10 Air Purifier Myths Debunked

Air Purifier Myths

Rumours about air purifiers circulate almost as much as the particles they catch! If you’ve ever wondered whether air purifiers can do everything they advertise, you’re not alone. People have a lot of questions about the effectiveness of air purifiers and whether they’re safe for use or not.
Luckily, the experts at Sanuvox are here to separate fact from fiction! In this article, find out the truth behind 10 common air purifier myths.

Myth 1: Air purifiers don’t do anything

Air purifiers are often misconstrued as a waste of money with no significant benefits. This couldn’t be further from the truth. Air purifiers are extremely effective at trapping all kinds of harmful airborne particles and pollutants commonly found in the home, from dust mites, pollen and mould spores to carbon monoxide, hydrocarbons, and viruses.

Just because air purifiers are quiet doesn’t mean they aren’t doing their job. If you need convincing, just pull out the filter and see for yourself how much debris the device is trapping!

Myth 2: Air purifiers are bad for you

Air purifiers are good for you! By removing particles from the air, air purifiers alleviate symptoms of allergies and asthma, reducing irritation and inflammation and improving sleep Air purifiers can also trap harmful pollutants that would otherwise lead to an increased risk of cancer, Alzheimer’s, heart, and lung disease.

Myth 3: Air purifiers emit ozone

If you’re worried that air purifiers are bad for you, it may be because you have heard that they emit ozone. This used to be true, but it’s not the case anymore. Before 2005, the most popular air purifiers were ionizers that were essentially ozone generators. In 2005, Consumers reports showing that these types of air purifiers not only did a poor job of cleaning the air, but also exposed users to potentially harmful ozone levels.

Technology has come a long way since then, and most ionizers now produce negligible amounts of ozone. Of course, if you are still concerned, you can simply select an air purifier that does not include an ionizer. Air purification systems that rely on filtration or UV light do not generate ozone.

 

Myth 4: Air purifiers give off dangerous radiation

In fact, just like every other electronic device in your home, air purifiers do emit small amounts of electromagnetic radiation. Microwaves, cell phones, TVs and air purifiers all irradiate some level of EMF (Electro Magnetic Frequencies) radiation. The important thing to note is that these levels are extremely small and not remotely dangerous for your health.

 

Myth 5: If you have AC, you don’t need an air purifier

Air conditioners and air purifiers have completely different functions. Air purifiers clean the air by filtering out 99.97% of airborne particles. Air conditioners cool the air but have no effect whatsoever on harmful particulates and pollutants. While some air conditioners are equipped with filters, these are not nearly fine enough to trap the harmful particles targeted by air purifiers.

 

Myth 6: Air purifiers lower humidity and dry the air

You may be thinking of dehumidifiers! It’s easy to confuse the two because dehumidifiers can also lessen allergens and mould, but they go about it differently. Warm, humid air creates a breeding ground for mould growth and makes it easier for allergens to circulate. Dehumidifiers help prevent this by removing the moisture from the air, while air purifiers have no effect on humidity but trap the particles that are already circulating. Dehumidifiers and residential air purifiers are being used together to optimize the air quality in your home!

 

Myth 7: If you clean your house, you don’t need an air purifier, and vice versa

Household cleaning focuses on surfaces, while air purifiers clean the air. No matter how much you dust, and vacuum, dander, pollen, and mould spores can still permeate the air in your house. On the flip side, if you neglect cleaning, the most powerful air purifier in the world won’t be able to keep up with the settling dust. Cleaning and home air purifiers should be used together for best results.

 

Myth 8: HEPA filters trap odours, gases and VOCs

While HEPA filters are the gold standard for air purification systems, they are designed to trap solid particulates larger than 0.3 microns. Gases, all VOCs (volatile organic compounds) and many viruses are small enough to pass right through. Activated carbon filters and UV lights are the only ones that show some effectiveness for these types of pollutants.

 

Myth 9: UV air purifiers don’t work

While HEPA filters are designed to trap particulates, UV air purifiers use high-intensity germicidal UV light to break down micro-organisms and any DNA or RNA bio-contaminants like viruses.  Some argue that the contaminated air doesn’t pass through the UV light for long enough to be properly purified, but scientifically conducted testing has confirmed that processes such as Sanuvox’s patented high-intensity J-lamps deliver a high enough dosage of UV light to effectively break down the contaminants.

 

Myth 10: All air purifiers are created equal

Just like any other device, think about computers or cars for example, there are a wide variety of air purifiers on the market. They differ in terms of the amount of space they cover, how much noise they make, and what kind of purification technology they use. When looking for an air purifier, it’s important to consider your needs and do your research to find one that suits you.

 

Fact: Air purifiers are safe, effective, and available at Sanuvox!

Air purifiers are safe and effective. They are not rendered unnecessary by air conditioning or thorough cleaning. Different varieties of air purifiers are effective at purifying different types of contaminants, so it’s important to consider that when choosing which one to buy.


If you have more questions about UV air purifiers or would like to talk to a Sanuvox representative about our products, contact us today!
Learn more about available products, markets, and applications and access our company’s blog and whitepapers on this website.
Follow Sanuvox Technologies on Facebook (@SanuvoxTechnologies) and on LinkedIn (@sanuvox-technologies).

*Full laboratory test report available upon demand.


 

Wildfires and their impact on our lives

wildfire

Huge fires broke out during this year. Most notably, California in the United States, British Columbia and northern Ontario have also suffered the consequences of Mother Nature. The great changes that our planet is undergoing are causing upheaval in wildlife in general, but also in living environments. Forest fires are usually the cause of poor air quality in affected cities. In addition, its odor is very persistent as it succeeds in becoming embedded in houses.

According to data compiled in the National Forest Database, more than 8,000 wildfires occur in Canada each year, destroying an average of over 2.1 million hectares. In addition, lightning causes almost 50% of all fires, but is responsible for some 85% of the area burned annually. (1)

cnfdb-data-wildfires

The figure above shows statistics extracted from the CNFDB and provides a comparison with those numbers reported annually to the National Forestry Database (NFD). This chart shows the high variability in both number of fires and area burned in Canada per year. Note that the data contained in the CNFDB are not complete nor are they without error. Not all fires have been mapped, and data accuracy varies due to different mapping techniques. This collection includes only data that has been contributed by the agencies. Data completeness and quality vary among agencies and between years.

What you need to know about fires

Wildfires often start unnoticed, but can spread at lightning speed and consume huge areas, igniting bushes, trees, houses and buildings in their path. Debris from such a fire can be thrown up to two kilometers away, while sparks and embers can ignite nearby homes and materials and cause extensive damage. (1) Large blazes devour entire regions, causing gigantic plumes of smoke to sweep across Canada from west to east.

Smoke has significant impacts on people’s health

Smoke is damaging to individuals and their health in general. Several factors explain these impacts such as the health of people, the amount of smoke embedded in the house and the concentration of smoke from nearby forest fires, as smoke is harmful. Clearly, the length of time you are exposed to this harmful smoke plays a critical role in the future impacts on your health. (2)

What are the components of smoke?

First, they are fine particles that scatter light and make smoke visible. These make it difficult to breathe and can cause a strong cough. These fine particles can penetrate deep into the lungs and worsen pre-existing heart and respiratory disease. (2)

In addition, smoke also contains a wide variety of volatile organic compounds (VOCs) which are invisible gas molecules. Several of these molecules are known to be carcinogenic. These are PAHs (Polycyclic Aromatic Hydrocarbons), such as pyrenes, benzenes, and dioxins. They also often come in the form of aerosols, i.e. liquids suspended in the air, commonly known as creosotes. These are substances that are particularly harmful to health.

Who is most likely to be affected by these fumes?

Young children, the elderly, and people with heart or lung conditions such as asthma, chronic bronchitis, emphysema and congestive heart failure are more susceptible to the harmful effects of exposure to smoke. People who participate in sports or do strenuous work outdoors may also be more vulnerable because they breathe more deeply and quickly. The denser the smoke and the longer the exposure, the greater the risk to those affected. (2)

What are the symptoms of exposure to smoke?

Exposure to smoke can cause eye irritation, tears, coughing, and a runny nose (runny nose). If the smoke lasts for several days to several weeks or if it is really thick, it can result in lung problems and persistent cough (2)

Sanuvox and its air purification products

Various solutions are proposed to you to remedy these smoke problems by Sanuvox. Indeed, installing air purification systems can treat the air in these rooms and eliminate odors and smoke particulates.Unlike its competitors, Sanuvox does not use expensive activated carbon filters which have the disadvantage of saturating very quickly and becoming useless. The patented UV process reduces odors and airborne smoke. A recirculation rate of 3 to 6 times per hour makes it possible to choose the appropriate equipment given the dimensions of the room.

EQUIPMENT USED

The air purification units are equipped with a blower, pre-filter, HEPA filter and UV sources of appropriate wavelengths.

PRINCIPLE OF OPERATION

This is to eliminate odors by oxidizing the odor molecules. This is the process that takes place naturally outside in the atmosphere with the rays of the sun. For example, the oxidation of hydrogen sulfide H2S produces a completely odorless H2O water molecule as well as a SO2 sulfur oxide molecule with an odor threshold of 5 mg / Nm3, which is 5,000 times less odorous than the initial hydrogen sulphide molecule.

Called “combustion” when it comes to conventional fuels, such a reaction then requires a high temperature to start. There is another way to initiate chemical oxidation reactions by using light with a high energy intensity photon source. This process of oxidation at room temperature is called “photolysis” or “photo-oxidation”.

The energy of photons, the particles that make up light, increases as the wavelength of light decreases. As a result, having a wavelength of 400 nm, purple photons are more energetic than red photons with a wavelength of 700 nm. The more energy photons have, the more they are like large caliber projectiles that can break the bonds that bind atoms together within a single molecule.

To initiate oxidation reactions, these bonds must first be broken to free the atoms which can then combine with oxygen atoms.

By using ultraviolet light sources with wavelengths of 254 nm for UV-C and 185 nm for UV-V, respectively, it is possible to emit photons with energies large enough to break and then to oxidize almost any scent molecule. Here is an example of the steps of the photo-oxidation process of hydrogen sulfide.

The first step is to break the chemical bond between the sulfur atom S and the hydrogen atom. The energy of this bond well known to chemists is 347 kJ / mole as shown in the attached Table 2, while the energy of UV-C photons is 470 kJ / mole. UV-C photons will therefore have no difficulty in breaking these bonds and momentarily releasing the sulfur and hydrogen atoms.

The second step is to provide oxygen atoms to react with the atoms thus released. Ambient air contains a lot of oxygen atoms (almost 21% of the air, the rest being nitrogen), but they ring in molecular pairs whose bond energy holding them together two by two is of 495 kJ / mole. In this case, UV-C photons (470 kJ / mol) lack the energy to break the bond and release oxygen atoms. It will be necessary to use a more energetic source of photons. This is the case with UV-V photons at wavelength 185 nm, whose energy of 646 kJ / mole easily exceeds the binding energy of an oxygen molecule (495 kJ / mole).

Once these two steps are completed, the oxidation reaction will occur at room temperature via a hydroxyl radical (OH *) which acts as a transmission intermediary for the free oxygen atoms.

The end result is the conversion and neutralization by oxidative effect of strongly odorous molecules into other molecules with little or no odor. The end products of this photo-oxidation process which in fact accelerates the natural process are then oxidized molecules and non-sticky dry ash particles which can now be captured by filters without causing them to clog. In this way, odors are removed by the oxidation process and the resulting dry particles are removed by filtration.

Table 2: Energy of Chemical Bonds

chemical bonds table

AIR FILTRATION AND LIMITS OF IONIZATION AGAINST SMOKE

Besides conventional filtration, there is another well-known way to remove solid particles from the air. Electrostatic filters, also often called air ionizers, have somehow this ability. Instead of mechanically capturing particles like conventional filters, the principle of operation of ionizers is to electrically charge the particles so that they migrate under the effect of electrical forces to neighboring surfaces. The same effect is achieved by rubbing a balloon on hair, then sticking it to a wall. After a while, however, the particle loses its charge and becomes airborne again. This is therefore a fairly temporary solution which in fact only stirs up the dust inside the house. This is why we dismissed ionizers as a method to really remove smoke particulates.

CONCLUSION

This article has described in detail the impact on the health, nature and composition of smoke in general and the inherent disadvantages of conventional filtration and air ionizers. Many years of experimental studies based on the chemical properties of smoke have shown that the smell of smoke cannot be removed without changing the structure of the molecules responsible for the odor. Besides thermal incineration, ultraviolet photooxidation has been shown to be the most effective way to achieve this by degrading these molecules through oxidation. Their oxidation makes smoke particles and aerosols dry and non-sticky, making them eligible candidates for standard filtration.

 

Thanks to the following references and sources:

(1) National Forest Database (BDNF)

(2) Government of Manitoba

(3) Hays, Gobbell, Ganick, Indoor Air Quality, McGraw-Hill, 1995, p.58.

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

(5) UWaterloo, Bond Lengths and Energies. n.d. Web. 21 Nov 2010.

(6) 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.

EPA / 600 / R-92/047, March 1992

Summary of McGill University study on UVGI published in The Lancet Medical Journal (2003)

Summary of McGill University study on UVGI published in The Lancet Medical Journal (2003).

Figure 1.

Environmental conditions: Bacterial and endotoxin measures in the ventilation system. Sterile coupons were installed on different surfaces of the ventilation system: on the cooling coils and drip pan (exposed to UVGI when turned ON), and on the filters (not exposed to UVGI). For bacterial count, coupons were collected and placed facing down on petri dishes, who were then incubated. For endotoxin count, coupons were then scraped with a sterile spatula, collected to sampling tubes, eluted, and assessed with an endotoxin measuring assay under standard lab procedures (LAL assay, KLARE protocol). Data is presented as the average number of bacterial colony forming units (CFU) per coupon that was formed on incubated petri dishes, and as the average of endotoxin units (EU) measured (n=4).

Figure 2.
Identification of microorganism growth obtained from coupons. Following the protocol procedures described in Figure 1., a sample from each petri dish was further incubated for one week. Microorganism colony types were then isolated, differentiated and counted with a stereomicroscope. Representative colonies were further selected and processed to identify species, either directly or by subsequent cultivation on selective media. The different species were then identified using standard manuals and protocols. Data is presented as the average number of colony forming units (CFU) per coupon.
 

Figure 3.
Office workers self-reported symptoms. UVGI systems were installed in the ventilation systems of 3 different office buildings, irradiating the cooling coils directly and the drip pans. None of the selected buildings had prior outbreaks of building-related illnesses. All workers of these office buildings were eligible to participate in the study. During the last week before UVGI systems were turned on, participants were asked to complete a previously validated self-administered questionnaire about demographics, working, medical, and personal information. UV systems were then either turned on (for 4 consecutive weeks) or off (for 12 consecutive weeks), while other HVAC parameters (humidity, heating, cooling, recirculation) were operated as usual. Every participant completed up to 6 questionnaires (following UVGI on/off periods). The participants and personnel handing the questionnaire were informed of the study goal but were not aware of the status of the UV lamps (on/off). Results are presented as the number of participants’ reported symptoms from the administered questionnaires. Responses from each trial was deemed an independent observation.

You wish to read the full study:

Understanding air filtration and UV disinfection in a medical environment

Health Europa reports on why UV purification is the most effective air disinfection method for medical, commercial and residential environments.

Evidence has accumulated over the years that following the standard guidelines and codes for designing healthcare facility ventilation systems is far from sufficient to ensure a sterile environment. Sterility is generally defined as 6 log (99.9999%) reduction of a population of microorganisms. This means that as little as one microorganism in a million is expected to survive after disinfection.

Traditional air filtration with high-efficiency particulate air (HEPA) filters or ultra-low penetration air (ULPA) filters have been widely adopted in the ventilation systems of hospitals, labs, and clinics, to control airborne pathogens. However, multiple studies have demonstrated that despite the use of such high-end filters, viral and bacterial airborne contamination are still ubiquitous in these ventilation systems. 

The most common explanation for underperforming filters often points to the filter rack seal joint’s bypass, filter puncture leakage, and poor general installation or maintenance. Although all these points remain valid and can always be improved, the physical cause is rooted in the fundamental fact that all filters show a significant drop in their capture efficiency for a certain range of particulate sizes – these can include both particles which are too small to be captured by interception and impaction and those which are too large to be removed via electrostatic and diffusion. This is simply a straightforward consequence of the fundamental principles of filtration physics. […]

Read more on Health Europe website.

Other articles that might interest you:

Using Sanuvox UVC technology to reduce the propagation of SARS-CoV-2 virus

Using Sanuvox UVC technology to reduce the propagation of SARS-CoV-2 virus

  • UVC irradiation (254 nm) is known for its germicidal properties. By disrupting their nucleic acids (DNA/RNA), it inactivates the reproductive capability of biological pathogens (molds, viruses, bacteria).1, 2

  • Sanuvox in-duct units have been demonstrated to be up to 99,97% effective at inactivating viruses and bacteria in the air in a study conducted by the EPA and Homeland Security 3. Bacteria and virus tested in the study (B.atrophaeus, S.marescens, MS2) are known to be more resistant to UVC than SARS-CoV-2 virus. 4,5
  • Many engineering and health agencies (ASHRAE, REHVA, CDC) now recognize that airborne transmission plays a major role in the propagation of SARS-CoV-2, the virus responsible for COVID-19. These agencies also recommend using UVGI as an effective method to mitigate the spread of the virus in indoor spaces. 6, 7, 8, 9
  • Because Sanuvox units are specified according to HVAC systems parameters, adequate UV output power is calculated using our proprietary software. As such, patented Biowall units can achieve the recommended 99% disinfection per pass regardless of air velocity.
 

1 https://www.fda.gov/medical-devices/coronavirus-covid-19-and-medical-devices/uv-lights-and-lamps-ultraviolet-c-radiation-disinfection-and-coronavirus

2 https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2789813/

3 https://cfpub.epa.gov/si/si_public_record_report.cfm?Lab=NHSRC&address=nhsrc/&dirEntryId=154947

https://www.springer.com/gp/book/9783642019982

5 https://www.researchgate.net/publication/339887436_2020_COVID-19_Coronavirus_Ultraviolet_Susceptibility

https://www.ashrae.org/about/news/2021/ashrae-epidemic-task-force-releases-updated-airborne-transmission-guidance

7 https://www.cdc.gov/coronavirus/2019-ncov/science/science-briefs/scientific-brief-sars-cov-2.html

https://www.rehva.eu/fileadmin/user_upload/REHVA_COVID-19_guidance_document_V4_09122020.pdf

9 https://www.ashrae.org/technical-resources/filtration-disinfection

Other articles that might interest you:

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 the 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 photocatalytic reaction, the titanium dioxide particles used in sunscreens are intentionally coated with silica. The addition of silica effectively neutralizes the photocatalytic 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 months, it will be down to 50% efficiency, 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 scientists 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-products. 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.

References

  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

INTRODUCTION

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.

COMPOSITION OF AIRBORNE DUST

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

INTERACTION OF UVC WITH KERATIN AND CYSTEINE

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
(kJ/mol)

C – C

347

C – H

413

C – N

305

C – O

358

C – S

259

N – H

391

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

H2S

Rotten egg, sewage-like

0.5 – 1.5

Ethyl Mercaptan

CH3CH2SH

Burnt match, sulfidic, earthy

1.1 – 1.8

Methyl Mercaptan

CH3SH

Rotten cabbage, burnt rubber

1.5

Diethyl Sulfide

CH3CH2SCH2CH3

Rubbery

0.9 – 1.3

Dimethyl Sulfide

CH3SCH3

Canned corn, cooked cabbage, asparagus

17 – 25

Diethyl Disulfide

CH3CH2SSCH2CH3

Garlic, burnt rubber

3.6 – 4.3

Dimethyl Disulfide

CH3SSCH3

Vegetal, cabbage, onion-like at high levels

9.8 – 10.2

Carbon Disulfide

CS2

Sweet, ethereal, slightly green, sulfidic

5

CALCULATION OF RESULTING SULFUR COMPOUNDS CONCENTRATION IN AIR

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:

Where:

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.

CONCLUSION

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.

ACKNOWLEDGMENTS

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

NOMENCLATURE

μg = micro gram

ppm = parts per million volumetric concentration

ppb = parts per billion volumetric concentration

nm = nanometer (10-9 m)

grain = lb/7,000

References

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.

INTRODUCTION

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.

COMPOSITION OF CIGARETTE SMOKE

Studies have shown that cigarette smoke contains over 3,800 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)

1.05E+12

3.50E+12

a) Solid particles and aerosols

(mg/cigarette)

(mg/cigarette)

Tar

20.80

44.10

Nicotine

0.92

1.69

Benzo (a) pyrene

3.50E-05

1.35E-04

Pyrene

2.70E-04

1.01E-03

Fluoranthene

2.72E-04

1.26E-03

Benzo (a) fluorene

1.84E-04

7.51E-04

Benzo (b/c) fluorene

6.90E-05

2.51E-04

Chrysene, benz (a) anthracene

1.91E-04

1.22E-03

Benzo (b,k,j) fluorenthrene

4.90E-05

2.60E-04

Benzo (e) pyrene

2.50E-05

1.35E-04

Perylene

9.00E-06

3.90E-05

Dibenz (a,j) anthracene

1.10E-05

4.10E-05

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

3.10E-05

1.04E-04

Benzo (g,h,i) perylene

3.90E-05

9.80E-05

Anthanthrene

2.20E-05

3.90E-05

Phenols (total)

2.28E-01

6.03E-01

Cadmium

1.25E-04

4.50E-04

Polonium 210, pCi

7.00E-02

1.30E-01

b) Gases and vapors

(mg/cigarette)

(mg/cigarette)

Water

7.50

298.00

Carbon monoxide

18.30

86.30

Ammonia

0.16

7.40

Carbon dioxide

63.50

79.50

NOx

0.014

0.051

Hydrogen Cyanide

0.240

0.160

Acrolein

0.084

0.000

Formaldehyde

0.000

1.440

Toluene

0.108

0.600

Acetone

0.578

1.450

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

 

AIR FILTRATION AND IONIZATION LIMITATIONS AGAINST TOBACCO SMOKE

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.

EFFECT OF ULTRAVIOLET LIGHT ON CIGARETTE SMOKE

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 (kJ/mol)

C – C

347

C – H

413

C – N

305

C – O

358

C – S

259

 N – H

391

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.

CONCLUSION

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.

 

ACKNOWLEDGMENTS

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.

References

  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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Effect of Germicidal UV on Plastic Materials

Effect of Germicidal UV on Plastic Materials

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

Introduction

Much of the effect of sunlight on materials has been attributed to the UV component (IESNA 2000), UV can fade some wall paints, wallpapers, and drapery fabrics (GE 1950). Some materials may have high UV reflectivity, like aluminum, or have high transmissivity, like quartz which absorbs very little UV. The absorption of UV by itself, is not necessarily an indicator that UV damage may occur, since it is the photochemistry which determines material effects. The total absorption is, therefore, an indicator of the potential for photodegradation in materials, while reflectivity can indicate protective effects.

UV photons energy vs. chemical bonds

When polymers are exposed to ultraviolet light, i.e. 100–400 nm wavelength, the photons energy exceeds the bond energy of the carbon bonds in the polymer or else exceeds the activation energy of chemical reaction (Moreau and Viswanathan 1976). The depth to which ultraviolet light penetrates the polymer creates a region of absorption where photochemical reaction may take place, and where photodegradation may occur. Since UV transmissivity tends to be very low for most materials, even at millimeter thicknesses, most of the photodegradation will occur on the immediate surface of a material, to a depth of typically less than 0.01 to 0.1 millimeter. For most common polymers the depth of UV penetration is typically about 0.025 mm to 0.050 mm i.e. 25 to 50 microns.

In the photodeterioration of paints, varnishes, and textiles, the quantum yield is several order of magnitudes less than unity (Feller 1994). For the bleaching of certain dyes the quantum yield has been reported to be about 0.002, meaning a thousand photons must be absorbed before two molecules are bleached. Quantum yields as low as 0.0001 (10,000 photons per molecule) have been reported for most plastics. High quality pure plastics are relatively resistant to UV but impurities and residual solvents in low-grade plastics are mainly responsible for their quick photodegradation.

Yellowing of polymers from ultraviolet exposure tends to be concentrated on the immediate surface. Surface yellowing tends to block UV and protects the inner plastic. The fading of pigments and dyes can be evaluated in terms of the loss in concentration over time (Feller 1994). The depth of discoloration is reduced by the presence of color pigments. As the concentration of pigments increases, the depth of discoloration or fading also decreases.

Plastic properties and protection against degradation

There are as many as thirteen different properties of plastics which can be used as indicators of photodegradation, including coloration, tensile strength, elongation, hardness, degree of polymerization, infrared absorbance, etc. Experimental data indicates the response of most of these properties to extended ultraviolet exposure results in data that can be effectively modeled with exponential decay curves of one or more orders.

Materials that would darken to UV after exposure create a thin UV-proof film on the surfaces of polymers like PVC. This would enable them to develop resistance to further UV exposure (Owen 1976).

The photochemical degradation of materials is a dose-dependent function that depends only on the quantum yield and the molar absorption coefficient at the irradiation wavelength (Bolton and Stefan 2002). It describes the susceptibility of a material to degrade under UV exposure. Associated with this there would be some limiting distance, a film thickness or penetration depth, to which UV would penetrate.

Based on several decades of use, experience has shown that within a few exceptions, the UV induced damages tend to remain superficial and do not generally affect the structural or mechanical integrity of thick plastic components. For critical components such as exposed electrical wire direct insulation coating, it is recommended to cover the wires with aluminum tape or run the wires inside protective metallic rigid or flex conduits according to good practice and general electrical codes prescriptions. Rubbers in general such as motor belts and conduits used in the HVAC industry have proven to stand germicidal UV very well over the last 20 years of cumulated field experience.

Screens of many electronic devices can be affected by UV degradation due to the grade of plastic used and the very thin film generally used. For such devices, protection is easily achieved by installing with a simple glass window of 3 mm thickness over the screen. Transmittivity of common amorphous glass approaches zero for below 370 nm wavelength.

References

IESNA. 2000. Lighting Handbook: Reference & Application IESNA HB-9-2000. New York: Illumination Engineering Society of North America.

GE. 1950. Germicidal Lamps and Applications. USA: General Electric. Report nr SMA TAB: VIII-B.

Moreau W, Viswanathan N. 1976. Applications of Radiation Sensitive Polymer Systems. In: Labana SS, editor. Ultraviolet Light Induced Reactions in Polymers. Washington, DC: Ameri- can Chemical Society, pp. 107–134.

Feller RL. 1994. Accelerated Aging: Photochemical and Thermal Aspects. Institute TGC, editor.

Ann Arbor, MI: Edwards Bros.

Bolton J, Stefan M. 2002. Fundamental photochemical approach to the concepts of fluence (UV Dose) and electrical energy efficiency in photochemical degradation reactions. Res Chem Intermed 28(7–8):857–870.

Owen ED. 1976. Photodegradation of Polyvinyl chloride. In: Labana SS, editor. Ultraviolet Light Induced Reactions in Polymers. Washington, DC: American Chemical Society, pp. 208–219.