Aerosol and Air Quality Research, 12: 295–303, 2012 Copyright © Taiwan Association for Aerosol Research ISSN: 1680-8584 print / 2071-1409 online doi: 10.4209/aaqr.2011.11.0193 Microwave-irradiation-assisted HVAC Filtration for Inactivation of Viral Aerosols Myung-Heui Woo1, Adam Grippin2, Chang-Yu Wu1*, Joseph Wander3 1 University of Florida, Department of Environmental Engineering Sciences, Gainesville, FL 32611, USA 2 University of Florida, Department of Chemical Engineering, Gainesville, FL 32611, USA 3 Air Force Research Laboratory, Tyndall Air Force Base, FL 32403, USA ABSTRACT Inactivation of collected viral aerosols is important for preventing a filter medium’s serving as a fomite. The focus of this study was to evaluate the inactivation efficiency (IE) achieved through filtration coupled with microwave irradiation. MS2 aerosolized through a Collison nebulizer was fed into the system and collected onto the filter. For in-flight microwave decontamination, microwave irradiation was applied to an HVAC (heating, ventilation and air conditioning) filter supported on a SiC disk for three cycles of selected irradiation times per 10 min (i.e., 1, 2.5, 5, and 10 min/10 min) at power levels ranging from 125 W to 375 W. The survival fraction (SF) on the substrate and the IE through the entire system were investigated to determine the efficacy of this approach. SF decreased and IE increased as microwave power level was increased (p = 0.02 and p < 0.01, respectively) or the application time was extended (p = 0.03 and p < 0.01, respectively). Both measures changed sharply above a threshold temperature of around 90°C and reached 2 logs at 116 and 109°C, respectively. The log SF and IE of –2.59 and 3.62, respectively, were observed when the operating condition of 375 W for 10 min/cycle was used and the SiC disk facilitated microwave absorption. When a quartz frit was used as a support instead of the SiC disk, log inactivation efficiencies of 0.8, 1.0, and 1.3 were measured at relative humidities of 30%, 60% and 90%, respectively, under the same irradiation conditions. Relative humidity is a significant parameter from 50–80°C (p = 0.01). The results demonstrate that microwave-assisted filtration systems can be used as an effective means for inactivating viruses. Keywords: HVAC filter; Mask; Microwave; MS2; Inactivation efficiency; Survival fractionˁʳ ʳ INTRODUCTION inactivation. For example, distortion of membrane structure and function (Phelan et al., 1994), altered enzyme activity Microwaves—electromagnetic waves with frequencies (Dreyfuss and Chipley, 1980), disruption of weak bonds between 300 MHz and 300 GHz—are widely applied in food (Betti et al., 2004), increased release of various substances processing, wood drying, plastic and rubber treating, curing (Woo et al., 2000; Celandroni et al., 2004; Campanha et and preheating ceramics as well as in cleanup processes (Park al., 2007), and increased ionic strength due to an increased et al., 2006). Microwaves are non-ionizing but sufficient to current within cells (Watanabe et al., 2000) have all been cause the molecules in matter to vibrate, thereby causing reported. However, all of the aforementioned research was friction, which is subsequently transformed into heat for conducted in the liquid, solid, or aqueous phase. various applications. Among the diverse applications, the use In recent years, microwave inactivation of airborne of microwaves for decontamination was studied soon after microorganisms has gained more interest because of microwaves became available. Goldblith and Wang (1967) increasing concerns about health-related issues, including and Fujikawa et al. (1992) compared the effect of microwave outbreaks of pathogenic airborne viruses (e.g., SARS, H1N1 irradiation on Escherichia coli (E. coli) and Bacillus subtilis and swine flu). For examples, Hamid et al. (2001) measured (B. subtilis). They concluded that the heat produced was a 90% inactivation efficiency (IE) by applying microwave key factor for inactivating the bacteria in solid and aqueous irradiation to airborne bacteria and fungi at 600 W for four phases. Meanwhile, there has been research demonstrating periods of 2.5 min, each separated by 5 min from the next. additional effects, beyond the purely thermal mode of Elhafi et al. (2004) demonstrated that infectious bronchitis virus, avian pneumovirus, Newcastle disease virus and avian influenza virus were inactivated on dried swabs in less than 20 s at 1250 W. In another study, Wu and Yao (2010a) * Corresponding author. Tel.: 1-352-392-0845; reported IEs of 65% and 6% against airborne B. subtilis var Fax: 1-352-392-3076 niger spore and Pseudomonas fluorescens, respectively, in E-mail address: [email protected] an air stream after exposure to microwaves at 700 W for 296 Woo et al., Aerosol and Air Quality Research, 12: 295–303, 2012 2 min. Wu and Yao (2010b) showed gene mutation MATHERIALS AND METHOD through polymerase chain reaction-denaturing gradient gel electrophoresis after microwave application. Test Filters and Agent Other recent studies (Heimbuch et al., 2010; Zhang et Two commercial HVAC filters made of polyethylene al., 2010) have focused on microwave inactivation of (PE) and polypropylene (PP) (Filter 1; 3M) and synthetic contaminated filters. Although filters are effective devices polymer (Filter 2; True Blue) were selected as test filters, for capturing bioaerosols—utilized to reduce the spread of and glass microfiber LydAir MG (Filter 3; Lydall) was used infectious viruses, both in virtually all modern heating, for comparison. MS2 (ATCC® 15597-B1™) was applied ventilation and air conditioning (HVAC) systems and in as a test agent. It is a surrogate for enteroviruses such as filtering facepiece respirators (FFRs) at healthcare facilities rotavirus because of their similar structural properties and and by first responders— they are limited as a preventive resistance to heat and chemicals (Brion et al., 1999; Prescott method because they inactivate neither viruses that pass et al., 2006). Freeze-dried MS2 was suspended in DI water through the filter nor those that are captured. As some with a titer of around 108–109 plaque-forming units (PFU)/mL pathogens have a low infectious or lethal dose, viruses that as the virus stock suspension. penetrate or reaerosolize have the opportunity to infect people the filter was intended to protect (McCrumb, 1961). Experimental System Heimbuch et al. (2010) reported that microwave-generated A microwave oven (Panasonic, NN-T945SF, 2.45 GHz, steam at 1250 W for 2 min induced a 5-log IE for H1N1 virus continuous irradiation) with two one-inch holes in the back collected on FFRs. Zhang et al. (2010) demonstrated that was used in this study. Because common filter holders could microwave irradiation could provide an adequate method not survive in the microwave, a custom-made quartz filter for inactivating B. subtilis endospores and E. coli via a holder was placed inside the microwave. To support the microwave-assisted nanofibrous air filtration system. filter material and to enhance heat transfer, a SiC disk was Indoor air quality is strongly dependent on the HVAC employed inside the quartz reactor. system. If infectious viruses can be inactivated while they The experimental set-up for testing the inactivation of circulate through the HVAC system, the risk of spreading the virus is shown in Fig. 1. Six L/min of dry air was passed viruses can be reduced. However, no research has been through a six-jet Collison nebulizer (Model CN25, BGI Inc., conducted to evaluate the applicability of the microwave MA) to aerosolize the viruses. A second air stream passed inactivation technology to commercial HVAC filters even through the humidifier and then rejoined the flow. After the though these filters are commonly used in hospitals and combined flow passed through the mixing chamber, it was residential buildings for collective protection. Therefore, split three equal ways, whence each stream proceeded toward the objective of this study was to evaluate the inactivation the filtration unit at 4 L/min, corresponding to a face velocity performance of microwave-irradiation assistance to HVAC of 5.3 cm/s, which is a standard face velocity for ventilation filtration systems during in-flight filtration against MS2 system testing (U.S. Army, 1998). Of the three flows, two bacteriophage (MS2). Key parameters examined were were directed to filter holders outside the microwave, one microwave power level, microwave application time, and with and one without an HVAC filter, as controls. The relative humidity. The thermal stability of the filter media third was equipped with an HVAC filter 47 mm in diameter was also investigated. (effective diameter 40 mm for the quartz reactor used) inside Fig. 1. Experimental set up for microwave-irradiation-assisted filtration. Woo et al., Aerosol and Air Quality Research, 12: 295–303, 2012 297 the microwave oven. The filters inside and outside the Viral aerosols penetrating the test filters under microwave microwave oven were labeled A and B, respectively. The irradiation were collected in BioSamplers containing 15 mL BioSamplers downstream of the microwave/filtration system of DI water. The IE through the microwave/filtration system and non-irradiated filter were labeled C and D, respectively. was obtained by comparing the viable MS2 concentration The BioSampler downstream of the empty filter holder in the two BioSamplers: (control) was labeled E. C For in-flight microwave decontamination, microwave IE = E (2) irradiation was applied for three 10-min cycles that included CC selected periods of irradiation—1, 2.5, 5 and 10 min/10 min—at three different microwave power levels, 125, 250 where CC and CE are the concentrations of viable viruses and 375 W. To select the microwave application conditions, collected in the BioSamplers C and E, respectively. the thermal stability of three test filters was analyzed by The filtration efficiency of the filter itself (1–CD/CE) was thermogravimetric analysis and simultaneous differential used to confirm the stability of this system after each test.