The aluminum giant Alcoa has designed a building panel that cleans both itself and the nearby air.
The secret to this feat is titanium dioxide, the same compound that allows sunscreen to block the sun. In this application, when the sun hits a coat of titanium dioxide, the compound begins spewing electrons. According to Alcoa, those electrons join with oxygen and water molecules in the air. This results in highly reactive molecules, which glom onto organic materials stuck to the building surface or near it — anything from bird droppings to smog particles — and chew them up.
These messes then wash off the building; the company says it only takes a light rain or the morning dew. All that nastiness then becomes someone else's problem, probably the guy who has to wash the sidewalk in the morning, or whoever's in charge of the sewer system. But that's not a problem for the executive on the 45th floor who's patting himself on the back for buying a smog-eating building.
Alcoa claims that covering 10,000 square feet of a building with their material is equivalent to planting 80 trees. (The company also says independent testing confirmed that its product does clean air, but as always with such corporate claims, some skepticism of the word "independent" is advised.)