Titanium dioxide (TiO2) is in everything these days, from toothpaste to latex paint. Valued for opacity, it’s also a powerful antimicrobial and the key ingredient in “smog-eating” building products. That’s because TiO2 acts as a catalyst in the presence of UV light, helping break down virtually any organic compound—whether it’s a bacterium or a formaldehyde molecule.
Photo courtesy Jonathan Raff
But TiO2’s reactivity may be harder to control than previously thought. New findings suggest the chemical could actually produce more smog-forming chemicals than it breaks down.
Studies of smog-eating coatings typically look at the direct conversion of the smog-forming compounds nitrogen oxide and nitric oxide (collectively called NOx) into nitrate on the building’s surface. In theory, the TiO2 coating neutralizes the NOx before it can turn into smog, and the nitrate trickles harmlessly away the next time it rains.
The problem with this scenario is that there are many other organic compounds in ambient air. These include ammonia—and NH3 spells trouble for smog-eating coatings, according to Jonathan Raff, Ph.D., assistant professor of chemistry at Indiana University–Bloomington. Based on Raff’s findings, published recently in the Journal of the American Chemistry Society, TiO2 converts small concentrations of ammonia into NOx with water as a catalyst (the reaction picks up speed at about 30 percent relative humidity and slows down at around 50 percent). So with widespread use of smog-eating technology, “you could potentially have fluxes of NOx coming off these surfaces that are 13 percent of urban sources,” meaning the coatings themselves would be producing a significant amount of the city’s overall smog, Raff says.
Ammonia in the air isn’t the only thing to worry about. Raff argues that a big-picture study should also look at ozone, ammonia, and nitrate concentrations on the surfaces of buildings, not just in the air. Researchers in France have already shown that nitrate deposited on smog-eating surfaces can “renoxify” if it isn’t washed away, but Raff says there are other problems as well, like the possibility that nitrate could turn into nitrous acid. “Nitrous acid is really important for kick-starting smog, especially in the morning hours,” he says.
Raff specializes in atmospheric chemistry, but he says there could be implications inside buildings as well since TiO2-containing products are also being marketed for cleaning indoor air.
“There are definitely situations where you can have a lot of ammonia indoors, maybe even higher than outdoors,” he says. Add to that the fact that indoor air has a lot more time to contact interior surfaces. “I think that this could potentially be a source of NOx indoors. That would be really a serious thing. That’s not healthy,” he concludes
Raff worries about the lack of field monitoring that would measure the actual effects of smog-eating coatings on the air but says the wider implications of his findings have not yet been determined. TiO2 coatings are still a niche product, so it’s unlikely that smog-eating buildings will make a dent in the overall urban air, he says.
But if the technology takes off, its potential benefits and potential dangers would likely increase in equal measure.