Judicious use of vegetation can yield rapid and sustained improvements in street-level air quality in dense urban areas. A 2012 study found that planting grass, climbing ivy, and other greenery in urban street canyons can reduce the concentrations of street level NO2 by up to 40% and PM by 60%, 8 times more than previously thought. Trees were also shown to be effective, but only if care is taken to avoid trapping pollutants beneath their crowns.