The next time you're in Segrate, near Milan, Italy, take an extra-deep breath. The main drag in Segrate has been repaved with cement coated with a chemical that actually scrubs the air, leaving it with 60% less nitric acid (a component of acid rain) than surrounding blocks. The discovery, like so many, was an accident: Italy's largest cement maker, Italcementi, was charged with making a white, self-cleaning concrete for a new church in Rome. Its solution was to spray titanium dioxide on the surface, which, it turned out, kept both the concrete and the air clean. The now-famous cemento mangiasmog (i.e., smog-eating cement), branded TX Active, is a sterling example of a new breed of "transmaterials" endowed with new properties, such as tiles that emit light or self-cleaning paints. Italcementi estimates that covering 15% of exterior surfaces with a titatium-dioxide coating would reduce air pollution by as much as 50%.