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PCO (Photocatalytic Oxidation) technology has attracted significant interest as an innovative method for air purification and disinfection. Its potential could be considerable, but experimental and regulatory reality reveals limitations that cannot be ignored.
THE TECHNOLOGY
The principle is simple: a catalyst (often titanium dioxide, TiO₂) irradiated with ultraviolet light generates oxidizing radicals capable of degrading volatile organic compounds (VOCs) and inactivating microorganisms.
This process generates highly reactive particles, called radicals, which are able to break down the substances present in the air. In practice, pollutants and unpleasant odors are transformed into harmless molecules such as water and carbon dioxide. When the air passes through the module, a photocatalytic process is activated that generates hydroxyl radicals and very small amounts of hydrogen peroxide (H₂O₂). These highly reactive molecules make it possible to sanitize not only the airflow but also the internal surfaces of the ducts, thanks to their effectiveness in breaking down bacteria, viruses, and other pathogens.
In addition, systems are often equipped with a bipolar ionization system (positive and negative), which amplifies odor reduction and is also reported to be effective against ultrafine particles.
BUT… DOES IT REALLY WORK THIS WAY?
Independent studies have shown that PCO technology can produce undesirable by-products, such as formaldehyde and acetaldehyde, especially when used under realistic conditions and with high airflows. The amounts of hydrogen peroxide generated are generally very low and, for this reason, the actual effectiveness of such concentrations in sanitizing large volumes of air in a short time is often lower than what manufacturers claim.Claims such as “effective against viruses, bacteria, mold, allergens, VOCs, and ultrafine particles” must also be viewed with caution: while significant reductions are recorded in laboratory settings, results in real-world contexts appear far less consistent. It is no coincidence that organizations such as ASHRAE and the EPA urge caution, emphasizing that there is no conclusive evidence to regard these technologies as equivalent to, or even superior to, HEPA filtration or properly sized UV-C disinfection.
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THEORETICAL EFFECTIVENESS AND POTENTIAL RISKS
- Ozone production: although many manufacturers claim that PCO systems are emission-free, several tests have shown the possibility of generating small amounts of ozone. Even at low concentrations, ozone is recognized as harmful to human health, capable of irritating the respiratory tract and worsening pre-existing conditions.
- Formaldehyde and secondary by-products: the degradation of VOCs through PCO can lead to the formation of intermediate by-products such as formaldehyde, a substance classified as carcinogenic. In this way, there is a risk of replacing one pollutant with another that is equally or even more dangerous.
- Questioned effectiveness: several studies and analyses of real-world environments have raised doubts about the effectiveness of PCO systems compared to results obtained under controlled laboratory conditions. Commercial claims that are often overly optimistic have led to legal disputes and critical positions from regulatory bodies.
- Lack of independent validation: many evaluations of PCO system performance are based on internal or non-standardized tests conducted by the manufacturers themselves. This lack of independent verification has fueled further concerns about the actual safety and effectiveness of the technology.
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ASHRAE’s Position
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UNREALISTIC COMMERCIAL CLAIMS
A particularly critical aspect is the spread of marketing claims that speak of air disinfection capacities of up to 4,000 m³/h. These statements find no support in physical laws nor in independent studies:
- Insufficient power: the UV power levels employed are so low that, even if perfectly converted into “active radiation,” they cannot guarantee complete microbial inactivation at such high flow rates.
- Exposure time: at 4,000 m³/h the contact time between air, light, and catalyst is extremely short (fractions of a second), insufficient for an effective reaction.
- Risk of false expectations: such claims lead users to believe in “total sanitization” that in reality does not occur, with the danger of reducing reliance on other proven measures (ventilation, filtration, properly dosed UV-C).
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Independent literature on the risks of PCO converges on several key points: under conditions close to real-life (indoor VOC mixtures, humidity, and short contact times), UV-PCO reactors can generate toxic by-products. Laboratory studies conducted on prototypes and commercial units in environmental chambers (e.g., work from the Lawrence Berkeley National Laboratory) have documented significant formation of these compounds when the oxidative reaction is incomplete; similar results appear in articles published in Building & Environment that, in addition to experimental evidence, include modeling based on dozens of tests, indicating that airflow, humidity, pollutant load, and reactor design are critical factors for secondary chemistry.
A more “chemical” line of research (e.g., Catalysis Today on toluene and other VOCs) has highlighted the need for specific toxicological assessments and continuous monitoring of by-products. At the level of technical-regulatory guidance, EPA and CARB documents warn that the devices may produce ozone or other secondary pollutants and recommend independent performance and safety verifications; ASHRAE position statements emphasize the need for testing in real buildings, with by-product measurements and emission limits.
At the level of technical-regulatory guidance, EPA and CARB documents warn that these devices may produce ozone or other secondary pollutants and recommend independent performance and safety verifications; ASHRAE position statements emphasize the need for testing in real buildings, with by-product measurements and emission limits.
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At the level of technical-regulatory guidance, EPA and CARB documents warn that these devices may produce ozone or other secondary pollutants and recommend independent performance and safety verifications; ASHRAE position statements emphasize the need for testing in real buildings, with by-product measurements and emission limits.
In summary, although PCO shows in laboratory the ability to degrade VOCs and inactivate microorganisms, the translation gap in the field—due to catalyst deactivation, surface “fouling,” and reduced residence times at typical HVAC flow rates—means that the claimed benefits are often accompanied by by-product risks and less clear-cut performance. This is why the most rigorous bibliography calls for standardized testing protocols, comprehensive by-product reports, and transparent comparison with established alternatives such as high-efficiency filtration, correctly sized UV-C, and ventilation.
References
- Performance of photocatalytic oxidizing air cleaners in different experimental setups – a review - ScienceDirect
- Ultraviolet photocatalytic oxidation technology for indoor volatile organic compound removal: A critical review with particular focus on byproduct formation and modeling - ScienceDirect
- Key parameters influencing the performance of photocatalytic oxidation (PCO) air purification under realistic indoor conditions - ScienceDirect
- Determination and risk assessment of by-products resulting from photocatalytic oxidation of toluene - ScienceDirect
- Photocatalytic oxidation technology for indoor air pollutants elimination: A review - ScienceDirect
- Modeling of by-products from photocatalytic oxidation (PCO) indoor air purifiers: A case study of ethanol - ScienceDirect
- Performance of ultraviolet photocatalytic oxidation for indoor air applications: Systematic experimental evaluation - ScienceDirect
- Evaluation of a Combined Ultraviolet Photocatalytic Oxidation (UVPCO) / Chemisorbent Air Cleaner for Indoor Air Applications lbnl-62202.pdf