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Wire Service Photo Gallery 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.
"It is also possible to make pavings that clean the air in cities." This is the idea: UV rays hitting the titanium dioxide trigger a catalyti c reaction that destroys the molecules of pollutants, including nitrogen oxides, which are emitted in the burning of fossil fuels and create smo g when combined with volatile organic compounds. Exposure to high levels of nitrogen oxides can trigger serious respirator y 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.
Cementa, another company partic ipating in the Swedish-Finnish project, said the byproducts of the react ion, called photocatalysis, are benign, though it depends on what substa nces 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 nanotec hnology -- science dedicated to building materials from the molecular le vel. 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 impa ct. In Rome, the Dives in Misericordia church, designed by US-based archite ct Richard Meier, is made of self-cleaning concrete that helps keep the surface shiny white. In Japan, several modern buildings, including the M arunouchi Building in downtown Tokyo, are covered with photocatalytic ti les to reduce discoloring from pollution.
Italcementi, maker of the concrete for the church in Rome. In a test in 2003, the company coated 75,000 square feet of road surface on the outskirts of Milan with photocatalytic cement. It found nitrogen oxide levels were reduced by up to 60 percent, depending on weather cond itions. A similar experiment in France found nitrogen oxide levels were 20 percen t to 80 percent lower in a wall plastered with photocatalytic cement tha n one with regular cement. "Now we want to find out if it works optimally and economically and make sure it has a long-lived effect that does not disappear after a couple o f years," said Eriksson of Cementa. Galimberti said Italcementi's products are 30 perc ent to 40 percent more expensive than regular concrete, and using the ex ternal air quality as a selling point doesn't necessarily appeal to buil ders with tight budgets. The company's sales pitch is that self-cleaning materials will save money in the long run. However, some scientists caution it's too soon to declare a titanium diox ide-fueled war on pollution. "Trying to clean up air pollution seems to me to be a stretch," said Reyn aldo Barreto, a chemistry professor at Purdue University in Indiana. But there's an awful lot of air and not a whole lot of surface."
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