About PCO: Photocatalytic Oxidation

About PCO: Photocatalytic Oxidation

By Normand Brais, P.Eng., M.A.Sc., Ph.D.

Common Titanium oxide base catalyst: TiO2

In chemistry, PCO is the acceleration of a photoreaction in the presence of a catalyst. In catalyzed photolysis, light is absorbed by an adsorbed substrate. The photocatalytic activity depends on the ability of the catalyst to create electron– hole pairs, which generate free radicals (hydroxyl radicals: OH) able to undergo oxidation reactions. Its comprehension has been made possible ever since the discovery of water electrolysis by means of the titanium dioxide. Commercial application of the process is called Advanced Oxidation Process (AOP) and is used for water treatment.

Titanium dioxide, particularly in the anatase form, is a photocatalyst under ultraviolet light. Recently it has been found that titanium dioxide, when spiked with nitrogen ions, or doped with metal oxide like tungsten trioxide, is also a photocatalyst under visible and UV light. The strong oxidative potential of the positive holes oxidizes water to create hydroxyl radicals. It can also oxidize oxygen or organic materials directly. Titanium dioxide is thus added to paints, cements, windows, tiles, or other products for sterilizing, deodorizing and antifouling properties and is also used as a hydrolysis catalyst.

Although this technology looks perfectly transposable to air, there is one main practical caveat that recently came to light: the titanium oxide is being “poisoned” by silica and its useful service life is severely impaired. After some longer time experience of this technology in air, it was observed that the PCO would gradually decay and lose most of its oxidative potential within a year or less.

The effect of silica as a titanium oxide neutralizer is well known in the sunscreen industry. Every sunscreen with a physical blocker contains titanium dioxide because of its strong UV light absorbing capabilities, thus preventing UV from reaching the skin. Sunscreens designed for infants or people with sensitive skin are often based on titanium dioxide and/or zinc oxide, as these mineral UV blockers are less likely to cause skin irritation than chemical UV absorber ingredients, such as avobenzone.

However, to avoid the creation of carcinogenic radicals on the skin due to the activity of photo catalytic reaction, the titanium dioxide particles used in sunscreens are intentionally coated with silica. The addition of silica effectively neutralizes the photo catalytic properties of the titanium oxide, making the sunscreen harmless.

Because silica is commonly found in household applications such as caulking and many other materials, the PCO titanium oxide is contaminated with silica and will lose half of its activity within three months. This means that after 6 month, it will be down to 50% efficiency and after 9 months, down to 25% efficient and after a year, down to 12.5% only. It will then cease to provide adequate performance as an air purification device. This is the main reason why serious companies are now taking a step back and even walking away from the marvelous promises of common titanium oxide based PCO as a solution for odor removal.

New Cobalt Photocatalytic Oxidation (Co-PCO)

Using UV light to achieve clean air and water resources through photocatalytic oxidation is a goal of scientist worldwide (1,2,3) over the last two decades. Photocatalysis is a widely generic term that applies to chemical oxidation reaction enabled by photon activated catalyst, commonly called PCO in the air purification industry.

PCO catalyst consists of a metal oxide semiconductor, usually titanium oxide (TiO2), with a band gap energy that allows the absorption of ultraviolet photons to generate electron hole pairs called “active sites” that can initiate the chemical reaction. For titanium oxide PCO, the energy band gap is centered on 360 nm photons, which is in the middle of the UV-A range (315-400 nm). This is quite far away from the UV-C range of common germicidal lamps emitting most of their photon energy at 254 nm wavelength and as such partially explains the rather deceiving efficiency of current titanium oxide based PCO air purifiers using low pressure mercury lamps. This low efficiency is mainly responsible for hazardous by-product formation such as formaldehyde. Another important barrier to the implementation of actual PCO is its short lifetime due to silica poisoning of the catalyst. Silica which is the main constituent of common sand is omnipresent in our daily environment. Siloxanes have been identified as the root cause of current PCO deactivation (4). As deactivation reduces the number of active sites available, incomplete oxidation becomes prevalent, promoting the production of by-products.

The fundamental effect of the addition of cobalt oxide is to shift the energy band gap of the catalyst toward higher energy photons closer to the 254 nm photons emitted by low pressure mercury lamps. With a capacity to absorb at higher energy, the cobalt enhanced catalyst provides enough photocatalytic activity to completely oxidize household VOCs (5,6) and avoid the transient formation of formaldehyde, acetaldehyde, and other incompletely oxidized by-product. It is worth noting that the higher energy active band gap of the Cobalt catalyst is much wider than the actual titanium oxide and was found to be almost insensitive to silica poisoning. Actual testing has shown no significant decline in the Cobalt catalyst activity after a full year in service.


  1. Peral,J.; Ollis, D.F. Heterogeneous photocatalytic oxidation of gas-phase organics for air purification: acetone,1-butanol, butyraldehyde,formaldehyde,and m-xylene oxidation. J.Catal. 1992, 136, 554-565.
  2. Dibble, L.; Raupp, G. Kinteics of the gas-solid heterogeneous photocatalytic oxidation of trichloroethylene by near UV illuminated titanium oxide. Catal. Lett., 1990,4, 345-354.
  3. Pichat,P.; Disdier, J.; Hoang-Van, C.; Mas, D.;Goutallier, G.; Gaysee, C. Purification/deodorization of indoor air and gaseous effluents by TiO2 photocatalysis. Catal today 2000, 63, 363-369.
  4. Warner, N.A.; Evenset, A.; Christensen, G., Gabrielsen, G.W.; Borga, K.; Leknes, H. Volatile siloxanes in the European arctic: Assessment of sources and spatial distribution. Env iron. Sci. Technol., 2010,4,7705-7710.
  5. Building Assessment Survey and Evaluation (BASE) study. Available online: http://www.epa.gov/iaq/base/index.html
  6. Hay, S.; Obee, T.; Luo, Z.; Jiang, T.;Meng, Y.; He, J.;Murphy, S.; Suib,S. The viability of photocatalysis for air purification. Molecules, 2015, 20, 1319-1356.

Root Cause of the Odor Generated by Germicidal UV Disinfection with Mobile Units

Root Cause of the Odor Generated by Germicidal UV Disinfection with Mobile Units

By Normand Brais, P.Eng., M.A.Sc., Ph.D. and  Benoit Despatis, Eng. ASHRAE Member


It has been often noticed by many users over the years that whenever a germicidal UV surface disinfection is performed in a room, there is almost always a strange odor left afterward. It is not the smell of ozone, which can be easily identified and measured. It is more like a slightly pungent smell similar to rotten eggs or burnt hair. It is actually easier to recognize the smell than to describe it. Up to now, no satisfactory explanation as to the origin of this peculiar odor has been provided.  Several working hypothesis have been explored to explain this awkward phenomena:

1) Off-gassing of wall surfaces such as paint or other volatile materials.

2) UV lamps end caps glue off-gassing.

3) UV lamps connectors or end rubber boots overheating.

4) Interaction of UV with airborne and surface-borne dust.

After several tests and experiments, the first three hypotheses were quickly ruled out as a potential root cause. Off-gassing of paint was eliminated after testing in a bare metal aluminum enclosure and witnessing the same odor.

The UV lamps end caps were completely removed and all the glue removed with no effect. The same was done for the lamps connectors and also showed no impact on the odor. However, while we were performing these tests, it was noticed that when the disinfection cycles were repeated several times in the same enclosure, the perceived odor level after each cycle seemed to be diminishing. This was the hint that leads us to focus our attention on the presence of dust particles in the air, what these particles consist of, and how UV can potentially alter them into perceptible odorous compounds.


Airborne dust in homes, offices, and other human environments typically contains up to 80% of dead human skin and squamous hair, the rest consists of small amounts of pollen, textile fibers, paper fibers, minerals from outdoor soil, and many other micron size materials which may be found in the local environment1,2. In a typical indoor environment, the airborne dust volumetric load is somewhere between 100 and 10,000 μg/m3 (0.000044 to 0.0044 grain/ft3) order of magnitude. The dust load depends upon the occupancy rate, type of human activity, air filtration system efficiency, etc. It is worth noting that the maximum acceptable ASHRAE level for total dust is 10,000 μg/m3 (0.0044 grain/ft3) and 3,000 μg/m3 (0.0013 grain/ft3) for PM10.

Since airborne dust is essentially dead human skin and squamous hair pieces, it is worth taking a closer look at the fundamental material they are made of. The main constituent of human skin is a molecular group called keratin. Keratin is a family of fibrous structural proteins. Keratin is the key structural material making up the outer layer of human skin. It is also the key structural component of hair and nails. Keratin monomers assemble into bundles to form intermediate filaments, which are tough and insoluble. Keratins encloses large amounts of the sulfur-containing amino acid cysteine, required for the disulfide bridges that confer additional strength and rigidity by permanent, thermally stable crosslinking; a role sulfur bridges also play in vulcanized rubber. Human hair is approximately 14% cysteine. Cysteine3 is an amino acid with the chemical formula HO2CCH(NH2)CH2SH. The pungent smell of burning hair and rubber is due to the sulfur by-products. The average composition of human hair consists of 45.2 % carbon, 27.9% oxygen, 6.6% hydrogen, 15.1% nitrogen and 5.2% sulphur.4


When high energy UV-C light photons hit a keratin/cysteine molecule, they have enough power to break their internal chemical bonds and shatter them into many smaller molecules. The energy of germicidal UV photons at 254 nm wavelength is 470 kJ/mole, a value greater than the energy of chemical bonds listed in Table 1. It is therefore quite clear that proteomic molecules such as keratin and cysteine can be broken up by germicidal UV irradiation but not by visible light, for which the average wavelength is 550 nm, and the maximum photon energy only 217 kJ/mol.


Table 1. Chemical Bonds Strength5

Chemical Bond

Chemical Bond Average Energy

C – C


C – H


C – N


C – O


C – S


N – H


Therefore, some of the chemical bonds between carbon atoms and hydrogen, nitrogen, oxygen and sulfur atoms will be broken by germicidal ultraviolet photons. Some of the resulting broken pieces of molecules following a sufficiently intense UV photon bombardment will contain sulfur and therefore fall into a category known as thiol molecules. Thiols are a family of sulfur compounds also called mercaptans. Their smell threshold is extremely low. The human nose can detect thiols at concentrations as low as 1 part per billion. The rotten egg-garlic smell is a dominant characteristic of mercaptans as shown in Table 2.

Burning skin emits a similar smell as thiols, while setting hair on fire produces a sulfurous odor. This is because the keratin in our hair contains large amounts of cysteine, a sulfur-containing amino acid. The smell of burnt hair can cling to nostrils for days.


Table 2. Reported Sensory Threshold for Thiol / Sulfur Compounds6

Compound Name

Chemical Formula

Sensory Description

Smell Threshold (ppb)

Hydrogen Sulfide


Rotten Egg, Sewage-like

0.5 – 1.5

Ethyl Mercaptan


Burnt match, sulfidic, earthy

1.1 – 1.8

Methyl Mercaptan


Rotten cabbage, burnt rubber


Diethyl Sulfide



0.9 – 1.3

Dimethyl Sulfide


Canned corn, cooked cabbage, asparagus

17 – 25

Diethyl Disulfide


Garlic, burnt rubber

3.6 – 4.3

Dimethyl Disulfide


Vegetal, cabbage, onion-like at high levels

9.8 – 10.2

Carbon Disulfide


Sweet, ethereal, slightly green, sulfidic


Carbon Disulfide


Sweet, ethereal, slightly green, sulfidic



Carbon Disulfide


Sweet, ethereal, slightly green, sulfidic




In order to confirm the hypothesis linking the origin of the post-UV disinfection smell to the presence of keratin and cysteine in the air dust, a straightforward molecular concentration calculation was performed.

Given the dust loading, and assuming that this dust consists of 80% skin or hair, both of these containing around 5% sulfur that will end up being broken down by UV into the smallest thiol molecules such as Methyl Mercaptan, Ethyl Mercaptan or even Hydrogen Sulfide, the concentration of Thiol can be estimated as follows:


Dustload = dust weight per unit air volume in μg/m3 (lb/ft3)

SK = % Sulfur in Keratin/Cysteine = 5%

%Skin_Hair = Skin and Hair mass fraction in the dust = 80%

ρThiol = Methyl Mercaptan density at normal ambient temperature and pressure = 1.974 kg/m3 (0.1232 lb/ft3)

Equation (1) shows that when the airborne dust load gets above 75 μg/m3 (0.000033 grain/ft3), which is frequently the case in occupied spaces, the level of thiol generated by the shattering of keratin proteins exceeds the smell threshold of 0.5 to 1.5 ppb. It follows that even in the case of a relatively clean environment with dust loading as low as 100 μg/m3 (0.000044 grain/ft3), the aftermath of the UV disinfection process will leave behind a concentration of 2 parts per billion, which is greater than the smell threshold level, thus leaving behind a perceptible smell. Plotting a graph of equation 1 and allowing the dust loading to go up to 1,000 μg/m3(0.00044 grain/ft3) shows that unless the dust does not contain much dead skin or hair squames, the UV disinfection of a room will almost always leave behind a thiol concentration that exceeds the smell threshold.

Figure 1. Thiol Concentration in ppb vs Dust Load

At maximum ASHRAE acceptable airborne dust loads of 10,000 μg/m3 (0.0044 grain/ft3), concentration of thiol could end up being as high as 200 ppb after UV disinfection. According to the US National Institute for Occupational Safety and Health7 (NIOSH), the IDLH (Immediate Danger to Life or Health) level for Methyl Mercaptan is 150 ppm i.e. 150,000 ppb. Also, according to CSST in Quebec as well as OSHA8 (Occupational Safety and Health Administration), the acceptable TLV-TWA (Threshold Limit Value-Time Weighted Average) level for 8hr exposure is 0.5 ppm i.e. 500 ppb. Consequently, the potential levels of thiol concentration generated by UV disinfection are safe even at the highest acceptable airborne dust level.


Given that human occupancy normally generates concentrations of dust well above 75 μg/m3 (0.000033 grain/ft3) and that this dust is mainly made of human dead skin and hair, which consist of keratin and cysteine molecules; and understanding that high energy UV-C photons can break-up these molecules into thiol molecules which have a very low smell threshold, this paper has revealed the root cause of the odor produced by UV disinfection9 of rooms. Given that the resulting potential concentrations of thiol molecules are negligible when compared to the published acceptable exposure limits, it is safe to enter a room after germicidal UV disinfection has been performed.


The authors are grateful to Dr. Wladyslaw Kowalski for data and editorial assistance.


μg = micro gram

ppm = parts per million volumetric concentration

ppb = parts per billion volumetric concentration

nm = nanometer (10-9 m)

grain = lb/7,000


Spengler, Samet, McCarthhy, Indoor Air Quality Handbook. McGraw-Hill, 2001.

Fergusson,J.E.,Forbes,E.A.,Schroeder,R.J., The Elemental Composition and Sources of House Dust and Street Dust, Science of the Total Environment, Vol.50,pp.217-221, Elsevier, April 1986.

Pure Appl. Chem. 56 (5), 1984: 595–624, Nomenclature and symbolism for amino acids and peptides (IUPAC-IUB Recommendations 1983)”, doi:10.1351/pac198456050595.

C.R. Robbins, Chemical and Physical Behavior of Human Hair, DOI 10.1007/978-3-642-25611-0_2, # Springer-Verlag Berlin Heidelberg 2012.

UWaterloo, Bond Lengths and Energies. n.d. Web. 21 Nov 2010. http://www.science.uwaterloo.ca/~cch…20/bondel.html

EPA. Reference Guide to Odor Thresholds for Hazardous Air Pollutants Listed in the Clean Air Act Ammndments of 1990. EPA/600/R-92/047, March 1992.

Odor Remediation of Environmental Tobacco Smoke

Odor Remediation of Environmental Tobacco Smoke

By Normand Brais, P.Eng., M.A.Sc., Ph.D.


Environmental Tobacco Smoke or ETS is a technical term which describes the contaminants released into the air when tobacco products burn or when smokers exhale. At room temperature, many of these compounds are gaseous but most are solid ash particulate and liquid droplets called aerosol.

Particles in tobacco smoke are especially problematic to remove not because of their small size (0.1 to 1 micron), but because they are coated with tar, nicotine, phenols, and many other pungent odorous compounds. They can remain airborne for hours after smoking stops.

Due to their aerosol coating, tobacco smoke particles are not dry but rather sticky and will inevitably clog the surface of any types of air filters, making them quickly wasted and thus ruling out the solution of simple filtration. Their stickiness makes them cling to walls, carpets, fabrics, and clothing, thus impregnating the environment with a lasting nasty smell.

This article describes those technical challenges and explores from a fundamental point of view the proper use of ultraviolet photo-oxidation process as a solution to remediate the odors caused by environmental tobacco smoke.


Studies have shown that cigarette smoke contains over 3800 chemical compounds. Some of these compounds are shown in Table 1 below. Cigarette smoke aerosols are essentially condensable gases resulting from incomplete combustion. Combustion being an oxidation process, those aerosols can be rendered less sticky and turned into dry ash by completing their oxidation. Their odors would even disappear if they could be fully oxidized down to water vapor and carbon dioxide, which are odorless compounds. If one could draw the smoke cloud directly into the combustion chamber of an industrial fume incinerator at 850 Celcius for two seconds, the odorous molecules cocktail listed in Table 1 would be completely oxidized and consequently odorless. Although it would work perfectly, this solution is obviously not economically sound.


Table 1. Chemical composition of cigarette smoke

Duration of smoke production (sec)

20 sec

550 sec

Characteristics or compound

Mainstream Smoke

Sidestream smoke

Particles (number per cigarette)



a) Solid particles and aerosols









Benzo (a) pyrene









Benzo (a)fluorene






Chrysene, benz(a)anthracene



Benzo (b,k,j) fluorenthrene



Benzo (e) pyrene






Dibenz (a,j) anthracene



Dibenz (a,h) anthracene, ideno-(2,3) pyrene



Benzo (g,h,i) perylene






Phenols (total)






Polonium 210, pCi



b) Gases and vapors






Carbon monoxide






Carbon dioxide






Hydrogen Cyanide















Source: Introduction to indoor air quality: a reference manual, EPA/40013-91/003



Inspection of Table 1 shows that filtration alone could not handle cigarette smoke aerosols. Past experience has shown that the very small sub-micron size of the particles requires expensive HEPA filters that become tar coated and consequently clogged very quickly.

Besides classic filtration, there is another well-known way to remove sub-micron particulates from the air. Electrostatic air filters also called air ionizers have this capability. Instead of capturing particles mechanically like classic filters, the idea behind electrostatic or electronic filtration is to electrically charge the particles so that they will migrate due to electrical forces toward nearby surfaces. The same effect is obtained by rubbing a balloon on one’s hair and then sticking it to a wall. Eventually, the balloon loses its charge and falls back to the floor.

Many of the popularly called “smoke eaters” use the electrostatic principle to collect smoke particles on metal plates. The effect of ionizers on the smoke particles in the air is the same except that they have no collecting plates and the charged particles end up sticking on the walls and surfaces of the room. It is worth noting that since the cigarette particles are sticky with tar, they will overtime coat all the room surfaces with pungent smelly yellow-brown tar extract.

Experiences with ionizers into small volumes like a hand jar is quite conclusive where the smoke particles of one cigarette can be easily dispersed toward to jar walls within 15 to 20 seconds. However when repeating the same experience in a larger volume like a 3m x 3m x 3m room, the time required to clear the air from the same amount of smoke goes up to several hours!

The explanation for this loss of effectiveness as the room size increases is rooted into basic fundamental physics of electrostatic forces: the Coulomb Law, which states that the electrical forces between charged particles decreases with the square of their distance. The Coulomb Law implies that when the distance is doubled, the electrical force is reduced by a factor of 4. When comparing the electrical forces in the small jar where the particles are within less than a few centimeters from one another and from a nearby wall with that of a room of a few meters wide, the electrostatic forces responsible for the dispersion of the smoke particles are down by the square of the ratio of 1 meter to 1 centimeter i.e. the square of 100 or 10,000 times less electrical force !

This fundamentally explains why experiment based on removing the same number of smoke particles in a normal size room takes several hours (10,000 + seconds) whereas the old sales-pitch demonstration videos performed in a hand size container takes seconds. Not only air ionization does not remove the odors due to the walls and surface tar coating effect, but their electrostatic actions are way too slow to have any significant cleaning effect except in a small jar. On top of their ineffectiveness, the fact that room surfaces will get gummy as they accumulate the electrically charged tar particles instead of using some internal cleanable capture plates like in all electrostatic smoke eater units, the air ionizers are in fact an ill-conceived version of an electrostatic smoke eater and an overall bad idea.


When ultraviolet UV-C light photons hit a tar or nicotine molecule, they carry enough impact energy to break the interatomic chemical bonds and shatter the molecule into many smaller molecules. The energy of germicidal UV photons at 254 nm wavelength is 470 kJ/mole, an energy greater than the energy of all the chemical bonds listed in Table 2. By comparison, visible light with an average wavelength of 550 nm has photon energy of only 217 kJ/mol.

It is therefore quite clear that some bonds within tar, nicotine and phenols molecules in the smoke can be broken down by UV-C irradiation but not by visible light.


Table 2. Chemical Bonds Strength4

Chemical Bond

Chemical Bond

Average Energy


C – C


C – H


C – N


C – O


C – S


 N – H


Therefore, the chemical bonds between carbon atoms and hydrogen, nitrogen, oxygen and sulfur atoms will be broken down by UVC ultraviolet photons, resulting broken pieces of molecules. Following this process, the broken molecules can now be further oxidized to complete their combustion and reduce their odor potential.

This oxidation can be accomplished by using a higher energy ultraviolet of 185 nm wavelength called UVV, where the second V stands for Vacuum. UVV photon have an energy of 645 kJ/mole but can only propagate into a vacuum because the dioxygen molecule in the air absorbs it and as a result gets broken up into monoatomic oxygen. At normal atmospheric pressure, UVV photons are almost totally absorbed within less than 5 mm away from a standard low pressure mercury quartz lamp UVV source. These free oxygen atoms generated by the UVV light are then able to react and complete the oxidation of the broken-down tar, nicotine, and phenols molecules.

The end products of this photo-oxidation process are then dry non-sticky ashes particles that can now be captured by adequate standard filters. This way the odors are eliminated by the oxidation process and the dry resulting particles removed by filtration.

The proper sizing to avoid oversizing of photo-oxidation system is of utmost importance. Should there be nothing to react with, the UVV generated oxygen atoms O* will react with dioxygen molecules O2 to produce ozone O3, another undesirable compound. Ozone is a not a stable molecule and will decompose naturally into normal dioxygen at ambient room temperature within 20 to 30 minutes depending upon relative humidity. The OSHA limit for 8 hours exposure is 0.05 ppm of ozone. Because the generation rate and the rate of decomposition of ozone in the absence of any smoke or other volatile contaminants in a given room size at an ambient temperature and ventilation rate can all be adequately calculated, it is possible to size an ultraviolet photo-oxidation system that will never exceed the OSHA safety limit.


This paper has described in detail the nature and composition of cigarette smoke and the consequential inherent shortcomings of classical filtration and electrostatic filters or air ionizers. Many years of experimental evidences backed by calculations based on cigarette smoke chemical composition show that the odor of cigarette smoke cannot be removed without altering the structure of the molecules responsible for the odors which are essentially tar, nicotine, and phenols. Besides thermal incineration, ultraviolet photo-oxidation has proven to be the most effective way to accomplish this by degrading and oxidizing those molecules. Their oxidation render the smoke particles dry and non-sticky which make them acceptable candidates for standard filtration. Care must be taken to adequately engineer the ultraviolet photo-oxidation system with respect to the room size and ventilation rates to keep the potential residual ozone well within the OSHA limit when the treated room becomes free from tobacco smoke.



The author is grateful to Francisco Doyon P.Eng. and Grégory Clément P.Eng. for sharing their experimental data on the effect of air ionizers on environmental tobacco smoke inside rooms of various scale.


  1. C.N. Davies, Cigarette smoke: generation and properties of the aerosol, J.Aerosol Sci. Vol 19, No.4, pp463-469, 1988.
  2. Hays, Gobbell, Ganick, Indoor Air Quality, McGraw-Hill,1995, p.58.
  3. Spengler, Samet, McCarthhy, Indoor Air Quality Handbook. McGraw-Hill, 2001.
  4. UWaterloo, Bond Lengths and Energies. n.d. Web. 21 Nov 2010.
  5. http://www.science.uwaterloo.ca/~cch…20/bondel.html EPA. Reference Guide to Odor Thresholds for Hazardous Air Pollutants Listed in the Clean Air Act Ammendments of 1990.
  6. EPA/600/R-92/047, March 1992.

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