Titanium dioxide: environmental white knight?

T itanium dioxide (TiO 2) is a multi-faceted compound. It's the stuff that makes toothpaste white and paint opaque. TiO 2 is also a potent photocatalyst that can break down almost any organic compound, and a number of companies are seeking to capitalize on TiO 2 's reactivity by developing a wide range of environmentally beneficial products. During the late 1970s, scientists began to realize that the propensity of TiO 2 to absorb energy from the UV end of the solar spectrum and then react with water vapor to produce oxygen could be used to create surfaces that were, for all practical purposes, self-cleaning. According to Daniel Blake, principal scientist at the Department of Energy's National Renewable Energy Laboratory in Golden, Colorado, when TiO 2 is exposed to UV light of a wavelength below 385 nanometers in the presence of water vapor, two highly reactive substances are formed: hydroxyl radicals [OH] and a superoxide ion [O 2-1 ]. Says Blake, " These are highly reactive chemical species. Hydroxyl radicals are very strong oxidizers and will attack all kinds of organic materials, including those that make up living cells. " Toto Frontier USA, based in Long Beach, California, is using TiO 2 to make self-cleaning ceramic tiles for use in hospitals, public restrooms, and other settings where cleanliness is vitally important, as well as on hard-to-reach rooftops, where simple sunlight and rainwater can help prolong the life of the roof. The tiles are made by applying a one-micron layer of TiO 2 to ceramic blanks immediately as they exit the kiln. (Toto has filed a patent for their specially developed " short fire " kiln, which makes the process more economically competitive, although Harvey Malloy, director of sales and marketing for the company, admits that the tiles still cost 10–20% more than regular ceramic tiles.) The result is a photocatalytic coating that bonds with the tile and remains in place for the normal life of the tile itself. This coating has several intriguing properties. First, it can break down organic matter into carbon dioxide (CO 2) and water. When organic matter lands on a TiO 2-coated tile and water is applied, the hydroxyl radicals break down the cell wall and outer membrane , allowing cell contents to leak out and TiO 2 particles to enter, thereby causing cell damage and death. Hydroxyl radicals are highly reactive and thus short-lived. Superoxide ions, …