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Increased sanitization potency of hydrogen peroxide with synergistic O3 and intense pulsed light for non-woven polypropylene

Supplies of respiratory masks have recently become a concern due to the onset of the SARS-CoV-2 pandemic. Sanitization and reuse of masks can alleviate high mask consumption and production stresses. In the present work, improved sanitization potency of vaporous hydrogen peroxide (VHP) treatment of resilient bacterial spores while retaining polymeric filter performance was explored. A batch fumigation chamber with hydrogen peroxide (H2O2) vapor and ozone (O3) is featured, followed by intense pulsed light (IPL) flash treatments. A resilient bacterial indicator, Geobacillus stearothermophilus (G. stearothermophilus), was utilized to compare the efficacy of various H2O2 concentrations in combination with O3 and IPL. It was found that exposure to 30 minutes of 4.01 L min−1 0.03% H2O2 aqueous vapor and 3 g h−1 O3 followed by 10 IPL flashes per side completely inactivated G. stearothermophilus. The xenon sourced IPL irradiation was found to synergistically enhance radical production and strengthen the complementary biocidal interaction of H2O2 with O3. Due to the synergistic effects, H2O2 was able to sanitize at a diluted concentration of 0.03% H2O2. The physical properties, such as surface potential, tensile strength, hydrophobicity, and filtration efficiency of >300 nm saline water aerosol of fibrous polypropylene (PP) sheets, were maintained. In addition, no residue of sanitizers was detected, thus confirming the biosafety and applicability of this method to disposable masks. Performance was benchmarked and compared with commercially available processes. The synergistic regime was found to achieve sterilization of G. stearothermophilus at drastically reduced H2O2 concentrations and in ambient conditions relative to commercial methods.


Introduction
The coronavirus 2019  pandemic caused by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has resulted in a dramatic shi in everyday routine. Additional lawfully enforced measures such as travel restrictions, physical distancing, and mask-wearing guidelines have placed unique economic and societal stresses worldwide. Mask wearing has now become ubiquitous with normal life. These unprecedented measures are a necessary response to the over 98.2 million cumulative worldwide COVID-19 cases, of which around 3.2 million have died as of April 2021. 1 The World Health Organization (WHO) estimates that approximately 89 million medical-grade masks per month will be required to respond to the COVID-19 pandemic. 2 Due to the high demands for personal protective equipment (PPE) for high-risk persons (healthcare workers, elderly persons, persons with underlying health conditions), the public has been advised to utilize nonmedical grade masks.
The PPE recommendation from various institutions emphasizes primarily wearing a face cover designed to cover the mouth and nose. Several types of masks and face-shields have been deployed for this purpose. High-risk persons require high efficiency PPEs such as N95 grade masks to protect from continuous exposure or due to increased vulnerability. These respirators help prevent exposure to droplets suspended in the air that may contain SARS-CoV-2 and thus require a tting test to ensure tight sealing. Furthermore, these cannot be conventionally washed, which limits their disinfection avenues. To conserve higher grade PPEs, surgical or loose-tting disposable masks have been recommended to the public as these can be more easily fabricated. Popular materials for fabricating these masks also include cotton, silk, chitosan, and polypropylene (PP). A recent study evaluated the efficacy of various cottonbased masks and found cotton-silk as one of the more effective fabric combinations. 3 However, both N95 respirators and surgical masks are oen fabricated using PP bers. These PP based non-woven masks are suitable for mass production while retaining breathing comfortability due to a less dense (thinner) lter piece.
Sanitization and reuse of PPEs such as facepiece respirators are possible and outlined by various studies. [4][5][6][7][8] This would also minimize contaminated wastes designated for landlls. Filtration efficiency of polymeric masks is reliant on various mechanisms including electrostatic capture. 6 Electrostatically dependent or electret lters can achieve ltration of smaller particles while minimizing ber density. Electrostatic properties on PP based masks are necessary to effectively capture sub-300 nm droplets or even possibly the virus itself which has a mean size around 100 nm. 9 Therefore, sanitization methods for polymeric masks must preserve or regenerate the electrostatic potential while maintaining structural integrity. Three methods of sanitization are recommended by the Center for Disease Control (CDC): vaporous hydrogen peroxide (H 2 O 2 ) (VHP) treatment, ultraviolet germicidal irradiation (UGI), and moist heat treatment at 60 C. These methods have shown at least 99.99% antimicrobial efficacy against contaminants such as Geobacillus stearothermophilus (G. stearothermophilus) spores, Inuenza A (H1N1), SARS-CoV, and Avian inuenza A virus (H5N1). These treatment methods have also produced sanitized N95 grade lters with acceptable ltration efficiency post sanitization. Other high temperature-based sterilization methods, such as microwave or autoclaves, are not recommended as they can melt PP bers. The use of alcohol is also not recommended as alcohol treatment can strip electrostatic charge properties from electret-based lters.
However, the CDC advises the public not to sanitize and reuse loose-tting disposable masks. Most disposable masks are not intended for reuse as they cannot be deemed safe nor reliably resistant to sanitization methods. Household sanitization methods, such as soaking and washing, have previously shown a severe loss in ltration efficiency despite no physical damage to the bers. 6 Therefore, contactless treatments, such as UGI or heat, are recommended but require a lengthy treatment cycle. 6 UGI appropriate lamps are widely available for off-the-shelf use; however, the UGI processes can be lengthy and easily obstructed. Heated sanitization methods that require controlling humidity levels and steam treatment has been proven to inoculate various viruses, including the SARS-CoV-2. 10 For example, simple disinfection via microwave generated steam is available in most households. However, continuous steam treatment runs the risk of degenerating a masks surface charge. Ou et al. 6 and Pirker et al. 11 found ltration efficiency losses in electret-based masks aer 5 to 10 cycles of steam exposure for 30 min, which was attributed to the condensation of water droplets that can strip surface charge. Furthermore, Ou et al. 6 observed an unacceptable loss in ltration efficiency in surgical and procedure masks aer being immersed in isopropyl alcohol (IPA). 6 A shi in the most-penetrating particle size (MPPS) to a much larger size range aer IPA immersion indicated a loss in electrostatic charge-based action, which is a typical observation when alcohol and soap-based sanitization methods are used.
The group found acceptable ltration performance from masks that were sanitized by VHP, UGI, and thermal methods. However, certain limitations exist in industrial-scale sanitization. UGI treatment has limited throughput due to the required surface area exposure to lamp irradiation. Moist heat treatment requires precise control of humidity to avoid long treatment times. 6 H 2 O 2 vapour treatment requires consumption of H 2 O 2 as well as possible toxic residues le on the mask. In addition, quantitative direct surface electrostatic charge observations before and aer treatment of non-medical masks and their correlation to lter efficacy are lacking.
The efficacy of H 2 O 2 can be further improved by introducing a UV irradiation source that can provide the necessary energy to drive the breakdown of the H 2 O 2 molecules into biocidal reactive oxygen species (ROS). This method is commonly referred to as an advanced oxidation process and is typically used for wastewater treatment in which free radicals attack and mineralize contaminants such as sewage or pharmaceutical waste. 12 Alkawareek et al. 13 utilized a composition of H 2 O 2 and silver nanoparticles (AgNPs) with strong antibacterial effects at a concentration 100 times lower than the commonly used 3-6% for disinfecting surfaces. They attributed the increased antibacterial efficacy to a Fenton-like synergistic effect of H 2 O 2 with AgNPs. Escherichia coli and Staphylococcus aureus were used as model organisms with exposure to H 2 O 2 and AgNPs in a liquid medium. This signicantly improved sterilization; however, its application was mostly focused towards hard surface disinfection. 13 Antibacterial effects of H 2 O 2 were also studied in recent works by using G. stearothermophilus as a model organism. It was demonstrated that H 2 O 2 fogging was able to kill the spores; however, longer contact times ranging from 60 minutes to 200 minutes and higher concentration of H 2 O 2 ranging from 5% to 35% were required. 14,15 These conditions highlight the need for an efficient H 2 O 2 based sterilization approach to reduce both processing time and the concentration of H 2 O 2 . 15 In response to the above challenges, we aimed to develop a novel sanitization method that increases the potency of VHP while retaining lter performance aer treatment. PP based lter sheets was selected as the mask analogue. A simulated contaminant in the form of G. stearothermophilus was used to assess the biocidal potency. In addition, PP lter sheets can simulate potential changes in PP based masks aer exposure to various sanitization regimes. We chose G. stearothermophilus due to its extremophile classication, making it hard to sterilize thus resulting in its widespread use for sanitization benchmarking. 16 The goal is to produce an optimized sanitization condition that increases the potency of H 2 O 2 during VHP treatment while ensuring safe reuse of sanitized lters. Synergistic elements O 3 , and IPL were introduced to improve the potency of VHP sanitization. This novel combination was explored at decreasing aqueous H 2 O 2 concentrations. Sanitization efficacy and biosafety was evaluated by 24 hour incubation of treated spore discs and FT-IR analysis of PP sheets, respectively. Retention of PP ltration efficiency was evaluated with ltration tests of >300 nm saline droplets and electrostatic surface potential measurements before and aer sanitization. In addition, mechanical strength of the PP sheets before and aer treatment was quantied via tensile testing. By introducing synergistic elements O 3 and IPL, biocidal potency was improved; this facilitates lower H 2 O 2 consumption which allows scaling to larger operations or augmenting existing methods to reduce the sanitization time.

Sanitization experimental setup
G. stearothermophilus spore discs were subjected to various sanitization processes to optimize the synergetic biocidal effect of H 2 O 2 , O 3 , and IPL. Glass ber discs (9 mm in diameter) laden with G. stearothermophilus spores (2.1 Â 10 6 cells per disc) were used for this purpose (GFTS-6, Crosstex, Hauppauge, NY, USA). Fumigation and IPL treatment of indicator spores and PP sheets were performed in a custom 13.4 L capacity acrylic cylindrical chamber (24.5 cm in diameter and 28.5 cm in height), as shown in Fig. 1. The chamber features two perpendicular inlets for O 3 and H 2 O 2 vapour with an outlet to a condenser (Fig. 1). The H 2 O 2 vapour and O 3 were generated via a single jet 6.89 kPa collision nebulizer (CN241 Collision 1-Jet MRE type, BGI Inc, Waltham, MA, USA) and a 3 g h À1 O 3 generator, respectively. To observe the limits of sanitization efficacy with and without synergistic elements, 3% v/v H 2 O 2 was diluted to lower volume concentrations with distilled water prior to being fed to the nebulizer. Individual ow controllers (Omega FMA5520A, Omega Engineering, Norwalk, CT, USA) were used to maintain and measure the H 2 O 2 and O 3 ows. The H 2 O 2 ow was controlled at 4.01 L min À1 while O 3 was allowed to ow freely. The O 3 ow was measured at 5.50 L min À1 thus assuming performance at the rated 3 g h À1 indicates about 0.42% of the ow is O 3 by volume. The chamber was enclosed with a removable 32 mm thick glass cover, which supported a 4800 W s xenon ash lamp (Speedotron model 4803, Speedotron, Chicago, IL, USA) with a 6 ms irradiation pulse. The xenon ash lamp provided a broadband light source, characterized in a previous study, with wavelengths between 300 to 900 nm. 17

Sanitization exposure and spore incubation
The G. stearothermophilus-laden berglass spore discs and 0.4 mm thick PP lter sheets were treated with various Once fumigated, the chamber was evacuated, and the samples were placed 20 cm below the xenon lamp for IPL exposure. All the samples were handled with sterilized steel tweezers and placed with their radial faces perpendicular to the ashlight, irradiated, and then ipped for exposure on both sides. Aer the G. stearothermophilus spore-laden discs were exposed to various sanitization conditions, the spores were incubated to expose any surviving bacterium. The spore samples were immersed in tryptic soy broth (TSB) and incubated at 37 C for 24 hours. Incomplete inactivation of spores was evident from a visible increase in turbidity of the TSB aer incubation. Successful or total inactivation of spores aer treatment was evident from the retention of TSB clarity aer 24 hour incubation (Fig. 2).

Physical characterizations
Aer identifying the optimal sanitization regime that utilizes the minimum amount of H 2 O 2 , the effect of this optimized condition on PP lter sheets is examined. PP lter sheet's tensile strength, surface residue, and surface potential was examined before and aer exposure to the sanitization processes. The physical integrity of the PP samples was examined by performing tensile tests (ESM303, Mark-10, Copiague, NY, USA). 100 mm Â 15 mm rectangular PP sheet samples with average ber diameters of 15 mm (Fig. 3) were mounted on the Mark-10 tensile tester and subjected to a strain rate of 15.0 mm min À1 . As all sheet samples were prepared with the same cross-sectional dimensions, earlier (strain value) or lower yield stress was regarded as evidence of mechanical degradation.
For surface residue and surface potential experiments, 1 cm Â 3 cm PP sheets were cut and used. Attenuated total reection (ATR) FT-IR analysis was performed to identify any sanitizer residue le aer treatment. To observe changes in surface potential due to the sanitization treatment, a non-contact digital static eld meter (DESCO 19492, DESCO, Chino, CA, USA) was used to measure the static surface potential of the PP sheets.

Filtration efficiency test
The ltration efficiency of the PP samples before and aer sanitization treatment was evaluated using 300 nm to 3000 nm saline water aerosol. As shown in Fig. 4, the air from the laboratory compressed air supply was passed through a regulator set to 138 kPa. This regulator was used to feed both the ionizer (SMC IZN10, SMC Corporation, Tokyo, Japan) and nebulizer. The aerosol generated by the nebulizer was passed through a desiccant dryer before being introduced into the mixing chamber. To provide the required air volume for the particle counter (Kanomax 3905, Kanomax USA, Andover, NJ, USA), an additional regulator was used to supply air to the mixing chamber in combination with the air supplied by the ionizer and nebulizer. This supply line was also connected to open air to balance the inward and outward airow in the mixing chamber. Solenoid valves were used to maintain consistent sampling of air from upstream and downstream of the test sample.
These experimental characterizations evaluated the limits of biocidal efficacy for the synergistic VHP process. By introducing O 3 and IPL, sanitization of G. stearothermophilus spore discs at diluted H 2 O 2 concentrations can be achieved. Mask integrity tests were used to infer conservation of ltration efficacy and mechanical strength aer exposure to the process. Retention of biosafety and ltration of smaller particles was examined by utilizing ATR FT-IR and surface potential characterizations, respectively.

Sanitization with various conditions
Vaporous 3% H 2 O 2 aqueous solution was unable to sanitize the spore discs aer 30 min of 4.01 L min À1 fumigation while 24 h full immersion in aqueous 3% H 2 O 2 was able to fully  sanitize spore discs. However, full immersion or washing of polymeric lters is not advised as it can deteriorate the surface potential. Loss of surface potential will reduce the ltration efficiency of smaller particles. The mechanism in which washing the electret lters causes surface potential loss is further explored in Section 4.2. Additionally, the present O 3 generator rated at 3 g h À1 was unable to sanitize the spore discs on it own aer 30 min fumigation. This lack of biocidal performance is attributed to the high stability of gaseous O 3 and insufficient exposure time or the lack of humidity control. Furthermore, IPL exposure alone (10 ashes per side) was unable to sanitize the spore discs. The poor performance of the IPL treatment is attributed to the emission spectrum from the xenon light source that was characterized in a previous study. 17 The present xenon ash lamp lacks emission components in the strongest UV light biocidal range of 100-280 nm, known as the UVC range. UVC irradiation, when absorbed by cells or microbe's DNA or RNA, induces damage or mutation mediated by the generation of free radical species. Consequently, this affects the microorganism's ability to replicate. 18 The present IPL source lacks a signicant amount of the UVC component and is too brief to provide sufficient sanitizing action on the resilient spores on its own.  optimized synergistic condition presents a 67.8% decrease in H 2 O 2 consumption. Although not present in Table 1, the reduction of ash number was investigated. However, ash pulses lower than 10 per side resulted in incubation with 3% H 2 O 2 and O 3 . Thus, 10 ash pulses (6 ms) per side was kept standard in this study. The optimal sanitization condition of 4.01 L min À1 of 0.03% H 2 O 2 vapor and 3 g h À1 O 3 exposure for 30 minutes followed by ten ashes per substrate/sheet side will be referred to as the optimized condition/regime.

Filtration efficiency and surface potential
It is important to note that the saline droplet ltration test was performed to conrm the absence of signicant mechanical damage and loss in ltration efficiency of larger particles (>300 nm). Surface potential changes would incur a severe loss in ltration efficiency of particle sizes below the 300 nm range, which the present particle counter cannot detect. 6,11,19 Degeneration of the mechanical integrity of PP has not been observed in most vaporous hydrogen peroxide experiments. 20,21 PP has generally been considered compatible with H 2 O 2 , and serious mechanical degeneration of PP is only a concern with high heat or intense mechanical washing. Due to the heat generated from the IPL xenon-lamp, the masks must be evaluated for thermal damage. Triplicate experiments revealed PP sheets retained the ltration efficiency of 300 nm to 3000 nm saline droplets aer exposure to the optimized regime with deviations within measurement error (Fig. 5a).
Polypropylene ranks relatively negative in the triboelectric series and is likely to interact strongly with aqueous H 2 O 2 . This phenomenon was conrmed by comparing surface charge measurements before and aer full immersion of PP sheets in 3% aqueous H 2 O 2 of PP sheets. The PP samples soaked in 3% H 2 O 2 followed by 12 h of drying exhibited 74% decrease in surface potential magnitude (Fig. 5b). This contrasted with the 1.6% and 5% surface potential change on PP sheets treated with 30 min of 0.03% H 2 O 2 vapour and the optimized 0.03% H 2 O 2 vapor + O 3 + IPL condition, respectively. At a constant pressure and temperature, the dew point of a vaporous mixture of water and H 2 O 2 is dependent on H 2 O 2 concentration. At an increasing concentration of H 2 O 2 , the maximum water vapour saturation pressure will decrease and will facilitate condensation. By utilizing a dilute 0.03% aqueous H 2 O 2 vapours, potential condensation was minimized.
It should be noted that certain systems utilize condensation to improve the decontamination performance of vaporous H 2 O 2 . However, the biocidal enhancement of condensed H 2 O 2 is still debated and certain studies even suggest a dry  environment facilitates greater H 2 O 2 biocidal action. 22,23 It should also be noted that the static eld meter used for surface potential measurements only provide a potential value proportional to the actual surface potential. However, the static eld meter is still able to capture the polarity and relative magnitude changes in electrostatic potential of PP sample surfaces. Furthermore, despite precautions during handling, the initial magnitude of the sheets differs due to changes in surface potential during storage. In addition to taking precaution during handling to minimize affecting the surface potential, all experiments were performed in triplicate and sourced from the same roll of PP lter sheet.

Tensile strength and biosafety
Tensile strength tests were performed to examine any losses in mechanical strength aer sanitization (Fig. 6a). Triplicate tests found yield stresses of 1.75 AE 0.17 and 1.70 AE 0.25 MPa for optimized condition treated and untreated PP sheets, respectively; these values were observed at 3.03% and 3.97% strain for optimized condition treated and untreated PP sheets, respectively. The treated PP sample showed slight embrittlement evidenced by an increase of the modulus of elasticity compared to the non-treated PP sample. It should be noted that aer the yield stress, signicant tearing was observed. The strain region past the yield point was considered as a total failure of the PP sheet integrity. H 2 O 2 overall is required to be present at relatively high concentrations as well as in heated environments to cause any serious decay in PP integrity by oxidative processes. 24 This is also found to be true for both excessive O 3 and UV-light exposure. 11,25,26 However, the proposed room temperature sanitization method minimized physical damage to the PP bers.
To conrm the removal of any residual sanitizers, FT-IR spectra of non-woven PP sheets before and aer sanitization were analyzed for the presence of H 2 O 2 and O 3 species. The optimized sanitization condition of 0.03% H 2 O 2 vapour exposure and 3 g h À1 O 3 fumigation for 30 min was performed on PP sheets, with and without IPL irradiation. As shown in Fig. 6b, none of the PP sheet samples provided detectable amounts of H 2 O 2 and O 3 in their ATR FT-IR spectra, performed in triplicate.
The control untreated PP sheet exhibited strong peaks in the 3000-2800 cm À1 range attributed to CH 3 asymmetric and symmetric vibrations. In addition, distinct 1460 cm À1 and 1378 cm À1 peaks associated with CH 3 asymmetric and symmetric deformations were also present. 27 When normalized based on the PP control spectrum, neither bands or peaks associated with H 2 O 2 and O 3 were observed in any of the samples, including fully immersed and direct sprayed. This was further corroborated by the lack of any 3200-3150 cm À1 bands representing O-H bonds. In addition, the near identical spectra suggest little change in the chemical structure of the PP sheets. This was also indicated by an only minor embrittlement observed during tensile tests aer exposure to the optimized sanitization regime. This can be the result of the relatively short lifetime of H 2 O 2 and O 3 on surfaces before they decompose to benign products such as water and oxygen. 22,23 PP has also been reported to have limited interactions and good chemical compatibility with present sanitizers. This was especially evident in ambient temperatures and short exposure times. 25,28 The lack of interaction between the PP sheets and present aqueous sanitizers is further reduced by the high hydrophobicity of PP. To verify the minimal chemical and physical effects of the optimized regime, contact angle measurements were taken with distilled water droplets. Retention of hydrophobicity in PP sheets was observed by measuring their contact angles with static water droplets prior to and post sanitization. The initial contact angle was measured to be 146 while the contact angle post sanitization was measured to be 133 . This small decrease in the contact angle suggested that there may be a slight increase in the wetting effect but the overall hydrophobicity of the PP sheets was retained.  oxidizing nature due to the generation of the ROS which include superoxide ðO *À 2 Þ and hydroxide (OH*) radicals (Fig. 7). 29 The hydroxyl radicals can also be generated by the Fenton reaction. In the Fenton reaction, Fe 2+ in the presence of H 2 O 2 catalyzes the reaction to generate hydroxyl radicals (Fe 2+ being present in various amounts in cells bound to DNA). Previous studies have also suggested a time-dependent biocidal action of H 2 O 2 within cells comprising of two modes. In the rst mode, a low concentration of H 2 O 2 results in DNA damage to kill the cells. In the second mode, a high concentration of H 2 O 2 results in cell death by targeting other organelles. Within the cells, H 2 O 2 oxidizes several amino acid residues independent of the presence of metal ions. This causes modications of the protein side groups or, in some cases, complete cleavage of the protein backbone, leading to cell death. The ROS generated from H 2 O 2 also lead to cell membrane and cell lipids damage. Furthermore, Brandi et al. 30 found loss of cellular material was the cause of cell death when the bacteria cultures were treated with H 2 O 2 , indicating that H 2 O 2 disrupts the cell membrane.

Individual and synergistic sanitization mechanisms
O 3 exhibits oxidative properties, which has attracted its use for antibacterial applications. O 3 , oen generated by a corona discharge device or UV light, is relatively stable in the gaseous phase mixed with oxygen or air. By simply sourcing ambient air, O 3 has the potential to be a readily available sanitizer that can easily penetrate porous structures. [31][32][33] Typically, O 3 is not considered an effective sanitizer below 50% relative humidity. 32 When dissolved in water, O 3 decays to form biocidal hydroxyl radicals (Fig. 7). Alone in the gaseous phase, O 3 is hypothesized to damage the cell membrane by reacting with: proteins, respiratory enzymes, unsaturated fatty acids, peptidoglycans, nucleic acids, spore coats, and virus capsids. 34 Thus, the present sanitization chamber with no environmental humidity control did not effectively utilize O 3 as a sanitizer with 30 min of exposure.
Previous studies found the requirement of H 2 O 2 concentration in the presence of visible light to be several folds lower than the minimum inhibitory concentration of H 2 O 2 for various bacteria strains. 35 It was also demonstrated that the mechanism of action did not involve the generation of more hydroxyl radicals but involved an 'activation' of H 2 O 2 through light exposure. Physiological changes in the bacteria cells resulting from visible light exposure can also increase the vulnerability to lower concentrations of H 2 O 2 . Additionally, the photo-Fenton reaction was attributed to the improvement of the biocidal performance of H 2 O 2 in the presence of light. The H 2 O 2 interaction with intracellular Fe 2+ is enhanced under blue light exposure, leading to irrecoverable damage of several proteins, nucleic acids, and various organelles. 35 This was attributed to advanced oxidation and the peroxide process, which enhances the production of reactive hydroxyl radicals and trioxidane (H 2 O 3 ), a potent biocidal agent. 8,36 Photolytic mechanisms of H 2 O 3 production in the presence of H 2 O 2 and O 3 were previously studied by isolation in argon matrices. Further analysis of H 2 O 3 production in a peroxide system by Xu et al. 37 indicates an initial [(HOO)(HOOO)-7r] complex formation. In the presence of irradiation such as IPL, [(HOO)(HOOO)-7r] produces [(HOO)(HO)] complex which rapidly close to form H 2 O 3 . As stated earlier, H 2 O 2 showed synergistic behavior with blue light exposure to result in a higher biocidal efficacy. The advanced oxidation process involving a combination of H 2 O 2 , O 3 , and light exposure, however, has not been studied in-depth in the context of sanitization. A few studies, however, have used a H 2 O 2 /O 3 /UV exposure system for chemical oxidation of municipal landll leachate and oxidation of organic compounds. 38,39 When UV exposure was introduced, the decomposition of O 3 was accelerated, and H 2 O 2 was formed as an intermediate species. In parallel, H 2 O 2 decomposed and produced hydroxyl radical species, which were further activated by light exposure. Both reactions occurred simultaneously in the advanced oxidation process using a O 3 /H 2 O 2 /light exposure system. The presence of H 2 O 2 drove the UV-assisted O 3 decay process, resulting in a higher and more rapid yield of hydroxyl radical species. These species, in turn, exhibited biocidal characteristics. It was evident from these investigations that the advanced oxidation processes involving O 3 /UV exposure and H 2 O 2 /light exposure enhances biocidal activity.

Filtration mechanism and physical properties
The ltration mechanism of various brous polymeric lters utilizes electrostatic forces. The bers on these lters hold an electrostatic charge which allows them to capture particles smaller than the lter pore size (typically sub-300 nm). This allows for effective but relatively thin lters that can retain user comfort and breathability. 11,19 Previous studies have reported signicant losses in ltration efficiency attributed to a loss in surface charge while visible mechanical damage was not present. 6 Loss in electrostatic charge during vaporous treatment or liquid washing can be attributed to the triboelectric effect. When water or sanitizer droplets condense, they carry and eventually remove a large magnitude of surface charge (see Fig. 8).
Although PP ber density is kept at a minimum by utilizing electrostatic mechanisms, ber density is still crucial in capturing larger particles or droplets. Mechanical degradation compromises both the ltration efficiency and resistance against stresses from regular handling and wear. The present sanitizing agents can damage the mechanical integrity of PP in a variety of ways. High concentration of O 3 has been found to attack and degrade the polymeric backbone of PP. 26 Brief exposure of H 2 O 2 at diluted concentrations can minimize the rate of oxidative attack on PP. In addition, IPL exposure specically in the biocidal UVC range has been found to have little affect on the mechanical properties of PP. 11 However, PP is susceptible to damage from heat which the broadband ashes do provide. Although brief, the introduction of heat can not only thermally damage PP but catalyze and intensify the oxidative attack of H 2 O 2 on PP. 24 For the reuse of polymeric-based masks using the presented novel sanitization process, the lters must also be biosafe.

Current limitations and future prospects
The optimised sanitization condition of 30 min exposure of 4.01 L min À1 0.03% H 2 O 2 and 3 g h À1 O 3 followed by 10 IPL ashes per side was able to fully sterilize G. stearothermophilus spore discs. All aspects crucial to polymeric lter performance such as: electrostatic charge, biosafety, mechanical strength, hydrophobicity, and ltration efficiency were retained. It must be noted that although the optimized sanitization regime was successful on PP lter sheets, a different interaction may occur on other mask components such as disposable mask ear loops and metallic or plastic nose clips. Previous studies have found degeneration of ear loops' wearability aer 20 to 50 cycles of sanitization. 20,21,43 However, the present H 2 O 2 concentration is not only reduced by 67.7.% relative to commercial products 15 but processed at ambient conditions for a short duration. These conditions are not expected to cause signicant damage to other mask components.
For future investigations, the effect of the present sanitization condition on other mask components such as ear loops and nose clips should be characterized. In addition, much of the surface potential of electret-based lters can be lost during normal usage; in consideration of this, an additional re-charging stage can be implemented, such as exposure to corona discharge.
Although ltration efficiency of sub-300 nm particles was inferred by surface potential, proper ltration efficiency tests on commercial facemasks (surgical, N95 grade, and dense cartridge based) is recommended. In addition, the current sanitization condition can be applied to disinfect a multitude of surfaces. Although O 3 generation may prove cumbersome, backpack or handheld nebulization of H 2 O 2 is currently utilized to disinfect high-touch surfaces. A simple modication of the process by incorporating a ash lamp can enhance the biocidal effect of H 2 O 2 ; this modication will also extend existing supplies of aqueous H 2 O 2 as the synergistic regime achieves potent disinfection at diluted H 2 O 2 concentrations.
Ideally, the present study should have incorporated viral samples, especially the SARS-CoV-2 virus. However, due to regulatory and time limitations, commercially available G. stearothermophilus was utilized as a model organism. These spore discs have been previously utilized to test the biocidal efficacy of commercial sanitization equipment as well as other vaporous H 2 O 2 processes. In addition, IPL exposure requires surfaces free from obstruction. The disposable masks will have to be unfolded and properly oriented to achieve optimal IPL surface exposure. Although these limitations of IPL exposure affect scalability, the ashing regime only takes a few seconds. In comparison, the Battelle Critical Care Decontamination System™ (CCDS™) is a proprietary N95 mask decontamination service; this process was quoted to achieve decontamination of 80 000 masks per day using VHP exposure for 2.5 h 44 It is important to note that the proprietary CCDS™ VHP process parameters were not disclosed. However, if assuming the methodology is similar to the Battelle report on VHP decontamination for N95 masks (where G. stearothermophilus was also utilized), comparisons can be made. 20 The optimized VHP process in the report quotes a 20 min gassing phase at 2 g min À1 H 2 O 2 followed by 150 min of dwell time at 0.5 g min À1 H 2 O 2 . Thus, the optimized synergistic sanitization method presents a potential 68.6% reduction in H 2 O 2 consumption relative to the Battelle CCDS™ process. In addition, the novel synergistic sanitization regime achieved decontamination at 30 min of fumigation followed by a few seconds of IPL irradiation. The novel synergistic also did not require dehumidication prior to sanitization nor post sanitization drying.

Conclusions
In the present study, a novel sanitization method was able to increase the sanitization potency of VHP treatment on polymeric-based lters such as PP. Aer exposure to the optimized sanitization regime, little to no change in important performance metrics was observed on PP lter sheets including: saline droplet ltration efficiency, surface potential, tensile strength, and biosafety. By introducing 3 g h À1 O 3 ow during VHP fumigation followed by 10 IPL ashes (on both disc faces), complete inactivation of G. stearothermophilus spore discs was achieved. Initial tests showed nebulized 3% H 2 O 2 aqueous solution was unable to sanitize the spore discs; however, introduction of O 3 and IPL synergistic elements achieved inactivation of indicator spores at aqueous H 2 O 2 feed concentrations as low as 0.03%. The increased sanitization strength was attributed to: introduction of biocidal species H 2 O 3 ; improved H 2 O 2 biocidal performance via the photo-Fenton reaction; and the increased population of previously present biocidal ROS via advanced oxidation mechanisms. Aer exposure to the optimized regime, no severe loss of 300 nm to 3000 nm net neutral saline water aerosol droplets was observed. A small 5% loss in magnitude of surface potential was observed (preserving surface potential is critical for the capture of sub-300 nm particles). Triplicate tensile tests revealed slight embrittlement as yield strain decreased from 3.97% (yield stress: 1.70 AE 0.25 MPa) to 3.03% (yield stress: 1.75 AE 0.17 MPa) before and aer treatment, respectively. In addition, ATR FT-IR characterization revealed near identical spectra before and aer sanitization indicating biosafety and no signicant structural changes. In comparison to the commercially available Bioquell BQ-50, the optimized regime required 67.8% less H 2 O 2 to sanitize the indicator spores. The potent synergistic effect provides a pathway to sanitize and re-use polymeric based lters with minimal H 2 O 2 consumption. With no active temperature or humidity control requirements, the optimized sanitization regime has competitive potential for scalability.

Author contributions
All authors contributed to the writing of the paper. Authors Robin Jeong and Hitendra Kumar optimized the sanitization regime and performed mechanical characterizations; in addition, these authors explored the underlying mechanisms and reported them in the present paper. Author Steven Jones performed provided consultation regarding indicator spore choice as well as incubation methodology. Dr Jones performed incubation of the spore discs aer sanitization in the Zymetrix facilities at University of Calgary. Author Allen Sandwell constructed and developed the methodology of the sanitization chamber and ltration efficiency setups. Authors Keekyoung Kim and Simon S. Park rst conceptualized the project and provided continued supervision and writing editing/review.

Conflicts of interest
The authors have no conicts of interest to declare.