A road that cleans the air

Take crushed limestone, add some gravel, throw in a bit of cement and you’ve got the basic recipe for concrete. Then add a white coating of titanium dioxide and you’ve got a powerful air scrubber that’s now helping to clean air in cities across the globe.

Titanium dioxide is a naturally occurring photocatalytic chemical that reacts with sunlight to remove nitrogen oxides—car exhaust pollutants that cause smog and acid rain—from the atmosphere by turning them into nitrates that can be washed away by rain. Tests show that when added to concrete it removes anywhere from 35 to 60 per cent of those chemicals from surrounding air, and, because titanium dioxide also breaks down dirt, it makes concrete self-cleaning.

“[It] could be a very feasible solution for inner-city areas where they have a problem with air pollution,” Jos Brouwers, a researcher studying photocatalytic concrete at the University of Eindhoven in the Netherlands, told CNN. “You can apply it very easily in normal production. It doesn’t require any maintenance; it doesn’t wear off with normal use.”

His team paved a city road in the eastern Netherlands with bricks coated with the chemical, and monitored it for two years. Although the project was originally met with skepticism, the results spoke for themselves: the pollution around the test road was reduced by almost half.

The new development isn’t a panacea. It costs about 50 per cent more than regular concrete, and the nitrates it produces accelerate algae growth in nearby bodies of water, which can be harmful to aquatic life. Brouwers says this isn’t a problem. “If you look at the total pavement costs, where the stone is one part—there is also labour, foundations, etc., to calculate—then you are only looking at a slightly higher cost,” he says. And some experts say the amount of nitrates produced is relatively small and far less harmful than the pollutants the concrete sequesters.

The new material may soon become a valuable tool in the fight against pollution. It’s already been installed in the U.S., France and Italy. Now, Brouwer says, more governments “need to be convinced it is a feasible technology.”