The Use of UVC Irradiation to Sterilize Filtering Facepiece Masks Limiting Airborne Cross-Infection

International Journal of Environmental Research and Public Health

Article The Use of UVC Irradiation to Sterilize Filtering Facepiece Masks Limiting Airborne Cross-Infection

Wojciech Kierat 1,* , Weronika Augustyn 2, Piotr Koper 3, Miroslawa Pawlyta 4 , Arkadiusz Chrusciel 5 and Bernard Wyrwol 1

1 Department of Digital Systems, Silesian University of Technology, 44-100 Gliwice, Poland; [email protected] 2 Department of Environmental Biotechnology, Silesian University of Technology, 44-100 Gliwice, Poland; [email protected] 3 Department of Heating, Ventilation and Dust Removal Technology, Silesian University of Technology, 44-100 Gliwice, Poland; [email protected] 4 Department of Engineering Materials and Biomaterials, Silesian University of Technology, 44-100 Gliwice, Poland; [email protected] 5 MEXEO Institute of Technology, 47-225 Kedzierzyn-Kozle, Poland; [email protected] * Correspondence: [email protected]

Received: 2 September 2020; Accepted: 7 October 2020; Published: 11 October 2020

Abstract: In addition to looking for effective drugs and a vaccine, which are necessary to save and protect human health, it is also important to limit, or at least to slow, the spread of coronavirus. One important element in this action is the use of individual protective devices such as filtering facepiece masks. Currently, masks that use a mechanical filter, such as a HEPA (High Efficiency Particulate Air) filter, are often used. In some countries that do not have a well-developed healthcare system or in exceptional situations, there is a real and pressing need to restore filters for reuse. This article presents technical details for a very simple device for sterilization, including of HEPA polymer filters. The results of biological and microscopic tests confirming the effectiveness of the sterilization performed in the device are presented. The compact and portable design of the device also allows its use to disinfect other small surfaces, for example a small fragment of a floor, table, or bed.

Keywords: ultraviolet germicidal irradiation; UVGI; HEPA ﬁlter sterilization; SARS-CoV-2

1. Introduction Ongoing in every corner of the world, the fight against the CoVID19 virus pandemic presents scientists with the challenge of seeking an effective drug or vaccine to either eliminate, or at least significantly reduce, the growing disease caused by the pathogen. Within the wide range of actions taken in view of the threat of a pandemic, solutions aimed at limiting or slowing down the speed of spreading of the virus play an important role. In the opinion of virology specialists, it is advisable to use personal protective equipment such as filtering face masks. In some countries, the use of such masks is not only a recommendation but even a legally sanctioned order. Unfortunately, the availability of protective filtering facepiece masks with sufficiently good filtration properties is limited. Unavailability of masks has been signaled by medical staff who are providing help to infected people. Considering the lack of a viable possibility of very quickly and sufficiently fully equipping medical services with personal protective devices, including filtering facepiece masks equipped with effective air filters, a good solution is to restore filters for use by sterilization. Extended use or re-use of filtering facepiece masks is not recommended by the World Health Organization (WHO), European Centre for Disease Prevention and Control (ECDC),

Int. J. Environ. Res. Public Health 2020, 17, 7396; doi:10.3390/ijerph17207396 www.mdpi.com/journal/ijerph Int. J. Environ. Res. Public Health 2020, 17, 7396 2 of 14 the US Centers for Disease Control and Prevention (CDC), and Public Health England (PHE). However, in exceptional circumstances, it is allowed to depart from this rule under certain conditions [1]. The most important requirements are that the mask is not mechanically damaged, is not dirty (blood, secretions, body fluids), and that the sterilization process is carried out by properly trained personnel. The mentioned solution should be treated as a last-resort measure. One of the possible methods of sterilizing air filters is exposure to ultraviolet radiation (UVGI—ultraviolet germicidal disinfection) with a wavelength range from 100 to 280 nm (UVC). The effectiveness of this sterilization method is confirmed by studies [2] in which severe acute respiratory coronavirus syndrome (SARS-CoV) and Middle East respiratory syndrome coronavirus (MERS-CoV) were inactivated. The method is based on inactivation of the coronavirus by damaging deoxyribonucleic acid, which prevents the reproduction of pathogen cells. Looking at the general similarities between the currently spreading SARS-CoV-2 and SARS-CoV and MERS-CoV, it can be presumed that UVGI is also effective in deactivating SARS-CoV-2. The problem of deactivating the pathogen by UVC radiation can be also found in many recent publications [3–6]. The primary purpose of our research is to develop the device and determine its effectiveness. The possibility of using the UVGI method has recently been proposed and generally described [7]. However, no specific calculations or sufficient technical details have been presented, which are needed in order to construct such a device. The main issue in the design of the device was to determine the required minimum exposure time to disinfect the surface, so as to effectively deactivate the pathogen-containing particles, as well as the maximum exposure time, exceeding which could cause permanent damage to the filter surface. As demonstrated in previous studies [6,8,9], a sterilization dose of approximately 1 J/cm2 is sufficient to sterilize the surface of the material. It has also been shown [8,10] that increasing the radiation dose above 1 J/cm2 does not bring significant benefits. Polymers are often used for the production of air filters and a high dose of ultraviolet radiation may result in a deterioration of their filtering properties [11]. The degree of sensitivity of the filter to UV radiation depends on the quality and technology of the particular polymer. Test results showed that for some polymers, their properties deteriorated by 1% when exposed to a radiation dose of 100 J/cm2 [11,12]. Determining the exposure time based on the assumed range of radiation dose variability requires taking into account the power and spatial distribution of the ultraviolet radiation and the distance of the sterilized object from the radiator or radiators. Sterilization is a process aimed at the complete destruction and elimination of all living microorganisms and their spore forms. The use of biological indicators is considered the most reliable, easy, and quick method of sterilization process control. The indicators are described in the EN ISO 11138-1: 2017 standard, concerning the sterilization of products in health care [13]. The biological indicator system contains a population of spores of a non-pathogenic microorganism, appropriate for a given type of sterilization, with the highest resistance to the action of a particular sterilizing agent. Microorganisms are most often placed on paper strips and stainless-steel discs [14,15]. Microorganisms vary widely in their resistance to sterilization and/or disinfection. With the exception of prions, bacterial spores have the greatest innate immunity. Subsequently, the relative scale of resistance includes mycobacteria, non-enveloped viruses, Gram-positive bacteria, fungi, and Gram-negative bacteria. The envelope viruses, which include the coronaviruses, are the most sensitive to disinfection processes, see Figure1[16,17]. In the case of using UVC radiation, it has been shown that the elimination of bacterial spores requires a radiation dose 3 times higher than for the inactivation of polio- and rotaviruses [18]. In turn, enveloped viruses, under the influence of reactive oxygen species induced by UVC radiation, are more susceptible to inactivation than viruses without envelopes. The lipids building the envelope undergo peroxidation [19]. Therefore, it can be concluded that the dose of radiation eliminating bacterial spores will also be destructive to enveloped viruses. Int. J. Environ. Res. Public Health 2020, 17, 7396 3 of 14

FigureFigure 1. Resistance 1. Resistance of of microorganisms microorganisms to to ster sterilizationilization and/or and /disinfectionor disinfection [16,17]. [16 ,17].

Due toIn their the case small of sizeusing and UVC ease radiation, of use, it paper has been indicator shown that strips the containing eliminationBacillus of bacterial atrophaeus spores ATCC requires a radiation dose 3 times higher than for the inactivation of polio- and rotaviruses [18]. In 9372 (the bacterial strains of B. atrophaeus from American Type Culture Collection, global biological turn, enveloped viruses, under the influence of reactive oxygen species induced by UVC radiation, materialsare resourcemore susceptible and standards to inactivation organization, than viruses no with 9372)out sporesenvelopes. were The selected lipids building for testing. the envelope Although the strain isundergo dedicated peroxidation primarily [19]. to assessingTherefore, theit can eff ectivenessbe concluded of sterilizationthat the dose withof radiation dry hot eliminating air, it can also be used inbacterial the case spores of sterilization will also be de withstructive UVC to radiation enveloped [20 viruses.,21]. Bacillus atrophaeus sporulation occurs as a result of theDue depletion to their small of essential size and nutrients. ease of use, This paper process indicator results strips incontaining extremely Bacillus stress-resistant atrophaeus spores that canATCC be successfully 9372 (the bacterial used tostrains assess of B. the atrophaeus effectiveness from American of sterilization Type Culture with dryCollection, air, ethylene global oxide, or microwavesbiological materials [22]. B. atrophaeusresource andspores standards were organizat also usedion, no to 9372) test thespores eff ectivenesswere selected of for the testing. sterilization of air circulationAlthough the systems strain is dedicated with the primarily use of HEPAto assess anding the UVC effectiveness filters [20 of]. sterilization Due to non-pathogenicity,with dry hot ease ofair, breeding, it can also and be at used the in same the time,case of high sterilizat resistance,ion withB. UVC atrophaeus radiationspores [20,21]. play Bacillus a fundamental atrophaeus role in monitoringsporulation a series occurs of new as a result sterilization of the depletion and disinfection of essential nutrients. products This [23 ].process results in extremely stress-resistant spores that can be successfully used to assess the effectiveness of sterilization with Thedry main air, purposeethylene oxide, of this or work microwaves was to develop [22]. B. aatrophaeus methodology spores for were the designalso used of portable to test the sterilizers that caneffectiveness be used toof the sterilize sterilization various of air objects circulation and systems surfaces, with inthe particular use of HEPA for and sterilizing UVC filters masks [20]. with HEPADue filters. to non-pathogenicity, As part of the work, ease aof methodbreeding, of and determining at the same thetime, spatial high resistance, distribution B. atrophaeus of the radiation intensityspores incident play a on fundamental the surfaces role of in sterilizedmonitoring objects a series wasof new demonstrated. sterilization and The disinfection aim of the products study was to determine[23]. the minimum exposure time that allows for effective deactivation of the pathogen and the maximumThe time, main which purpose if exceeded, of this work may was cause to develo deteriorationp a methodology of the filtering for the design properties of portable of the HEPA filters.sterilizers The effectiveness that can be of used the to device sterilize was various checked objects by and performing surfaces, in appropriate particular for biological sterilizing tests.masks As part of the work,with HEPA microscopic filters. As tests part were of the also work, carried a method out to of assess determining the eff ectsthe spatial of ultraviolet distribution radiation of the on the surfaceradiation structure intensity of a filter incident made on of the a polymersurfaces of material. sterilized objects was demonstrated. The aim of the study was to determine the minimum exposure time that allows for effective deactivation of the 2. Methodpathogen and the maximum time, which if exceeded, may cause deterioration of the filtering properties of the HEPA filters. The effectiveness of the device was checked by performing 2.1. Constructionappropriate ofbiological the Sterilizer tests. As part of the work, microscopic tests were also carried out to assess the effects of ultraviolet radiation on the surface structure of a filter made of a polymer material. CommerciallyInt. J. Environ. available Res. Public Health emitters 2020, 17 can, x be used as the UVC radiation source, for example2 of 15 HNS L 55 W 2. Method 2G11 manufacturedCommercially by OSRAM available or TUVemitters PL-L can be 55 used W/ as4P the HF UVC 1CT radiation/25 manufactured source, for example by PhillipsHNS L (Figure2). The main technical55 W 2G11 parameters manufactured for by OSRAM both emitters or TUV PL-L are 55W/4P almost HF the 1CT/25 same manufactured and are asby Phillips follows: electrical 2.1. Construction of the Sterilizer power consumption(Figure 2). 55 The W, main dominant technical parameters wavelength for both 254 emitters nm, and are almost radiant the fluxsame 17and W are at as a follows: wavelength range electrical power consumption 55W, dominant wavelength 254 nm, and radiant flux 17W at a from 200 to 280wavelength nm. range from 200 to 280 nm.

Int. J. Environ. Res. Public Health 2020, 17, x; doi: www.mdpi.com/journal/ijerph

Figure 2. ShapeFigure and 2. Shape dimensions and dimensions of the of TUVthe TUV PL-L PL-L 5555W/4P W/4P HF HF1CT/25 1CT emitter./25 emitter. The active The surface active is surface is marked purple.marked purple.

The housing for the sterilizer was made of aluminum. This material was chosen as it is light, durable, and resistant to corrosion and UV radiation. Moreover, it is easy to be worked with and widely available in different shapes. The construction and dimensions of the completed sterilizer are presented in Figure 3. The emitters were mounted at a distance of 15 cm from each other and 15 cm from the surface to be sterilized. Figure 4 presents photos of the sterilizer.

Int. J. Environ. Res. Public Health 2020, 17, x 2 of 15

Commercially available emitters can be used as the UVC radiation source, for example HNS L 55 W 2G11 manufactured by OSRAM or TUV PL-L 55W/4P HF 1CT/25 manufactured by Phillips (Figure 2). The main technical parameters for both emitters are almost the same and are as follows: electrical power consumption 55W, dominant wavelength 254 nm, and radiant flux 17W at a wavelength range from 200 to 280 nm.

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Figure 2. Shape and dimensions of the TUV PL-L 55W/4P HF 1CT/25 emitter. The active surface is The housingmarked for the purple. sterilizer was made of aluminum. This material was chosen as it is light, durable, and resistant to corrosion and UV radiation. Moreover, it is easy to be worked with and The housing for the sterilizer was made of aluminum. This material was chosen as it is light, widely availabledurable, in di andﬀerent resistant shapes. to corrosion The and construction UV radiation. and Moreover, dimensions it is easy ofto be the worked completed with and sterilizer are presented in Figurewidely available3. The in emitters different shapes. were mountedThe constructi aton a and distance dimensions of of 15 the cm completed from each sterilizer other are and 15 cm from the surfacepresented to be in sterilized. Figure 3. The Figure emitters4 were presents mounted photos at a distance of the of 15 sterilizer. cm from each other and 15 cm from the surface to be sterilized. Figure 4 presents photos of the sterilizer.

Int. J. Environ. Res. Public Health 2020, 17, x 3 of 15

Figure 3.FigureDimensions 3. Dimensions and and 3D 3D visualization visualization of the of thecompleted completed sterilizer. sterilizer.

FigureFigure 4. Photographs 4. Photographs of the the built built sterilizer. sterilizer.

In order to determine the required minimum and allowable maximum surface sterilization times In orderin to the determine UVGI method, the first, required the dose minimum of ultraviolet and radiation allowable reaching maximum the surface from surface the source sterilization of times in the UVGIradiation method, should ﬁrst, be the determined. dose of The ultraviolet proposed solu radiationtion uses emitters, reaching which the emit surface light evenly from from the source of radiation shouldtheir entire be determined. active surface. Although The proposed the UVC radiation solution source uses consists emitters, of two which parallel emittubes placed light evenly from close to each other Figure 2in order to simplify calculations, both tubes will be treated as an emitter their entire activein the form surface. of a single Although tube of length the UVCL. For this radiation type of source, source it can consists be assumed of that two the parallel radiation tubes placed close to eachdensity other Figuredistribution2 in is orderaligned to along simplify the longitudinal calculations, axis of boththe emitter tubes and will that be on treated any small as an emitter in the form offragment a single of the tube emitter of lengththe radiationL. For propagates this type equally of in source, all radial itdirections. can be assumed that the radiation density distribution2.2. Determining is aligned the Spatial along Distribution the longitudinal of the Direct Radiation axis of Intensity the emitter and that on any small fragment of the emitter theFigure radiation 5 shows propagates a single radiator equally illuminating in all the radial surface directions. to be sterilized by an element at a distance R from the radiator axis. The length of the active surface of the emitter is L. 2.2. Determining the Spatial Distribution of the Direct Radiation Intensity Figure5 shows a single radiator illuminating the surface to be sterilized by an element at a distance R from the radiator axis. The length of the active surface of the emitter is L. L y

Coordinates of α Coordinates of i expossed surface UV emitter (xi, 0, zi) (xL, yL, 0) |xi-xL| v Ai L y Y

Figure 5. A fragment of the radiator illuminating a fragment of the sterilized surface.

Each unit fragment of the sterilized surface Ai is illuminated from each unit cylindrical fragment of the emitter. The radiation of the emitter falls on the sterilized surface located at a distance Ri from the source and at an angle of αi. Taking into account the coordinates of the unit radiation source [xL, yL, zL] and the coordinates of the unit surface [xi, 0, zi], the distance Ri can be determined from the equation

Int. J. Environ. Res. Public Health 2020, 17, x 3 of 15

Figure 3. Dimensions and 3D visualization of the completed sterilizer.

Figure 4. Photographs of the built sterilizer.

In order to determine the required minimum and allowable maximum surface sterilization times in the UVGI method, first, the dose of ultraviolet radiation reaching the surface from the source of radiation should be determined. The proposed solution uses emitters, which emit light evenly from their entire active surface. Although the UVC radiation source consists of two parallel tubes placed close to each other Figure 2in order to simplify calculations, both tubes will be treated as an emitter in the form of a single tube of length L. For this type of source, it can be assumed that the radiation density distribution is aligned along the longitudinal axis of the emitter and that on any small fragment of the emitter the radiation propagates equally in all radial directions.

2.2. Determining the Spatial Distribution of the Direct Radiation Intensity Int. J. Environ. Res. Public Health 2020, 17, 7396 5 of 14 Figure 5 shows a single radiator illuminating the surface to be sterilized by an element at a distance R from the radiator axis. The length of the active surface of the emitter is L. L y

Coordinates of α Coordinates of i expossed surface UV emitter (xi, 0, zi) (xL, yL, 0) |xi-xL| v Ai L y Y

Figure 5. AFigure fragment 5. A fragment of the of the radiator radiator illuminatingilluminating a fragment a fragment of the sterilized of the sterilizedsurface. surface.

Each unit fragment of the sterilized surface Ai is illuminated from each unit cylindrical fragment Each unit fragment of the sterilized surface Ai is illuminated from each unit cylindrical fragment of the emitter. The radiation of the emitter falls on the sterilized surface located at a distance Ri from of the emitter. The radiation of the emitter falls on the sterilized surface located at a distance Ri from the the source and at an angle of αi. Taking into account the coordinates of the unit radiation source [xL, source and atyL, an zL] angleand the of coordinatesαi. Taking of the into unit account surface [x thei, 0, coordinateszi], the distance ofRi can the be unit determined radiation from source the [xL, yL, zL] equation and the coordinates of the unit surface [xi, 0, zi], the distance Ri can be determined from the equation q 2 2 2 R = (x xL) + yL + (z zL) (1) i i − i −

The angle of incidence αi can be determined from the equation      y  1 L  αi = tan−  q  (2)  2 2   (x xL) + (z zL)  i − i − Assuming an even emission of radiant ﬂux ∅ along the entire axis of the emitter of length L, the ﬂux 0 emitted by a unit cylindrical fragment of the active surface of the emitter can be determined from ∅dzL the equation ∅ ∅0 = (3) dzL L E Irradiance e0,i,dz received from a unit cylindrical fragment of the emitter, and reaching the unit surface Ai, can be determined from

0 ∅dzL E0 = sin αi (4) e,i,dz 4π R 2 · i E E L The total irradiance e,i on a unit area surface is the integral of e0,i,dz over the length of the active area of the emitter is ZL = Ee,i Ee0,i,dz dzL (5) 0

2.3. The Method of Verifying the Effectiveness of Sterilization The studies used standard bio-indicators in the form of strips, which were inoculated with B. atrophaeus spores (ATCC 9372) at >106 spores/strip (ACE test, Fukuzawa Shoji Co., Ltd., Yokohama, Japan). The product meets the requirements of the ISO 13485 standard regarding quality management systems in the medical devices industry. Biological indicators are supplemented with Soybean Casein Digest broth, which acts as a culture medium, enabling germination and growth of microorganisms that Int. J. Environ. Res. Public Health 2020, 17, 7396 6 of 14 have survived the sterilization process. During the growth of microorganisms, enzymatic reactions take place that make the pH of the medium acidic [24]. The sterilization efficiency is assessed on the basis of the color change of the sample, from red to yellow, due to the presence of the pH indicator—phenol red. The mask, consisting of 9 layers, was cut into pieces measuring 4 2 cm. The fragments consisted × of 2 outer layers (layers 1 and 9) and 7 internal layers (Figure6a). The front of the mask in some places has been provided with a stiffener, which is also a second layer. (Figure6b). Indicator strips were placed on the surface and under the 1st, 2nd, 3rd, 4th, 5th, and 8th layer of the mask (Figure6c). The tests were carried out on parts of the mask with and without the stiffener. The prepared samples were placed on petri dishes at a distance of 15 cm from the radiators and irradiated for 15 min. After this time, the indicator papers were placed in culture media and incubated in an incubator at 37 ◦C for 7 days. The procedureInt. J. Environ. was performed Res. Public Health in 2020 four, 17, replicationsx for each layer of the mask. Additionally,5 control of 15 tests Int. J. Environ. Res. Public Health 2020, 17, x 5 of 15 were carried out, where the mask fragments were not irradiated, and the UVC lamps were turned off.

(a(a) ) (b()b ) (c) (c)

Figure 6. FragmentFigure 6. ofFragment a protective of a protective mask mask separated separated into into layerslayers (a ();a );additional additional stiffening stiﬀ eninglayer of layerthe of the face Figure 6. Fragment of a protective mask separated into layers (a); additional stiffening layer of the mask (b); anface indicator mask (b); strip an indicator located strip on located the surfaceon the surface of the of the mask mask fragmentfragment (c). ( c). face mask (b); an indicator strip located on the surface of the mask fragment (c). 3. Results and Discussion 3. Results3. and Results Discussion and Discussion Spatial distributions of the direct radiation irradiance originating from a single active emitter are Spatial distributionsshownSpatial in Figuredistributions 7. of The the calculations of directthe direct radiationand radiation charts were irradiance made using originating originating a program from written a from single in athe active single LabView emitter active are emitter are shownshown inenvironment, Figure in Figure7. in 7. Theaccordance The calculationscalculations with the andtheoretical and charts charts a weressumptions made were and using made formulas a program using presented awritten program in the in chapter, the writtenLabView in the Methods. LabView environment,environment, in inaccordance accordance with withthe theoretical the theoretical assumptions assumptions and formulas and presented formulas in the presented chapter, in the Methods. chapter, Methods.

Figure 7. Spatial distributions of the direct radiation irradiance originating from a single active emitter. Figure 7. Spatial distributions of the direct radiation irradiance originating from a single active emitter.

Figure 8 shows the spatial distribution of the direct radiation irradiance Ee from five Figure 7. Spatial distributions of the direct radiation irradiance originating from a single active Figure8 simultaneouslyshows the spatial operating distribution radiation emitters. of the direct radiation irradiance Ee from ﬁve simultaneously emitter. operating radiation emitters. The distanceFigure 15 cm8 shows between the thespatial emitters distribution was chosenof the asdirect a compromise radiation irradiance between Ee thefrom two five solutions. An arrangementsimultaneously of emitters operating very radiation close together emitters. allows for a uniform distribution of irradiance but only on a small working area. The arrangement of emitters at some greater distance increases the size of the

Int. J. Environ. Res. Public Health 2020, 17, 7396 7 of 14

Int. J. Environ. Res. Public Health 2020, 17, x 6 of 15 working area, which generally allows one to sterilize more items. Increasing the distance, however, causes the appearance of larger diﬀerences in the irradiance in poorly and heavily lit places.

Figure 8. The spatial distribution of the direct radiation irradiance over the working area of the sterilizer.

The distance of 15 cm between emitters has been related to the assumed nominal distance of Figure 8. The spatial distribution of the direct radiation irradiance over the working area of the sterilized objectssterilizer. from emitters. The distance of 15 cm between the object and the emitter seems to be optimal. For currently commercially produced UVC radiation emitters, placing the object at a distanceThe of distance 15 cm 15 allows cm between a 1 J/cm the2 emittersdose to was be chos delivereden as a compromise to the surface between within the two a few solutions. minutes, which isAn not arrangement a long time. of emitters Increasing very the close distance together from allows the for source a uniform will extenddistribution this time,of irradiance which seemsbut undesirable.only onOn a small the working other hand, area. decreasingThe arrangement the distanceof emitters will at some shorten greater the distance exposure increases time, the but size will increaseof the the unevenness working area, of thewhich exposure. generally This allows unevenness one to sterilize of the more exposure items. will Increasing be due the to two distance, reasons. however, causes the appearance of larger differences in the irradiance in poorly and heavily lit places. First, radiation reaching the flat surface will be emitted mainly from one direction, from the nearest lamp. Radiation fromThe otherdistance emitters of 15 cm will between reach theemitters surface has frombeen related a very smallto the angle.assumed Secondly, nominal distance the sterilized of surfacesterilized is not perfectly objects from flat. emitters. For a The fragment distance protruding of 15 cm between 1 cm the above object the and remaining the emitter surface seems to of be the object, foroptimal. a distance For currently of 5 cm fromcommercially the emitter, produced it will UV meanC radiation a 20% change emitters, in distance.placing the At object a distance at a of 15 cm, itdistance will be of only 15 cm 7%. allows Therefore, a 1J / cm² a distancedose to be of de 15livered cm between to the surface the object within and a few emitters minutes, was which adopted. is not a long time. Increasing the distance from the source will extend this time, which seems In order to bring to each fragment of the sterilized surface lighting with a similar intensity but coming undesirable. On the other hand, decreasing the distance will shorten the exposure time, but will from different directions, the distance between the emitters was also assumed to be 15 cm. increase the unevenness of the exposure. This unevenness of the exposure will be due to two reasons. First, radiation reaching the flat surface will be emitted mainly from one direction, from the nearest 3.1. Determining the Minimum Required Exposure Time lamp. Radiation from other emitters will reach the surface from a very small angle. Secondly, the Tosterilized determine surface the minimumis not perfectly time, flat. it For was a assumedfragment protruding that the sterilized 1 cm above surface the remaining will be illuminatedsurface of only bythe direct object, radiation. for a distance This of 5 assumescm from the the emitter, absence it will of mean any reflectiona 20% change of in the distance. radiation At a distance insidethe sterilizer.of This15 cm, corresponds it will be only to an7%. extreme Therefore, worst-case a distance scenario. of 15 cm A between limited possibilitythe object and of reflectionemitters was could, adopted. In order to bring to each fragment of the sterilized surface lighting with a similar intensity for example, result from extreme pollution of the internal surface of the device housing. but coming from different directions, the distance between the emitters was also assumed to be 15 The presented spatial distributions of direct radiation (Figure8) show that in the corners of cm. the area where sterilization is carried out, the irradiance is the smallest and its value is equal to 2 2 Ee,MIN =3.1.1.94 Determining mW/cm the. ThisMinimum means Required that Exposure to ensure Time a minimum dose of UVC radiation of 1 J/cm , regardless of where the sterilized item was placed, the minimum exposure time should not be less than To determine the minimum time, it was assumed that the sterilized surface will be illuminated only by direct radiation. This assumes theJ absence mWof any reflection of the radiation inside the sterilizer. This corresponds TtoMIN an =extreme1 worst-ca/ 1.94se scenario. 516 A s limited possibility of reflection (6) cm2 cm2 could, for example, result from extreme pollution of the internal surface of the device housing.

3.2. DeterminingThe presented the Maximum spatial Allowed distributions Exposure of direct Time radiation (Figure 8) show that in the corners of the Inarea order where to determine sterilization the is maximum carried out, exposure the irradi time,ance itis wasthe smallest assumed and that its allvalue radiation is equal generated to Ee,MIN = by 1,94 mW/cm². This means that to ensure a minimum dose of UVC radiation of 1 J/cm², regardless of all emitters will reach the surface on which the objects to be sterilized will be located without any loss. where the sterilized item was placed, the minimum exposure time should not be less than Such a case is completely unreal, but nevertheless it allows one to determine the theoretical value of the absolute maximum radiation dose. Illumination of the surface to be sterilized with all the radiation generated by the emitters would be possible under the condition of perfect lossless reflection of waves by the internal surfaces of the device housing, as well as under the condition of full absorption of Int. J. Environ. Res. Public Health 2020, 17, 7396 8 of 14 radiation on the surface. For five emitters, each generating a flux of 17 W, and with the sterilizer working area of size 50 cm 60 cm, the maximum radiation intensity E can be determined from × e,MAX 5 17 W mW Ee,MAX = · = 28.33 (7) 50 cm 60 cm cm2 · So that the maximum value of the radiation dose does not cause a significant deterioration in the properties of the polymer filter material, it should be less than 100 J/cm2. The exposure time should therefore not be longer than

J mW TMAX = 100 / 28.33 3530 s (8) cm2 cm2

3.3. Validation of the Calculation of the Spatial Distribution In order to verify the correctness of the determined spatial distribution of the irradiation, a series of measurements were made in which the irradiation Ee,measured was measured at several selected points of the sterilizer’s working area. Due to the symmetry of the device structure and the symmetries of the determined spatial distribution, it was decided to take measurements in only a quarter of the working area. The locations of the selected points are shown in Figure9. The measurements were made with the HD2302 m with the LP 471 UVC probe manufactured by DeltaOhm. Considering all factors affecting measurement accuracy, the expanded uncertainty of measurement (at 0.95 confidence level) was around 10% of the measured value. The uncertainty of measurement of radiation intensity for the measuring system used during tests depends on the following factors: calibration accuracy, effectiveness of temperature compensation of indications, aging effect of the photosensitive element of the probe (which was important in the case of the measuring probe used), and accuracy of correction of the cosine sensitivity of the probe directional sensitivity. The results of the measurements are summarizedInt. in J. TableEnviron. Res.1. Public Health 2020, 17, x 8 of 15

P1 P2 P3 P4 P5 P6 P7

Figure 9.FigureLocation 9. Location of irradiation of irradiation measurement points points during duringvalidation. validation.

Table 1. ResultsTable 1. Results of irradiation of irradiation measurement measurement during during validation validation procedure. procedure.

Point Ee,calculated, ideal case Ee, measured Ee,calculated, worst case Point Ee,calculated,ideal case Ee,measured Ee,calculated,worst case mW / cm² P1 mW4.8 /cm2 3.70 P1 P2 4.64.8 3.35 3.70 P3 4.6 2.13 P2P4 28.33 4.4 4.6 3.68 3.35 P3P5 4.1 4.6 3.33 2.13 P4P6 28.33 3.8 4.4 3.42 3.68 P5P7 3.3 4.1 1.94 3.33 The measuredP6 radiation values for individual measuring 3.8 points P1–P7 3.42 are smaller than the maximum limit value calculated for the ideal reflection case, and higher than the value calculated for P7 3.3 1.94 direct radiation, i.e., for the complete absence of reflection. The measurement results show that the reflected radiation also reaches the surface on which sterilization is carried out. The amount of this The measuredradiation depends radiation primarily values on what for individualmaterial the sterilizer measuring housing points is made P1–P7 of. The are sterilizer smaller than the housing, assembled for the purposes of testing, was made of aluminum, because of its good maximum limitmechanical value properties, calculated resistance for to the ultraviolet ideal reﬂectionradiation, and case, good radiation and higher reflecting than capability. the value In calculated for direct radiation,this case, the i.e., measurements for the complete and theoretical absence values of (Table reﬂection. 1) show that The approximately measurement 30% of results the show that radiation intensity is increased by reflected and diffused radiation, thus lost radiation not directly illuminating the surface can be estimated at around 70%. As research shows [25], good results can be also achieved by placing aluminum foil on the inner walls of the sterilizer housing. Further development of the device structure may be directed at increasing the reflection coefficient inside the sterilizer.

3.4. Verification of the Effectiveness of Sterilization During the measurements verifying the effectiveness of sterilization, its outer casing was removed from the sterilizer, so that only direct radiation reached the sterilized test element. This configuration corresponded to the most unfavorable conditions of the sterilization process in which the intensity of radiation was limited. The minimum dose of radiation delivered to the system should be 1 J/cm2, which according to the calculations, required irradiation for at least 9 min. Considering the fact that the maximum time that will not destroy the filtration properties of the polymeric material is 55 min, the tests were carried out for 15 min so that each mask could be sterilized three times. It is clear that the components could be effectively sterilized in a shorter time as there are actually additional contributing factors, such as reflection of the radiation inside the housing or placing the sample in a well-lit place.

First, a control series was performed during which all steps of the verification procedure were performed, except that the lamps emitting UVC radiation were not turned on. During this control

Int. J. Environ. Res. Public Health 2020, 17, 7396 9 of 14 the reflected radiation also reaches the surface on which sterilization is carried out. The amount of this radiation depends primarily on what material the sterilizer housing is made of. The sterilizer housing, assembled for the purposes of testing, was made of aluminum, because of its good mechanical properties, resistance to ultraviolet radiation, and good radiation reflecting capability. In this case, the measurements and theoretical values (Table1) show that approximately 30% of the radiation intensity is increased by reflected and diffused radiation, thus lost radiation not directly illuminating the surface can be estimated at around 70%. As research shows [25], good results can be also achieved by placing aluminum foil on the inner walls of the sterilizer housing. Further development of the device structure may be directed at increasing the reflection coefficient inside the sterilizer.

3.4. Verification of the Effectiveness of Sterilization During the measurements verifying the effectiveness of sterilization, its outer casing was removed from the sterilizer, so that only direct radiation reached the sterilized test element. This configuration corresponded to the most unfavorable conditions of the sterilization process in which the intensity Int. J. Environ. Res. Public Health 2020, 17, x 9 of 15 of radiation was limited. The minimum dose of radiation delivered to the system should be 1 J/cm2, which accordingtest, no to sterilization the calculations, was performed. required All microbial irradiation tests for forthe controls at least showed 9 min. a positive Considering result. In the fact that all samples, growth of B. atrophaeus caused the medium to change color from red to yellow. the maximumUnambiguous time that results will notwere destroy obtained for the four filtration repetitions, properties where the in ofdicator the polymericstrips were placed material on is 55 min, the tests werethe carried surface outand under for 15 the min 1st and so that8th layers each of mask the mask. could The betests sterilized included fragments three times. of the mask It is clear that the components couldwith and be without effectively stiffening sterilized (Figure 10a). in a shorter time as there are actually additional contributing factors, such as reflectionThe main tests of were the radiationthen carried insideout in which the housingthe lamps emitting or placing UVC radiation the sample were turned in a well-lit place. First, a controlon, i.e., the series system was operating performed normally. during In the case which of sterilization all steps of ofthe theparts verification of the mask without procedure were the stiffener, the test for the presence of B. atrophaeus was negative. The color of the culture medium performed, exceptremained that red in the all combinations lamps emitting of the location UVC of radiation the indicator were strips, not i.e., on turned the surface on. and During under this control test, no sterilizationthe 1st, 2nd, was 3rd, 4th, performed. 5th, and 8th layer All of microbial the mask. Clear tests results for were the obtained controls for showedfour replicates a positive in result. In all samples,each growthcombination. of (FigureB. atrophaeus 10b). In the causedcase of ster theilization medium of mask fragments to change containing color a from stiffener, red to yellow. UVC radiation hardly penetrates through the stiffening of the mask. Indicator tests placed on the Unambiguoussurface results and wereunder the obtained 1st layer forshowed four a negative repetitions, result for where the presence the indicator of microorganisms strips were(Figure placed on the surface and under10b), but the the tests 1st andplaced 8th under layers the 2nd of stiffening the mask. and 3rd The layer tests of the included mask showed fragments a positive result of the mask with and without(Figure stiffening 10a). (Figure 10a).

(a) (b)

Figure 10. TheFigure indicator 10. The indicator papers papers with withB. B. atrophaeus atrophaeus sporsporeses (ATCC (ATCC 9372) placed 9372) in culture placed media, in after culture media, after incubation.incubation. Positive Positive result—microbial result—microbial growth, growth, (a). (Negativea). Negative result—no result—no microbes, microbes,effective effective sterilization (b). sterilization (b). 3.5. Laboratory Microscopy Tests of HEPA Filters The main tests were then carried out in which the lamps emitting UVC radiation were turned on, i.e., the systemThe filter was consisted operating of several normally. internal layers In (resembling the case thin of paper) sterilization and two outer of thelayers. parts After of the mask cutting out a filter fragment with scissors, the layers could easily be separated. Scanning electron without the stimicroscopyffener, the(SEM) test and for transmission the presence electron of microscopyB. atrophaeus (TEM) waswere negative.used to assess The the coloreffect of of the culture medium remainedradiation redon the in filter all combinations structure. SEM imaging of the was location performed of theon a indicatorFEI Helios NanoLab strips, i.e.,™ 600i on the surface and under themicroscope. 1st, 2nd, Fragments 3rd, 4th, of the 5th, filter and layers 8th (from layer the of outer the and mask. inner Clear parts) with results an area were of 5mm obtained x for four 5mm were glued to the microscope table with a conductive carbon tape. Samples were not further replicates in eachprepared combination. (they were not (Figure sputtered 10 withb). conductive In the case material). of sterilization In addition,of the mask morphology fragments of the containing a stiffener, UVCinner radiation part (single hardly layer) was penetrates observed in througha FEI Titan the80-300 sti TEMffening microscope. of the Before mask. being Indicator placed in tests placed on the surfacethe and microscope under column, the 1st a fragment layer showedof the filter alayer negative with an area result of about for 1 the mm presence x 1mm was ofsecurely microorganisms sandwiched between the two halves of the folding microscopy grid. (Figure 10b), but the tests placed under the 2nd stiffening and 3rd layer of the mask showed a positive Imaging results confirmed that the filter consists of entangled polymer fibers with free spaces result (Figure 10a). between them. In the outer layer (Figure 11a), the fibers have similar thicknesses (diameter about 30 μm). The inner layers (Figure 11b) are made of polymer fibers with heterogeneous diameters ranging from 100 nm to 10 μm. After irradiation, the morphology of the fibers did not change (Figure 11c and Figure 11d).

Int. J. Environ. Res. Public Health 2020, 17, 7396 10 of 14

3.5. Laboratory Microscopy Tests of HEPA Filters The filter consisted of several internal layers (resembling thin paper) and two outer layers. After cutting out a filter fragment with scissors, the layers could easily be separated. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to assess the effect of radiation on the filter structure. SEM imaging was performed on a FEI Helios NanoLab™ 600i microscope. Fragments of the filter layers (from the outer and inner parts) with an area of 5 mm 5 mm × were glued to the microscope table with a conductive carbon tape. Samples were not further prepared (they were not sputtered with conductive material). In addition, the morphology of the inner part (single layer) was observed in a FEI Titan 80-300 TEM microscope. Before being placed in the microscope column, a fragment of the filter layer with an area of about 1 mm 1 mm was securely sandwiched × between the two halves of the folding microscopy grid. Imaging results confirmed that the filter consists of entangled polymer fibers with free spaces between them. In the outer layer (Figure 11a), the fibers have similar thicknesses (diameter about Int. J. Environ. Res. Public Health 2020, 17, x 10 of 15 30 µm). The inner layers (Figure 11b) are made of polymer fibers with heterogeneous diameters ranging µ from 100 nm to 10 m. After irradiation, the morphology of the fibers did not change (Figure 11c,d).

(a) (b)

FigureFigure 11.11. SEM images of filters. ﬁlters. Outer Outer layer layer before before (a) (a) an andd after after (b) (b )sterilization. sterilization. Internal Internal layer layer before before (c(c))and and afterafter ((d)d) sterilization.sterilization.

TheThe radiationradiation usedused diddid notnot damagedamage the material structure. No No changes changes in in shape, shape, thickness thickness or or fiberfiber arrangementarrangement werewere observed.observed. In In addition, addition, th thee size size of of the the free free spaces spaces between between them them has has not not changedchanged significantly.significantly. Particular Particular attention attention was was paid paid to the to most the mostsensitive sensitive fibers with fibers a diameter with a diameter below below1 μm. 1Theµm. tests The confirmed tests confirmed that nanofibers that nanofibers are present are presentin material in material before (Figure before 12a) (Figure and 12aftera) and(Figure after (Figure12b) irradiation. 12b) irradiation.

(a) (b)

Int. J. Environ. Res. Public Health 2020, 17, x 10 of 15

(a) (b)

Figure 11. SEM images of filters. Outer layer before (a) and after (b) sterilization. Internal layer before (c) and after (d) sterilization.

The radiation used did not damage the material structure. No changes in shape, thickness or fiber arrangement were observed. In addition, the size of the free spaces between them has not changed significantly. Particular attention was paid to the most sensitive fibers with a diameter below Int.1 J. μ Environ.m. The Res. tests Public confirmed Health 2020 that, 17, nanofibers 7396 are present in material before (Figure 12a) and after 11 (Figure of 14 12b) irradiation.

(a) (b)

FigureFigure 12. 12.TEM TEM images images of theof the internal internal layer layer of theof the ﬁlter filter before before (a) (a) and and after after (b) (b) sterilization. sterilization.

4. Conclusions

This article presents a design of a simple sterilizer allowing the deactivation of pathogenic particles by the UVGI method. Its use is an alternative solution when it is not possible to replace filtering face masks, because it allows one to restore the filters for reuse. The designed device can also be used to sterilize a fragment of the floor, bed, table, or other objects potentially covered with pathogens. Taking into account the fact that sterilization is carried out due to the presence of a very dangerous pathogen, it is required that the effectiveness of the device for the sterilization of various objects should be confirmed by separate tests. The presented device can also be used for disinfecting other types of masks. In this case, it involves the use of holders dedicated to each type of mask, allowing the mask to be properly spread and stretched so that the entire surface of the mask can be exposed. As part of the work, theoretical simulations of surface irradiation by a set of radiators were performed. Based on them, the placement of radiators and chamber dimensions were proposed to obtain the appropriate radiation intensity on the working surface. The methodology given can be used to design a chamber of any size and any number of radiators. As recommended by virologists, the minimum dose of UVC radiation inactivating the SARS-COV-2 pathogen should be 1 J/cm2. Following these assumptions, a minimum exposure time has been set, which allows one to eliminate the pathogen, and a maximum time that will not destroy the filtration properties of the polymer material in masks. These times are 9 and 55 min for the designed chamber, respectively. The recommended sterilization time is 15 min, which gives the possibility of sterilizing one mask three times. The results of microbiological tests show that UVC radiation eliminates pathogens in all layers of the HEPA filter. Masks without stiffening or other impenetrable layers can be successfully sterilized using UVC radiation. The only limitation is the presence of a non-transparent stiffening on the face of the mask, which inhibits the penetration of radiation to the next layers. The problem may be solved by exposing the mask from both sides. Another possibility is to introduce an additional sterilization method. Possibilities are seen in the use of gaseous chlorine dioxide [26]. The authors have plans to apply for an implementation project, which would allow the implementation and beginning of production of portable sterilizers utilizing two combined methods: UVC and chemical. Measurements of radiation intensity in the chamber showed that the value of irradiation is greater by the value resulting from reflectivity and diffuse reflectivity of rays from the upper inner surface of the chamber and the side walls. This makes it possible to conclude that using the specified minimum exposure time you will be sure of the pathogen’s deactivation. Int. J. Environ. Res. Public Health 2020, 17, 7396 12 of 14

Based on these experiences, directions for further work were determined, including research on the phenomena of reflectivity and diffuse reflectivity of radiation in the chamber and the possibility of using LED UVC emitters, closed sterilization chamber design, and a sterilization process control system. In response to the demand reported to the authors of this article by the head of hospital infectious disease wards, a portable compact sterilizer using UVGI ultraviolet radiation was developed and manufactured. Use of this device requires some precautions. Personnel using the device must avoid prolonged exposure of exposed parts of the body, especially the face, to radiation produced in the device. This radiation can lead to burns or reduced vision [27,28].

5. Limitations During the tests, the effectiveness of sterilization was checked for only one type of mask, which was provided by the state material reserve agency to all infectious hospitals in Poland. Although, in the opinion of the authors, this type of mask is most representative of the various masks used in hospitals, additional testing of other types of masks should be performed. The sterilization efficiency was not tested for used masks; only new masks were used in the experiments. The studies did not carry out tests with different sterilization times. The sterilizer operation was checked always for a time of approximately 15 min. This time was assumed as the recommended sterilization time.

Author Contributions: Conceptualization, W.K. and A.C.; methodology, W.K., W.A., and A.C.; software, W.K.; validation, W.K., W.A., M.P., and A.C.; formal analysis, W.K., W.A., and B.W.; investigation, W.K., W.A., P.K., M.P., and B.W.; resources, W.K., P.K., and A.C.; data curation, W.K., W.A., and A.C.; writing—original draft preparation, W.K.; writing—review and editing, W.K., W.A., M.P., A.C., and B.W.; visualization, W.K., W.A., and P.K.; supervision, W.K. All authors have read and agreed to the published version of the manuscript. Funding: The works were supported by the National Centre for Research and Development, Poland, POIR/EFRR 2014-2020, Project No. 01.01.01-00-1104/17-00, and by the Implementation Doctorate Program—2nd edition at the Faculty of Energy and Environmental Engineering of the Silesian University of Technology, and from the funds of the Faculty of Automatic Control, Electronics and Computer Science of the Silesian University of Technology. Acknowledgments: The authors acknowledge the Heads of the Faculty of Automatic Control, Electronics and Computer Science of the Silesian University of Technology and the Heads of MEXEO Institute of Technology. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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