From catalytic converters to alternative fuels, the fight against big-city smog has for years been fought inside combustion engines and exhaust pipes.
Now, scientists are taking the fight to the streets by developing “smart” building materials designed to clean the air with a little help from the elements.
Using technology already available for self-cleaning windows and bathroom tiles, scientists hope to paint cities with materials that dissolve and wash away pollutants when exposed to sun and rain.
“Among other things, we want to construct concrete walls that break down vehicle exhausts in road tunnels,” said Karin Pettersson, a spokeswoman for Swedish construction giant Skanska. “It is also possible to make pavings that clean the air in cities.”
The Stockholm-based company is part of a $1.7 million Swedish-Finnish project to develop catalytic cement and concrete products coated with titanium dioxide, a compound often used in white paint and toothpaste that can become highly reactive when exposed to ultraviolet light.
This is the idea: UV rays hitting the titanium dioxide trigger a catalytic reaction that destroys the molecules of pollutants, including nitrogen oxides, which are emitted in the burning of fossil fuels and create smog when combined with volatile organic compounds.
Exposure to high levels of nitrogen oxides can trigger serious respiratory problems, including lung damage.
The catalytic reaction also prevents bacteria and dirt from sticking to a surface, making them easily removed by a splash of water or rain.
Bo-Erik Eriksson, head of research at Cementa, another company participating in the Swedish-Finnish project, said the byproducts of the reaction, called photocatalysis, are benign, though it depends on what substances are involved: Organic compounds are broken down into carbon dioxide and water, while the nitrogen oxides yield nitrate salts.
Research in the field has been made possible by the revolution in nanotechnology — science dedicated to building materials from the molecular level. The catalytic properties of titanium dioxide become active when it is applied in a very thin layer, or in microscopic particles.
A range of self-cleaning products coated with titanium dioxide, including windows and ceramic tiles, are already on the market, but the focus has mostly been on their practical value rather than the environmental impact.