Buildings, roads and sidewalks have developed an appetite for air pollution. Researchers in Japan and Hong Kong are testing construction materials coated with titanium dioxide--the stuff of white paint and toothpaste--to see how well they can fight pollution. Better known as a pigment for whiteness, titanium dioxide can clear the air because it is an efficient photocatalyst: it speeds the breakdown of water vapor by ultraviolet light. The results of this reaction are hydroxyl radicals, which attack both inorganic and organic compounds, and turn them into molecules that can be harmlessly washed away with the next rainfall. But it wouldn't work to smear toothpaste on the sidewalk--the titanium dioxide crystals in such applications are too large (about 20 to 250 nanometers wide). The width of the pollution-fighting form is about seven nanometers, offering much more surface area for photocatalysis.

In the early 1970s researchers from the University of Tokyo described titanium dioxide's photocatalytic abilities. Since then, scientists have exploited the compound to kill bacteria on hospital surfaces and to treat contaminated water. Fighting nitrogen oxide on the streets is the latest twist. In Hong Kong, concrete slabs coated with titanium dioxide removed up to 90 percent of nitrogen oxides, most commonly spewed from older cars and diesel trucks and a contributor to smog, acid rain and other environmental headaches. In taking care of the contaminants, a coating of titanium dioxide did in minutes what the environment does in months, says Jimmy Chai-Mei Yu, a chemist at the Chinese University of Hong Kong. Moreover, he adds, because titanium dioxide is a catalyst, it could last forever.

This article was originally published with the title Paving Out Pollution.

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