This article in our series about air purification technologies concerns photocatalysis-based air purifiers. Do they keep their promises ?
What is photocatalysis ?
Photocatalysis is the decomposition and the degradation of pollutants through the use of light rays on the surface of a catalyst, generally titanium dioxide. It can eliminate VOCs, inorganic pollutants and microorganisms. The process results in water and carbon dioxide.
Its efficiency has not been proven
Most photocatalysis-based air purifiers manufacturers assert that their air purifiers eliminate 99% of all VOCs, PAHs, viruses and bacteria up to a size of 0,01 µm. There is an accumulation of research findings conducted in laboratories that confirm the depolluting properties of this technology. But tests under real-life conditions call into question that efficiency. According to the ADEME, photocatalysis is efficient only under certain conditions. Experiments reveal performance gaps depending on mixtures of air pollutants and airflows.
Under real-life conditions, photocatalysis-based air purifiers efficiency depends on a variety of factors :
- The environmental conditions : temperature and humidity
- The parameters of the lights : power, spectrum:(UV-A, UV-C)
- The nature of the photocatalyst
- The wear of the photocatalyst
- The number of passages of the air that has to be cleaned on the active surfaces
- The nature and the concentration of pollutants
The photocatalysis-based air purifiers innocuity called into question
Manufacturers most often use titanium dioxide (TiO2) as catalyst. It is known to be effective against VOCs, gas, odors, mold, fungus, bacteria and viruses. According to the International Agency for Research on Cancer, titanium dioxide is also a carcinogen. Results from animal studies (ARC, 2006 & Yamashita et al, 2011) indicate that titanium dioxide causes lung cancer and impedes fetal development.
Above all, degradation mechanisms involve dangerous by-products. It is difficult to guarantee that the photocatalytic reaction will be complete, as indicated in the picture. The factors listed can lead to a partial reaction and the emission of toxic by-products. Among them, ketones and aldehydes which have irritant and toxic properties.