It's a fact that trees can absorb CO2 from the air and mitigate the impact of air pollution, but in many large cities there simply isn't room for much green space. In Italy, during 2012, there were an estimated 84,000 pollution related deaths. Milan was, at one point, the most polluted city in Europe. To combat the problem, designers of a pair of sky scraper apartment buildings engineered the terraces of the buildings to hold hundreds of trees. The pair of buildings, known as the Bosco Verticale (translated it means Vertical Forest), has large trees on all sides.
Rather than just having a window box of flowers that look pretty, the Bosco Verticale trees are functional. They can reduce the amount of fine particulate matter floating around, almost sucking the pollution out of the air, making the city cleaner and the air easier on the lungs. Between the two buildings there are over 25,000 trees, and scientists and arborists are monitoring them closely to see what impact they are having on the pollution problem. Other cities that have air quality issues are using "living walls" and similar building technology to reduce their CO2 as well.