A team of University of California, Riverside's Bourns College of Engineering students created a new titanium dioxide roof tile coating that when applied to an average-sized residential roof breaks down the same amount of smog-causing nitrogen oxides per year as a car driven 17,703km.

They calculated 21 tonnes of nitrogen oxides would be eliminated daily if tiles on one million roofs were coated with their titanium dioxide mixture.

They also found it would cost only about USD 5 for enough titanium dioxide to coat an average-sized residential roof.

Nitrogen oxides are formed when certain fuels are burned at high temperatures. Nitrogen oxides then react with volatile organic compounds in the presence of sunlight to create smog.

Currently, there are other roofing tiles on the market that help reduce pollution from nitrogen oxides. However, there is little data about claims that they reduce smog.

The students set out to change that. They coated two identical off-the-shelf clay tiles with different amounts of titanium dioxide, a common compound found in everything from paint to food to cosmetics.

The tiles were then placed inside a miniature atmospheric chamber that the students built out of wood, Teflon and PVC piping.

The chamber was connected to a source of nitrogen oxides and a device that reads concentrations of nitrogen oxides.

They used ultraviolet light to simulate sunlight, which activates the titanium dioxide and allows it to break down the nitrogen oxides.

They found the titanium dioxide coated tiles removed between 88 per cent and 97 per cent of the nitrogen oxides.

They also found there wasn't much of a difference in nitrogen oxide removal when different amounts of the coating were applied, despite one having about 12 times as much titanium dioxide coating.