Trees, bushes and other greenery growing in the concrete-and-glass canyons of cities can reduce levels of two of the most worrisome air pollutants by eight times more than previously believed, a new study has found. A report on the research appears in the ACS journal Environmental Science & Technology.

Thomas Pugh and colleagues explain that concentrations of nitrogen dioxide (NO₂) and microscopic particulate matter (PM) — both of which can be harmful to human health — exceed safe levels on the streets of many cities. Past research suggested that trees and other green plants can improve urban air quality by removing those pollutants from the air. However, the improvement seemed to be small, a reduction of less than 5 percent. The new study sought a better understanding of the effects of green plants in the sometimes stagnant air of city streets, which the authors term "urban street canyons."

The study concluded that judicious placement of grass, climbing ivy and other plants in urban canyons can reduce the concentration at street level of NO₂ by as much as 40 percent and PM by 60 percent, much more than previously believed. The authors even suggest building plant-covered "green billboards" in these urban canyons to increase the amount of foliage. Trees were also shown to be effective, but only if care is taken to avoid trapping pollutants beneath their crowns.