The limited availability of N95 respirators during the SARS-CoV-2 pandemic has forced many healthcare workers to reuse respirators designed for one-time use. The Center for Disease Control (CDC) identified ultraviolet germicidal irradiation (UVGI) as one of 4 most promising methods for N95 decontamination during a crisis capacity situation (1) .
Although the efficacy of UVGI for decontamination of other viruses, such as influenza, has been investigated (2) (3) (4) , very few studies directly evaluate UV-C mediated inactivation of SARS-CoV-2 on N95 respirators (5, 6) . Furthermore, most studies are performed using small, flat mask coupons that do not recapitulate angular incidence and shadowing effects caused by the 3D structure of the masks (7).
We created a UVGI device for N95 decontamination designed to address these factors via high levels of reflection and enable ease of use via straightforward fixturing and application. The decontamination chamber consists of a metal reflecting box containing high power, commercially available UV LEDs with driver circuitry on metal core printed PCBs mounted on the sidewalls. The LEDs are arrayed in a fashion to optimize exposure dose uniformity across the surface of an N95 respirator and were calibrated to deliver a minimum irradiance of 1 mW/cm 2 across all locations of the mask (see Appendix).
We tested this device for decontamination of SARS-CoV-2 on two masks models, 3M 1860 and 3M 8210. We analyzed decontamination of 5 different inoculated mask locations (center, top, bottom, right cheek and strap, Figure, panel A). Masks were exposed to UV-C for 0, 300 or 600 seconds. The minimal doses received at each location were greater than 300 and 600 mJ/cm 2 for the 300 and 600 second exposures, respectively. All rights reserved. No reuse allowed without permission. preprint (which was not certified by peer review) is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity.
The copyright holder for this this version posted November 28, 2020. ; https://doi.org/10.1101/2020.11.24.20237917 doi: medRxiv preprint Industry standards consider 3 log10 reductions as effective for decontamination. Both UV-C doses achieved a 5 log10 reduction in virus on the aluminum control coupons ( Figure, panel B and Table) , validating the efficacy of our UV-C device to eliminate SARS-CoV-2 on non-porous material. However, the 300 second exposure was insufficient for decontamination when averaging locations across the masks ( Figure, panel B and Table) . The 600 second exposure effectively decontaminated the 3M 1860 masks but failed to decontaminate 3M 8210 masks Table) . Notably, there was little difference between the 300 and 600 second doses on the 8210 masks regardless of location, suggesting that increased exposure time does not achieve higher levels of decontamination of this mask surface While the reduction averaged across the entire mask was greater than 3 log10 for the 3M Table) . Irradiation doses were calculated using a representative N95 mask with integrated irradiance sensors (Appendix). We determined that the smaller reduction in viral titer at the bottom location correlates with a lower irradiation dose received at this location. We hypothesize that this is due to the strap material and potential shadowing effects caused by twists in the strap during exposure to UV-C.
These results suggest that mask material is a major factor in the ability of UV-C to decontaminate an N95 respirator, and that the 1860 facepiece (hydrophobic polypropylene shell) All rights reserved. No reuse allowed without permission. preprint (which was not certified by peer review) is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity.
The copyright holder for this this version posted November 28, 2020. ; https://doi.org/10.1101/2020.11.24.20237917 doi: medRxiv preprint is more appropriate for UV-C decontamination than the 1860 strap (braided polyisoprene), the 8210 facepiece (polyester), or the 8210 straps (thermoplastic elastomers).
While UV-C is an attractive method for decontamination of PPE when applied at appropriate doses that do not compromise material integrity and device functionality, our findings suggest that efficacy for individual mask models should be evaluated for a given UV-C device. Our results as well as the recent study by Ozog et al. (6) indicate that while the facepieces of some mask models can be successfully decontaminated using UV-C, others are incompatible with this method of SARS-CoV-2 decontamination. Important factors to consider are the 3D structure of the mask and corresponding differences in irradiation dose received in some mask locations which can significantly influence the efficacy of decontamination. The straps may be particularly difficult to decontaminate and may require the use of a secondary method of decontamination for the straps in addition to UVGI (8). However, UVC LED technology is improving rapidly and future devices will offer higher irradiation levels, improving penetration of UVGI and/or shortening exposure times. The identification of existing N95 models that are most suited for UV-C based decontamination or the creation of new mask models for this purpose would be important milestones that could help mitigate future N95
shortages.
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The copyright holder for this this version posted November 28, 2020. ; https://doi.org/10.1101/2020.11.24.20237917 doi: medRxiv preprint preprint (which was not certified by peer review) is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity.
The copyright holder for this this version posted November 28, 2020. InfectionControl.tips. 2020;5:1-9. All rights reserved. No reuse allowed without permission. preprint (which was not certified by peer review) is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity.
The copyright holder for this this version posted November 28, 2020. ; https://doi.org/10.1101/2020.11.24.20237917 doi: medRxiv preprint inoculated with 50ul of 8e 7 TCID50/ml virus, applied as three aliquots of 16.7ul. Inoculated masks were allowed to dry for 3.5 hours at room temperature in a biosafety cabinet before masks were exposed to UV-C irradiation. UV tape was adhered to masks to confirm irradiation. preprint (which was not certified by peer review) is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity.
The copyright holder for this this version posted November 28, 2020. ; https://doi.org/10.1101/2020.11.24.20237917 doi: medRxiv preprint
UV-C dose measurements. The UVC LEDs were commercially available products from Bolb, Inc.: Surface Mount Type SMD6060, with peak emission wavelength of 272 nm, full-width-athalf-maximum of 9.5 nm, and outputs of 100 mW (250 mA) or 140 mW (350 mA) per LED.
Website: www.bolb.co. The LED emission is narrow, at a wavelength of 272 nm. The range of irradiance incidents upon the surface on 3M 1860 respirators was 1.5 to 3.0 mW/cm 2 . To account for non-uniformity across the surface of a respirator, the irradiance at each inoculation site was measured using a custom N95 respirator with calibrated sensors (Appendix Figure 1 ).
Virus preparation and stock titration: The SARS-CoV-2 strain used was USA-WA1/2020. Viral was assessed by TCID50 assay using Vero-E6 cells and was determined to be 8 x 10 7 TCID50/ml. All rights reserved. No reuse allowed without permission. preprint (which was not certified by peer review) is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity.
The copyright holder for this this version posted November 28, 2020. ; https://doi.org/10.1101/2020.11.24.20237917 doi: medRxiv preprint Mask inoculation: All mask locations (center, top, bottom, right, aluminum coupon and strap) were inoculated with 3 aliquots of 16.67ul, for a total of 50ul, of virus stock. Masks were left to dry for 3.5 hours in a biosafety cabinet. Straps of 3M 1860 masks were inoculated with 50ul of SARS-CoV-2 only 10 minutes before irradiation because optimization experiments showed that virus viability on this material decreased with excessive drying (unpublished data). Desiccation on mask facepieces for 3.5 hours did not significantly affect virus concentration (Appendix A picture of the mask was taken after irradiation to document UV tape change of color.
Virus titration: Inoculated regions of the mask were cut out using 12mm biopsy punches. Mask punches, strap pieces and aluminum coupons were incubated in 1.4ml (mask punches and aluminum coupons) or 2ml (strap pieces) of DMEM (Sigma-Aldrich) supplemented with 10% fetal bovine serum, 100 U/mL penicillin, and 100 μg/mL streptomycin for a minimum of 30 minutes. Virus was quantified by TCID50 assay by incubating Vero E6 cells in 96 well plates with 10-fold serial dilutions in 8-fold of incubation media. Five days after inoculation, cytopathic effect was scored visually, defined as any virus induced cell death or change in cell morphology and the TCID50 was calculated.
All rights reserved. No reuse allowed without permission. preprint (which was not certified by peer review) is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity.
The copyright holder for this this version posted November 28, 2020. ; https://doi.org/10.1101/2020.11.24.20237917 doi: medRxiv preprint