Do photocatalytic finishes really clean the air?

570 Broome closeup
© 570 Broome

No, there is no magic bullet, we have to stop pollution at the source.

Inhabitat sports an intriguing headline: This NYC skyscraper will clean the air “at a rate of 500 trees.” Lacy Cooke writes that 570 Broome, a new condo in New York City, is clad in a new facade material that "will have an equivalent impact to removing 2,000 cars from roads for a year, or that of 500 trees."

570 Broome Exterior© 570 Broome

The tower is covered in Neolith sintered stone panels that are treated with Pureti," an aqueous and titanium dioxide nanoparticle-based treatment that is sprayed on Neolith to create a photocatalytic, self-cleaning and decontaminating effect." From the Neolith website:

When the surface enters into contact with sunlight (or some LED lights), titanium
dioxide particles are activated which use light energy to transform the moisture in the air into oxidizing agents which destroy the nitrogen dioxide particles and contaminating agents and transform them into water vapor and salt.

There is no mention of the photocatalytic panels on the 570 Broome website; the only reference I can find is an interview with the architect, Tahir Demircioglu, whotells New York Spaces that the building "features a state-of-the-art façade that is not only self cleaning but actually purifies the air through photocatalysis and superhydrophilicity."

fresh2.jpg

I was surprised to see all of this because I had thought that the idea that titanium dioxide photocatalytic finishes actually did any good had been pretty much debunked, which was pretty embarrassing since TreeHugger was full of posts about its wonders, back when the pictures were small. My favourite was a compact fluorescent bulb covered in the stuff, so that it cleaned the air while you used it. I gushed that it had "two favourite TreeHugger subjects, CFLs and Titanium Dioxide, rolled into one."

But then a 2013 study concluded that they did more harm than good, by also converting ammonia into nitrogen oxides.

"We show that uptake of atmospheric NH3 (ammonia) onto surfaces containing TiO2 (titanium dioxide) is not a permanent removal process, as previously thought, but rather a photochemical route for generating reactive oxides of nitrogen that play a role in air pollution and are associated with significant health effects," the authors write.

More recently, a big study, Paints and Surfaces for the Removal of Nitrogen Oxides, prepared for Departments of the environment in Scotland, Wales and Northern Ireland concluded that they were pretty much useless.

Taken as a whole, there is little current evidence to suggest the widespread use of photocatalytic surfaces will reduce ambient concentrations of NO2. Furthermore, there is a risk that these materials will result in the production of other undesirable species such as nitrous acid and formaldehyde, which can have wider impacts on atmospheric chemistry as well as adverse health impacts.

For a building that is mostly glass, the amount of surface matters too. The study notes that “It is not physically possible for large enough volumes of air to interact with the surface under normal atmospheric conditions and therefore this method will not remove sufficient molecules of NO2 to have a significant impact on ambient concentrations.” Also, "A field trial in London did not show strong evidence of reduced NOx concentrations due to the photocatalytic surface and it was difficult to discern any clear effect that could be attributed to the influence of the paint."

Yet another study by scientists in France and China found that "photocatalytic paints release significant quantities of nanoparticles and volatile organic compounds (VOCs) over their lifetime."

The British study suggests an alternative approach:

A 0.7% reduction in NOx concentration could be achieved by a 1.1% reduction in road traffic NOx emissions on the street based on a central London vehicle mix.

There is no such thing as pollution-sucking paint, there are no magic bullets. We just have to stop making pollution in the first place.

Do photocatalytic finishes really clean the air?
No, there is no magic bullet, we have to stop pollution at the source.

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