California Air Resources Board

SACRAMENTO – Research by the California Environmental Protection Agency's Air Resources Board (ARB) shows that informed selection and planting of trees can reduce urban air pollution.  In a study presented to the Board, over 1400 tree species were compiled and rated for their various pollution impacts and pollen production.

    ARB Chairman Alan Lloyd said, "People plant trees for many reasons: to beautify their home, provide shade, and reduce energy bills.  But, they may not be aware that certain trees can decrease air pollution and pollen counts.  Necessary information is now available to choose the most effective smog and pollen reducing species."

    When considering a new tree, there are three main concerns: intended use (large or small shading needs), climate of area, and the tree's emissions types and amounts.  Large scale planting can affect air quality through regional concentrations of ozone and fine particles.  To reduce ozone concentrations in urban areas it is particularly important to use low emitting species.   When selected appropriately, trees and other plants can improve local cooling, reduce energy use and slow the chemical reactions that lead to the formation of ozone, or urban smog.

    The differences in emission rates from one species to the next can vary significantly.  Some plant species can release as much as 10,000 times more biogenic (naturally formed) volatile organic compounds (VOCs) than the more atmospherically friendly "low emitters."  For more information about selecting appropriate trees, see Cal Poly San Luis Obipso's website Selectree.

    The ARB recommends that you consider the tree's capacity to emit biogenic VOCs before you plant.  These compounds are particularly important because they create ozone and fine particulate matter (PM 2.5), which are damaging to the human respiratory system.

    Plants both positively and negatively affect air quality by:

• Local cooling
• Removing some pollutants
• Emitting biogenic Volatile Organic Compounds
• Releasing potent allergens

    Low-emitters include the Chinese Hackberry, Avocado, Peach, Ashes, Sawleaf Zelkova and the Eastern Redbud.  A few of the high emitters include the London Plane, California Sycamore, Liquidamber, Chinese Sweet Gum, Goldenrain Tree, and the Scarlet, Red and Willow Oaks.