While much attention has been given to reducing air pollution and exhaust from cars, a new concept is taking hold in Paris, Frankfurt, Madrid and other cities across Europe: green walls.
The design calls for leafy vegetation fixed to the outside of a building. The benefits are many: It keeps "street canyons," streets and sidewalks between tall buildings, cool; it reduces nitrogen dioxide and particulate pollution in the air; and it takes advantage of the "more underutilized spaces in our urban areas," Paul Erlichman of a Toronto environmental design group.
The design also recycles water by collecting it at the bottom and returning it to the top.
See the story and photos at National Geographic.