Photocatalysis air purifier: a solution or a threat?

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.

In practice, a photocatalysis air purifier relies on three elements working together: a photocatalyst (most often titanium dioxide, TiO2), a source of UV light to activate it, and a flow of air that brings pollutants to the catalyst's surface. When UV photons hit the TiO2, they generate highly reactive oxidising species, such as hydroxyl radicals, at the surface. These radicals attack the organic molecules that come into contact with the catalyst and, ideally, break them down step by step into carbon dioxide and water.

The key word is ideally. This complete oxidation only happens when every condition is right: enough UV energy, enough contact time, a fresh catalyst and a manageable pollutant load. When the reaction stops halfway - which is common in a real room - the pollutant is not fully mineralised but transformed into intermediate compounds that can be as harmful as, or more harmful than, the starting molecule.

Photocatalysis air purifier efficiency has not been proven

Most photocatalysis air purifier 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
  

In its May 2022 opinion on indoor air purification by photocatalysis, the French agency ADEME reached a measured conclusion: some devices showed effectiveness in laboratory tests simulating realistic conditions, but current knowledge does not allow this effectiveness to be guaranteed across all real conditions of use. The report highlights performance gaps that depend on the mix of pollutants and the airflow. More strikingly, for light VOCs such as formaldehyde, acetaldehyde and acetone, the tested concentrations often barely changed after treatment - and in some cases increased.

Photocatalysis air purifier safety questioned by experts

Manufacturers most often use titanium dioxide (TiO ₂ ) as a 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, are ketones and aldehydes have irritant and toxic properties.

These concerns are echoed by public agencies. In its 2017 assessment of catalysis- and photocatalysis-based air-cleaning devices, the French agency ANSES concluded that the available scientific data were not sufficient to demonstrate either the effectiveness or the safety of these devices under real conditions of use. ANSES flagged two specific risks: the possible emission of nanoparticles by the photocatalytic material, particularly as it ages, and the formation of reaction by-products that may be more harmful than the pollutants being treated. Studies analysing photocatalytic oxidation air cleaners have identified formaldehyde and acetaldehyde as the most common of these by-products.

How a photocatalysis air purifier compares with other technologies

Put in perspective, photocatalysis is one option among several, each targeting a different kind of pollutant. The table below summarises how it compares with filter-based purifiers and with ionisation, on a few simple criteria.

The comparison makes the trade-offs clear. Against gases and VOCs, no air-treatment technology replaces reducing the sources and ventilating the room. Against particles - dust, pollen, mould spores, particles carrying bacteria or viruses - a particle-focused technology, such as a filterless air purifier, is the appropriate answer. TEQOYA ionisers act only on this particle pollution, by electrostatic precipitation and without any chemical reaction, so they generate no gaseous by-products. We make no claim on formaldehyde, VOCs or other gases: that is simply not what an ioniser does.

For more information, click here and read an excellent French article by Corinne Mandin and Séverine Kirschner.

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