Take ordinary cement. Mix in an agent called a photo-catalyzer (titanium dioxide, if you really want to know), which speeds up the natural process that breaks down smog into its component parts. Now start paving things with the stuff. That's what they're doing in Segrate, an Italian town near Milan. The smog-eating cement is called TX Active, and the Italian firm Italcementi spent 10 years developing it. Now there's a busy street in Segrate that's covered with it, and Italcementi claims it has reduced nitric oxides in the area by as much as 60%. Bonus: buildings made with TX Active stay cleaner too.