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.