Paving stones, asphalt or concrete that clean the air above them. It sounds too good to be true, but it does seem to work. Mono-nitrogen oxides (NO and NO2) represent a danger to public health, particularly in busy city centres. And these harmful substances could be removed almost completely from the air due to a Dutch invention.
After two years of tests in the south-eastern city of Hengelo, scientists from the Technical University of Eindhoven heaved a sigh of relief. Their air-cleaning street paving, which was greeted with scepticism at the time, appeared to work. The new road surface turns almost half of the mono-nitrogen oxides from car exhausts into harmless nitrates.
Mono-nitrogen oxides are responsible for acid rain, smog and respiratory illnesses. Ever stricter European regulations are forcing busy cities to do something about the problem.
Red brick road
The Castorweg is a very ordinary street in Hengelo. Two years ago, it was paved with red bricks. There are millions of roads just like it across the Netherlands. The red bricks may look ordinary to the naked eye, but as their inventor Götz Hüsken, who is of German origin, explains:
"The upper layer of the brick contains titanium dioxide, which is a photo catalyser essential for breaking down mono-nitrogen oxides more quickly..."
In lay-man’s terms this means the natural process in which the sun breaks down mono-nitrogen oxides is accelerated when these gases come into contact with the titanium dioxide in the road surface. This happens automatically: the wind and the turbulence caused by passing vehicles ensure that polluted air continually passes over the air-cleaning bricks.
The new road surface is therefore very suitable for cities with narrow streets and relatively tall buildings. Here pollution tends to linger between the buildings, but busy traffic provides a lot of air turbulence so that the mono-nitrogen oxides can be broken down.
Professor Jos Brouwers, leader of the air-cleaning road surface project, proudly talks about the good results of air quality measurements:
"Up to now, we have made three extensive measurements since the road was laid like this, and we have measured between 25 and 45-percent reductions of mono-nitrogen oxides..."
Mr Hüsken explains what happens to the poisonous mono-nitrogen oxides:
"They change into nitrates and are simply washed off the road by rain."
However, nitrates are not totally harmless. Isn’t extreme algae growth in lakes caused by too many nitrates? But Mr Hüsken quickly dismisses this idea. For a start, mono-nitrogen oxides in the air are far more harmful than nitrates. Secondly, the quantities of nitrates produced are tiny. And finally, they are washed away by rainwater via the sewage system to treatment plants which have been coping with nitrates for many years.
So at the end of the day, there are only a few disadvantages. The road surface performs just as well in terms of sound production and durability, and the air-cleaning layer of titanium dioxide can also be applied to concrete or asphalt. There is only one drawback and that is the price: an air-cleaning brick is one and a half times more expensive than an ordinary brick. But on the other hand, the material costs of a square metre of road only make up a small part of the total costs. The rest consists of digging costs, sewage systems, and more than anything else, man hours.
Professor Brouwers draws a clear conclusion:
"So, it is a little more expensive. But at least it is practical to apply and produces much cleaner air. That’s allowed to cost a little, isn’t it?"