The stones work by having a top layer impregnated with titanium dioxide, a chemical that when exposed to sunlight binds with oxides of nitrogen produced by cars and other pollution sources. The pollutants are transmuted into nitrates, which are not harmful to the environment, during the process. Then the next time the rain comes along, it washes the stones clean.
Hengelo, and specifically the Castorweg area, was chosen for its relatively high volume of automobile traffic. The project will divide the street in two sections, one of which will be paved with normal stones, and the other with the special air-cleaning units. Air quality will then be monitored to gauge how much impact the stones have in real-world applications.
Another feature of the stones is that they repel dirt, helping the road to stay clean with out the use of human interference through street sweepers or other means. The test is being made possible in part by the stone producer, Struyk Verwo Infra, and as part of its 'Effective Sustainability' program, the host province of Overijssel is providing a grant as well.
The road will be finished by late 2008, with the air quality monitoring program to start early next year. The university hopes to announce its first results by summer 2009.