Students from University California Riverside have designed a roof tile that breaks down smog particles present in the atmosphere. Roof tiles are coated with a layer of titanium– oxide, the titanium–oxide then interacts and breaks down nitrogen–oxide gas– what is emitted by motors that run on fossil fuels. Titanium–oxide is fairly abundant and very cheap, it will only cost about $5 to coat an average sized roof with titanium–oxide. | See pictures of the tiles on Inhabitat