Windows that clean themselves may seem too good to be true, but so-called photocatalytic-cleaning glass is likely to take over this annoying chore.

The new glass is made by fusing a light-sensitive coating of titanium dioxide into the glass.

When sunlight falls on the glass, the photocatalytic action gradually loosens dirt and other impurities.

The glass also causes rainwater to cover the surface evenly, suspend dirt and wash it away.

Titanium oxide is harmless to humans and is widely used as an ingredient in face powder and food additives, experts say.

While commercial application has only recently begun, Mitsubishi Research Institute predicts the market for photocatalytic materials and related products will expand to more than 1 trillion yen in the near future.

Nippon Sheet Glass Co. started marketing photocatalytic-cleaning glass under the brand name Cleartect across the country in February.

The Osaka-based company had been test marketing it for use in large office buildings and airports.

Cleartect costs about 9,000 yen more per sq. meter than regular glass. But the company decided to market it for use in residential houses after it was swamped with inquiries from individual customers, a company official said.

The company installed Cleartect in the houses of some of its employees and "it proved its effectiveness immediately," the official said.