The ongoing pandemic of SARS-CoV-2 (i.e. COVID-19) has severely stressed the worldwide healthcare system and has created unprecedented shortages of personal protective equipment (PPE) including N-95 filtering respirators (N-95s). Plasma sterilization is a known technique and works on viruses [1] . A factor of 30 reduction of human norovirus was obtained after a 5-minute treatment from a cold atmospheric-pressure air plasma by a FlatPlasTer 2.0™ device. The inactivation of the virus particles' functions occur through synergistic effects of the cold plasma-initiated air chemistry, which consists of nitric oxide (NO) (including its intermediates, NO radicals, NO -, and NO + , and adducts, NO 2 , NO 2 -, NO 3 , N 2 O 3 , N 2 O 4 , and ONOO -) and reactive oxygen species (including ozone, atomic oxygen, singlet oxygen, and oxygen ions). [1] In addition UV light was generated by this device. The main UV components of the emission spectrum (measured with an Avantes spectrometer, AvaSpec-2048) were in the wavelength range between 280 and 400 nm, which can be mainly attributed to the excitation of molecular nitrogen in air. That means, the UVC light was almost absent, whereas UVA and UVB were active plasma components. [2] Commercial plasma sterilization devices, such as the FlatPlasTer 2.0™, are not widely available and are costly. Few if any hospitals, nursing homes, or medical facilities have such a device. However, everyone has a microwave oven -often on every floor or even every single room. Microwave ovens have been used to make plasmas. There are numerous how-to videos using grapes on the internet [3] and scholarly articles explaining how they work [4] . Our lab specializes in using microwave sources to make atmospheric-pressure plasmas too for a variety of uses [5] .
The new idea described in this work is to create a plasma by making an antenna that has a length equal to some multiples of the wavelength of the microwaves in a microwave oven (some multiple of 12.2 cm) and bending it into a split ring with a small (2mm) gap, an intense electric field is generated in the gap. That electric field can ignite a plasma under the right conditions. Those conditions are created by adding a conducting liquid, such as saline solution, into the gap, and placing the antenna cap on a piece of ceramic. The conducting fluid electrically shorts the gap causing a large current to flow and heats the fluid quickly. The water is vaporized in a few seconds and a spark is generated in the gap. These first electrons then allow a plasma to be generated in the gap out of the air and other gasses and materials present.
This plasma is sustained as long as the microwave power is present. Plasmas generally generate UV light, but UV light alone is insufficient. Plasmas also produce radicals, particularly O radicals from the air and the water, and carbon-containing radicals from the ceramic surface.
Hydrogen peroxide (30%) is also added to the gap with the saline. This produces hydrogen peroxide vapor and OH radicals. Positive and negative ions and electrons are generated as well, which likely leads to ozone and NO compounds. All of these species are biologically active and will react away any mucus/saliva barriers and then destroy the cell walls of the biological contaminant.
The ability to turn a microwave oven into a decontamination device with common and some 30% hydrogen peroxide (at least 1ml or 1 tsp) in the antenna gap. 5) Turn the microwave on its highest power (normal operation) for 1 minute maximum. At first the liquid will start boiling. In about 15 seconds a popping sound and bright sparks will emanate from the gap. (Figure 1 , lower inset) Soon, a continuous plasma will form, and a loud buzzing sound will be heard. The continuous plasma should run for at least 30 seconds. Do not leave the mask in for more than 1 minute. If the mask is left in too long (about 2 minutes) the mask material around the metal nose piece may start to melt. A how-do video [6] is available which shows the complete process and includes additional safety tips.
Geobacillus stearothermophilus spore kits (EZS/5, Mesalabs) were used to determine the sporicidal efficacy of microwave plasma. The kit contained the bacterial spores (2྾10 6 CFU) on a piece of paper with culture medium in a separate glass ampule. The filter-paper was taken out of the kit and hung from the top of the microwave with three different distances (10, 20, and 30 cm) from the source of plasma (i.e., the antenna). After the decontamination treatment, the filterpaper was incubated at 60℃ for 24-hour with the culture medium. The sporicidal efficacy of >6 log 10 reduction of the experimental conditions was confirmed by the violet color being unchanged. Once the violet color of the culture medium changed to yellow, this meant the >6 log 10 reduction of spores was not achieved.
We used Tulane virus, which is in the calicivirus family and a surrogate for human norovirus, for testing. This virus is a mammalian virus within the same Baltimore classification as SARS-CoV-2 (i.e., (+)ssRNA). Tulane virus was received from Cincinnati Children's Hospital Medical Center. The MA104 cells were used as the host cells in virus propagation and plaque assay as described previously [7, 8] . 
Circular sections of the test respirator (2 circular sections of 47 mm diameter were cut out from each respirator) were loaded on to a filter holder (URG, Carrboro, NC, USA) and exposed to dried and charge-neutralized polydisperse 2% NaCl aerosols generated using a constant output atomizer (TSI Model 3076, MN, USA). The face velocity on the filter was maintained at 9.4 cm/s, which is equivalent to an effective flow rate of 85 lpm (NIOSH recommended most severe breathing flow rate) through the entire respirator area [surface area of the respirator was measured manually (~ 150 sq cm)]. NaCl particle number concentration was measured before and after connecting the test filter using a condensation particle counter (CPC, TSI Model 3022A; flow rate = 1.5 lpm). The particle filtration efficiency of the tested filter material was calculated using the equation below:
The filtration efficiency tests were performed twice on each of the respirator after 0,1, 3, 5 and 10 cycles of plasma exposure. Respirators exposed to microwave only, boiling 30% hydrogen peroxide and saline only were used as the controls.
The quantitative fit testing was performed at the University of Illinois following the modified ambient aerosol condensation nuclei counter quantitative fit testing protocol (1910.134 App A, OSHA). The N95 respirators which underwent 1, 3, 5, and 10 cycles of decontamination treatment were prepared.
A set of masks were also sent to the National Personal Protective Technology Laboratory run by the CDC for evaluation. Eleven respirators (3M model 1860) that were unworn and not subjected to any pathogenic microorganisms were submitted for evaluation. This included 7
respirators that were subjected to 3 cycles of the microwave plasma decontamination process and an additional 4 respirators that served as controls. The samples were tested using a modified version of the NIOSH Standard Test Procedure (STP) TEB-APR-STP-0059 to determine particulate filtration efficiency. The TSI, Inc. model 8130 using sodium chloride aerosol was used for the filtration evaluation. For the laboratory fit evaluation, a static manikin headform was used to quantify changes in manikin fit factor. The TSI, Inc. PortaCount® PRO+ 8038 in "N95
Enabled" mode was used for this evaluation. Additionally, tensile strength testing of the straps was performed to determine changes in strap integrity. The Instron® 5943 Tensile Tester was used for this evaluation.
Spectra were obtained from the plasma in the microwave oven by drilling a hole through the back of the oven and inserting an optical fiber. The fiber was connected to an Oceanoptics HR 2000+ ER Spectrometer and data between 200-1100 nm was taken at various intervals during the process. All decontamination experiments were conducted with a steel antenna made from a coat hanger, but to understand the origin of the spectral lies, additional spectra were taken when using an antenna made from Cu and Al.
The plasma generated by the microwave oven was effective to decontaminate both the spores and Tulane virus. As shown in Table 1 , the spore tests indicated that exposure to the plasma for 30 s inactivated the spores by >6-log 10 even in the far corner of the microwave oven (i.e., 30 cm away from the antenna). In the cases where the spore paper was hung in the same place, but a plasma was not generated (ie microwave's only), the 6-log 10 reduction was not achieved.
Tulane viruses inoculated both sides of the respirator were reduced by a factor of greater than 3-log 10 regardless of the inoculation sites. Neither microwave alone nor the microwave with hydrogen peroxide were able to inactivate greater than 3-log 10 reduction, which means the plasma generation was essential for the decontamination. Although the microwave and hydrogen peroxide are also known to inactivate the viruses, the treatment time seemed too short to attain the goal of 3-log 10 reduction.
The particle filtration efficiency of the respirator after multiple plasma exposure cycles are shown in Figure 2 . The initial particle filtration efficiency of the respirators before and after plasma exposure were very similar (i.e. ≥ 99 %). Furthermore, all the tested respirators passed the fit test after 1, 3 , 5 and 10 plasma exposure cycles. Collectively these results suggest that the decontamination of the N95 respirator by plasma exposure does not compromise the filtration efficiency or the fit of the N95 respirator. The filtration and fit tests done by the CDC's NPPTP lab confirmed these results. Tables 2, 3 and 4, show that there is no statistical difference between the masks which underwent 3 cycles of decontamination compared to the control samples. and 650 nm is likely due to CO and CO 2 excited states [9] . Every plasma heats the ceramic material under the gap to the point of becoming red hot. Since this is done in air, some combustion of the carbon-containing ceramic material is expected, and that may be the source for the carbon oxide compounds.
A few notes of caution need to accompany the description given here. First, if possible, one should dispose of contaminated masks and use new ones to assure optimal safety. If this procedure is used to decontaminate N-95 respirators, it is important that an actual plasma is made. It is quite obvious when the system goes into the continuous mode: A bright light is emitted, and a loud buzzing sound is made by the plasma. The antenna gets very hot -so hot that the ends glow. The material beneath the antenna gap also gets very hot and may glow red.
These surfaces should not be touched until they cool down. If plastic or other flammable material touches the red-hot glowing antenna or surface, it may catch on fire. This is likely why microwave manufacturers do not recommend that people put metal into microwave ovens. The metal gets hot, and if it is bent into the correct shape it can serve as an antenna and generate an atmospheric-pressure plasma as shown in this work. As shown by our results however, the plasma species generated are capable of decontamination in 30 seconds and anyone with a microwave oven and a few simple household items can convert it into a N-95 respirator decontamination unit for emergency use.