Stone washed

An 'air-purifying concrete' that uses titanium dioxide to remove harmful pollutants from the air is being tested in paving stones in

, the

.

Researchers from the University of Twente are paving a test road section in the town with air-purifying stones, the top layer of which will convert nitrogen oxide (NOx) from car exhaust fumes into harmless nitrates.

NOx is the pollutant that can cause acid rain and smog, and the researchers hope the problem can be partly solved by using the special stones.

'Our laboratory tests showed that typically 30 to 40 per cent of the NOx that flowed over the stones was degraded,' said Jos Brouwers, Twente's lead researcher.

Brouwers explained that titanium dioxide is a photocatalytic material that uses sunlight to convert the NOx in the air into harmless nitrates.

'Basically the photons from the UV light activate the titanium dioxide,' he said. 'OH ions are released and these react with the NOx which leaves nitrate salts in the stone.'