This blog post, originally shared in the Pharos Signal, includes information about parts of Pharos that are no longer available. Please use it for historical reference and for the other useful information it contains.
Over the course of our work with manufacturers, the Pharos team often learns of creative and innovative new building products. The latest: products that, in addition to performing their standard function in buildings, also claim the ability to actively improve the air quality where they are used and installed. Do these products represent the first wave of “regenerative materials” that actually improve their environment? Intrigued, but skeptical, we dug a little deeper into the science behind these technologies to find out whether or not “pollution-eating” products were more than just clever marketing.
The apparent wizardry behind some of these products is the photocatalysis of titanium dioxide. In nanoscale form, titanium dioxide in these products’ surface layers can react with ultraviolet light to catalyze a chemical reaction that breaks some pollutants down into non-hazardous molecules, carbon dioxide, and water.
Is this too good to be true? European researchers have confirmed that cement enriched with titanium dioxide can reduce airborne pollutant levels in a laboratory setting. After following up with some outdoor experimentation, they found that the “efficiency of the cements [was shown] to vary depending on the intensity of the sunlight and direction of the wind; variables that are difficult to control in the real world.”[1, 2] A separate 2002 field test is reported to have resulted in a 60% drop in the nitrogen oxide (NOx) concentration in the surrounding air, but HBN has yet to find the details of that field test.
However, a recent study indicates that the photocatalysis strategy may backfire in some conditions. Kebete et al make the case that research on this technology to date has focused on industrial environments with high levels of pollutants in the air. When they looked at how photocatalysis would work under more typical settings, they found that the reaction actually creates NOx rather than destroying it when humidity levels change, or when the atmosphere is cleaner. Without higher levels of pollutants to oxidize, researchers found that titanium dioxide would catalyze a chemical reaction with ammonia found in the atmosphere and produce up to a 33% increase in NOx levels. They concluded that the efficacy of photocatalysis in “eating” pollution is likely limited to low humidity/high concentration of pollutant environments seen in industrial settings.[3, 4]
But, assuming the product does succeed in eating some NOx and turning it into another substance, there must be a net reduction of NOx to make the venture worthwhile. In the case of photocatalytic concrete, the Portland Cement Association (PCA) claims that: “harmful smog can be turned into harmless compounds and washed away." The irony in this statement is large, since the US EPA has identified the kilns that make cement as major producers of smog-producing nitrogen oxides and sulfur dioxide.[6, 7] The PCA, representing cement companies in the US and Canada, has fought EPA’s efforts to restrict these emissions for years. We would like to see an assessment of how the capacity for neutralization of smog-forming compounds by photocatalytic cement compares to the production of those same compounds by the cement kiln.
Assuming a net gain in NOx reduction, we then have to ask what the NOx breaks down into, if it is truly harmless, and where it ends up. In this case, according to the PCA “photocatalytic concrete contains titanium dioxide particles that act as the catalyst for the natural breakdown of NOx into nitrates in sunlight. This occurs at the surface of the concrete, where the nitrates can be easily washed away.” Such properties have earned these products the title of being “self cleaning” and interest in developing new applications is high.
Of course, the concept of “washing away” pollution is an oversimplification. Adding these nitrates to storm water runoff could increase the eutrophication problems caused by phosphate and nitrate runoffs from agriculture, lawns, and waste facilities that already clog many lakes and streams with algae and starve them of oxygen.
Moreover, while the use of nano-particles is still largely unstudied for impacts on human health and environmental safety, there is some evidence that nano-scale titanium dioxide may be carcinogenic or cause genetic damage in animals. [9, 10] The possible release of titanium dioxide nanoparticles in the runoff along with the nitrates raises more potential health concerns.
Eager to apply this technology on a large scale, the Portland Cement Association sees the future in our transportation system: photocatalyzed-titanium-dioxide-treated cement highways where the road itself would act as an air purifier, collecting nitrogen oxide pollutants from passing tailpipes between rains.  Understanding the impact of runoff waters becomes more important with a project of this scale.
Applications of photocatalysis are not limited to cementitious products. Some self-cleaning building products that we’ve come across in our Pharos research include:
- NOx-Active roofing membrane by Siplast. Treated with a titanium dioxide compound that is advertised to decompose NOx in the air upon contact with the membrane.
- KNOxOUT paint by Boysen Paints. Reportedly uses titanium dioxide to convert NOx to nitric acid, which is then neutralized by other alkaline paint ingredients.
Products intended for indoor use eliminate concern about runoff, but given the indications that these products may need industrial environmental conditions to perform properly, we have concerns about their efficacy. In an interior setting, presumably windows would be the only source of UV light, and would only provide that light on sunlit days for a few minutes or hours per day, depending on orientation, time of the year, and outside obstructions.
Further, without rainfall to wash pollutants away, occupants would need to hose down their treated surfaces (or maybe wipe them down depending on the setting), both of which could require a fair investment of time, effort, and presumably some messiness.
Pollution-eating products could play an important role in creating a truly regenerative material economy. They could also end up being just another form of greenwash. We have lots of questions about these products, and will keep on with our research. We’ll keep you updated here on The Signal with more analysis as we do. If you’ve encountered self-cleaning or pollution-eating products, let us know. We’re curious about what’s out there.
Thank you to Susan Sabella, Jim Vallette and Tom Lent for their contributions to this post.
HBN has also written a general overview about this topic here.
 de Groot, Ute. “Self Cleaning Buildings.” European Research Media Center, 23 Feb2012. Available http://www.youris.com/Nano/Environment/SelfCleaning_Buildings.kl
 “Self Cleaning Buildings” video. Produced by the European Commission European Research Area, co-produced with DT-RTG of the European Commission. Available http://www.youris.com/Nano/Environment/SelfCleaning_Buildings.kl
 “Dangers of Claiming Green.” EDC Magazine online, 9 Jul 2013. Available http://www.edcmag.com/articles/95136-the-dangers-of-claiming-green
 Kebede M, Varner M, Scharko N, Gerber B and Raff J. “Photooxidation of Ammonia on TiO2 as a Source of NO and NO2 Under Atmospheric Conditions,” Journal of the American Chemical Society 2013, 135, 8606-8615.
 Portland Cement Association website. “Building a Better (Cleaner) World in the 21st Century,” available http://www.cement.org/tech/self_cleaning.asp
 US EPA. “Fact Sheet: Final Amendments to National Air Toxics Emission Standards and New Source Performance Standards for Portland Cement Manufacturing.” Available http://www.epa.gov/ttn/atw/pcem/pcem_fs_080910.pdf
 US EPA Press Release. “Settlement with Ash Grove Cement Company to Reduce Thousands of Tons of Air Emissions.” 19 Jun 2013. Available http://yosemite.epa.gov/opa/admpress.nsf/d0cf6618525a9efb85257359003fb69d/f3293fe684f4a43485257b8f00520c9e!OpenDocument
 Goode, Darren. “EPA Limits Mercury, Other Emissions from Cement Plants.” Blog post for The Hill, 9 Aug 2010. Available http://thehill.com/blogs/e2-wire/e2-wire/113381-epa-issues-mercury-other-emission-limit-for-cement-plants
 Trouiller B, Reliene R, Westbrook A, Solaimani P, and Scheistl R. “Titanium Dioxide Nanoparticles Induce DNA Damage and Genetic Instability In vivo in Mice.” Cancer Research 15 Nov 2009. Available http://cancerres.aacrjournals.org/content/69/22/8784.full
Takeda K, Suzuki K, Ishihara A, Kubo-Irie M, Fujimoto R, et al. “Nanoparticles Transferred from Pregnant Mice to Their Offspring Can Damage the Genital and Cranial Nerve Systems.” Journal of Health Science 11 Nov 2008. Available http://jhs.pharm.or.jp/data/55(1)/55_95.pdf