ASHRAE OR-16-C033-2016 Root Cause of the Odor Generated by Germicidal UV Disinfection with Mobile Units.pdf

1、Dr. Normand Brais is the founding vice-president of Sanuvox Technologies Inc, Montreal, Qc, Canada. Benoit Despatis is a research engineer at Sanuvox Technologies Inc, Montreal, Qc, Canada. Root Cause of the Odor Generated by Germicidal UV Disinfection with Mobile Units Normand Brais, Eng., PhD Beno

2、it Despatis, Eng. ASHRAE Member ABSTRACT Germicidal ultraviolet (UV) light has long been used successfully for the disinfection of water, air, and surfaces, and has become a common practice in the healthcare industry. There has been, however, an unresolved health concern with regard to the residual

3、odors that have often been noted after rooms have been disinfected, and no satisfactory explanation of these possible volatile organic compounds (VOCs) has previously been published. This study explains the residual odors in terms of thiol or mercaptan molecules that can be produced by the UV irradi

4、ation of keratin and cysteine. Keratin is a protein that is found in skin squames while cysteine is a similar molecule found in hair. They also both contain a significant amount of sulfur. Skin squames and hair particles are common contaminants of indoor environments and are present in airborne dust

5、 as well as being surface-borne. UV photons carry sufficient energy to break the chemical bonds of keratin and cysteine, as well as the chemical byproducts including volatile smaller sulfur-containing molecules that fall into the categories of thiols and mercaptans. The human nose is extremely sensi

6、tive to these molecules and can detect them at concentrations as low as 1 part per billion. The smell after UV disinfection is sometimes described as that of burning hair or the pungent odor of rotten eggs or garlic. The latter smell is characteristic of mercaptans. In an indoor environment where th

7、e dust loading in the air may typically be about 100 g/m3(0.000044 grain/ft3), the aftermath of the UV disinfection process will leave behind a concentration of mercaptans of about 2 ppb, or twice the smell threshold level. According to the CSST in Quebec and per OSHA, the safe level for 8 hours of

8、exposure to mercaptans is 500 ppb. Consequently, the actual level obtained after UV disinfection is negligible and therefore it is concluded that the VOCs responsible for the residual odor after UV disinfection do not pose a health hazard to humans. INTRODUCTION It has been often noticed by many use

9、rs 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.

10、 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

11、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 p

12、aint was eliminated after testing in a bare metal aluminum enclosure and withnessing 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 performi

13、ng 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 part

14、icles 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, text

15、ile 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 g/m3 (0.000044 to 0.0044 grain/ft3) order of magnitude. Th

16、e 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 g/m3 (0.0044 grain/ft3) and 3,000 g/m3 (0.0013 grain/ft3) for PM10. Since airborne dust is essentially dead

17、 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

18、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

19、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

20、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 b

21、reak 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 cyst

22、eine 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 39

23、1 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 fal

24、l 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

25、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. Report

26、ed 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

27、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, ether

28、eal, 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 pe

29、rformed. 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 Thi

30、ol can be estimated as follows: (1) Where: Dustload = dust weight per unit air volume in g/m3 (lb/ft3) SK = % Sulfur in Keratin/Cysteine = 5% %Skin_Hair = Skin and Hair mass fraction in the dust = 80% Thiol = Methyl Mercaptan density at normal ambient temperature and pressure = 1.974 kg/m3 (0.1232 l

31、b/ft3) Equation (1) shows that when the airborne dust load gets above 75 g/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

32、 relatively clean environment with dust loading as low as 100 g/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 o

33、f equation 1 and allowing the dust loading to go up to 1,000 g/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 Concen

34、tration in ppb vs Dust Load At maximum ASHRAE acceptable airborne dust loads of 10,000 g/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

35、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. C

36、onsequently, 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 g/m3 (0.000033 grain/ft3) and that this dust is mainly made

37、 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 r

38、ooms. 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

39、data and editorial assistance. NOMENCLATURE g = micro gram ppm = parts per million volumetric concentration ppb = parts per billion volumetric concentration nm = nanometer (10-9 m) grain = lb/7000 REFERENCES Spengler, Samet, McCarthhy, Indoor Air Quality Handbook. McGraw-Hill, 2001. Fergusson,J.E.,F

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41、 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

42、Odor Thresholds for Hazardous Air Pollutants Listed in the Clean Air Act Ammndments of 1990. EPA/600/R-92/047, March 1992. National Institute for Occupational Safety and Health, NIOSH pocket guide to chemical hazards. Washington, D.C. : U.S. G.P.O. (1985). RM-514001 ACGIH 1971. Methyl mercaptan (methanethiol). In: Documentation of the threshold limit values for substances in workroom air. 3rd ed. Cincinnati, OH: American Conference of Governmental Industrial Hygienists, pp. 167-168. ASHRAE Handbook-HVAC Applications, Chapter 60, 2011.