USFS Community Tree Guides can be accessed from the Resources - Archives page http://www.itreetools.org/resources/archives.php under the i-Tree Streets Reference City Tree Guides tab
Section for FAQs regarding i-Tree Streets (STRATUM)
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