Green Walls, Clean City Using Trees and Plants to Improve Urban Air Quality

green walls' width=Concentrations of nitrogen dioxide (NO2) and microscopic particulate matter (PM) exceed safe levels on many city streets, and both represent a big health problem. According to the World Health Organization, more than one million people worldwide die from the consequences of polluted air every year. Past research has suggested that trees and other green plants (aka “green infrastructure”) can improve urban air quality by removing these pollutants from the air; but only by a small percentage. When Thomas Pugh, Ph.D., of the Karlsruhe Institute of Technology in Germany and his colleagues took a second look at the effects of the judicious placement of grass, climbing ivy and other plants on city air quality, they found that levels of NO2 and PM were actually dramatically reduced, with PM reduced by 60% and NO2 by as much as 40%. Green walls were found to be the most effective green infrastructure project for cleaning the air.

“Big initiatives to fight air pollution, such as car-scrap bonuses, catalytic converters or the introduction of a city toll are not sufficient,” says Rob MacKenzie, professor of atmospheric science at the University of Birmingham. “Green walls may be an additional help. They are capable of cleaning the air that enters and remains in the city. When positioned strategically, they are a comparably simple means to locally solve problems.”

The study, published in the journal Environmental Science & Technology, also recommended incorporating “green billboards” into urban areas—an easy way to increase vegetation “where pollution is highest.”

“Green areas may be grown road by road without expensive or big initiatives,” Pugh says.

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