Cleaning the air of pollution is a traditional job for urban trees, but lately buildings, and even billboards, have got in on the act. However, it turns out we don't need to to substitute visual pollution for the atmospheric version. A study has revealed the potential of roofing tiles to turn every house into a battle station for clean air.

Titanium dioxide is a highly versatile compound, used in plastics, sunscreen and food. Titanium Dioxide's intense whiteness makes it particularly popular in paints, and it is the basis of a new generation of solar cells. 

However, its capacity to fight pollution comes from its status as a photocatalyst. When exposed to ultraviolet light TiO₂accelerates many chemical reactions including the oxidation of smog producing nitrogen oxides and volatile organic compounds (ironically most famous as “new paint smells”). The catalytic poetry billboard used nanoparticles of titanium dioxide to turn nitrogen oxides to soluble nitrates, and volatile organic compounds to fatty acids.

However, although it doesn't always seem like it, cities have far more roofing space than billboards. Students at the University of California, Riverside set out to test how roof tiles coated with TiO₂ affect nitrogen oxides in a chamber. They found that a thick coating could remove 97% of the target gasses. However, another tile, with just one twelfth the catalyst, removed 88%, suggesting a much more cost effective option. They estimate just $5 will cover the cost of coating for a typical new roof.

The students calculated that an average sized American roof could, with the thinner coating, remove 21 grams of nitrogen oxide a day. Over a year this comes to as much of this form of pollution as a typical car releases driving 18,000km. Smog fighting roofs already exist, but the students found there had been no reliable research on how well they work.

The team involved with this research are all graduating, but have left a set of ideas for further tests, such as coating concrete rather than roofing tiles and seeing how long the coating continues to work.

Sadly, to trap the carbon dioxide released by cars or other emission sources will require harder to install roofs. However, the lighter color of the coated tiles does have a small but useful cooling effect.