Milan, Italy is one of Europe’s most polluted cities, its air quality frequently breaching safety limits set by the EU and causing city officials to install a ventilation system in 2009 in an effort to reduce damage to Leonardo da Vinci’s The Last Supper that resides in the Santa Maria delle Grazie church. In 2003, a medical study compared breathing air in Milan to smoking almost a pack of cigarettes each day. On top of that, there is less space dedicated to vegetation in Milan than any other Italian city.

Short on space for increasing the presence of greenery, the city turned to architect Stefano Boeri to create the world’s first vertical forests, incorporated into the “Bosco Verticale” apartment towers, currently under construction and nearing completion.

The two residential towers, which are part of a rehabilitation project in the historic district between Via De Island Castillia and Confalonieri, loom 111 meters and 78 meters and will be home to over 900 trees that cover nearly 9,000 square meters of terrace space. They are 24 floors and 17 floors high with combined capacity for 730 trees, 5,000 shrubs, and 11,000 plants.

In addition to producing oxygen, mitigating smog, and providing an ecosystem for insects and birds, the trees and plants on the terraces will help to cool the apartments and reduce the energy costs for air-conditioning, especially when summers in Milan can get hotter than 100 degrees Fahrenheit.

The vertical forest idea has inspired a social housing tower in Spain, called the Torre Huerta, and a “Flower Tower” in Paris that features nearly 400 bamboo plants on its ledges.