Researchers at Eindhoven University of Technology (EUT) in The Netherlands are developing a road surface that can reduce the concentration of nitrogen oxides (NOx) in the air.

The concrete stones contain titanium dioxide, a photocatalytic substance that removes NOx from the air and converts them to harmless nitrate particles with the help of sunlight.

The converted nitrate is rinsed away by the rain and because the stones also break down algae and dirt, they always remain clean.

NOx is emitted from car exhausts and is a major contributor to smog and acid rain.

Initial testing on a 1000 square metre section of road in Hengelo showed that the air-purifying concrete reduced the concentration of NOx by 25 percent to 45 percent in the air between 0.5m and 1.5m above the road.

EUT officials said although the air-purifying stones are around 50 percent more expensive than standard concrete stones, the total cost of constructing a road with the stones is only 10 percent higher.

The paving stones are already available for use, with further testing planned for later this year.