When the Milan Expo opens next year, the centerpiece building will be a masterpiece of sustainable engineering. Designed around the idea of an urban forest, the new Palazzo Italia will generate its own electricity, and will be clad in materials specifically designed to clean the surrounding air. The designers, Nemesi & Partners, are using photocatalytic cement - basically, concrete that's been mixed with titanium oxide. When the building material comes into contact with ultraviolet light, the titanium oxide reacts with nitrogen dioxide in the air, converting the pollutant to a salt that can easily be washed away. The building will open in time for the Expo's launch next May, and we're already planning to book a trip over so that we can spend a day sniffing the air next to the building.