A start-up company called Green City Solutions has developed 4m high vertical plant walls with moss, called CityTrees. These moss installations feed on dirty air; they filter CO2, NOx and dust pollution and convert them to their own biomass. The company aims to tackle one of the biggest problems for people living in cities worldwide – air pollution. Every CityTree reduces local air pollution in a proximity until 50m by up to one third. One CityTree equals to 275 natural trees and can convert 240 tons of CO2 equivalents from dust pollution and soot per year. The cost of a CityTree is about 25,000 to 50,000 Euro, when 275 natural trees would cost about 1 million Euro. The CityTrees are self-sustaining with energy and water due to connected solar cells and rain water collection systems and need only few hours of maintenance per year. However the company doesn’t aim at replacing natural trees but complementing them in an effort to fight air pollution. Currently about 50 systems are installed all over the world (including Germany, Norway, Great Britain, France and the Netherlands).