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.
In a large city such as Milan, researchers have calculated – on the basis of test results – that covering 15% of visible urban surfaces with products containing TX Active would enable a reduction in pollution of approximately 50%.