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Review
. 2021 Jul 20;120(14):2927-2942.
doi: 10.1016/j.bpj.2021.02.039. Epub 2021 Mar 4.

Principles and practice for SARS-CoV-2 decontamination of N95 masks with UV-C

Thomas Huber  1 , Olivia Goldman  2 , Alexander E Epstein  3 , Gianna Stella  4 , Thomas P Sakmar  5
Affiliations
Review

Principles and practice for SARS-CoV-2 decontamination of N95 masks with UV-C

Thomas Huber et al. Biophys J. .

Abstract

A mainstay of personal protective equipment during the coronavirus disease 2019 pandemic is the N95 filtering facepiece respirator. N95 respirators are commonly used to protect healthcare workers from respiratory pathogens, including the novel coronavirus severe acute respiratory syndrome coronavirus 2, and are increasingly employed by other frontline workers and the general public. Under routine circumstances, these masks are disposable, single-use items, but extended use and reuse practices have been broadly enacted to alleviate critical supply shortages during the coronavirus disease 2019 pandemic. Although extended-time single use presents a low risk of pathogen transfer, repeated donning and doffing of potentially contaminated masks presents increased risk of pathogen transfer. Therefore, efficient and safe decontamination methods for N95 masks are needed to reduce the risk of reuse and mitigate local supply shortages. Here, we review the available literature concerning use of germicidal ultraviolet-C (UV-C) light to decontaminate N95 masks. We propose a practical method for repeated point-of-use decontamination using commercially available UV-C cross-linker boxes from molecular biology laboratories to expose each side of the mask to 800-1200 mJ/cm2 of UV-C. We measure the dose that penetrated to the interior of the respirators and model the potential germicidal action on coronaviruses. Our experimental results, in combination with modeled data, suggest that such a UV-C treatment cycle should induce a >3-log-order reduction in viral bioburden on the surface of the respirators and a 2-log-order reduction throughout the interior. We find that a dose 50-fold greater does not impair filtration or fit of 3M 8210 N95 masks, indicating that decontamination can be performed repeatedly. As such, UV-C germicidal irradiation is a practical strategy for small-scale point-of-use decontamination of N95s.

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Figures

Figure 1
Figure 1
The multilayer sandwich anatomy of the N95 mask using the 3M Company 8210 mask as an example. (A) Environmental interface. (B) User interface. (C) From left to right: inner layer (shell), middle layers (filter 2 and filter 1), and outer layer (coverweb). (D) Light microscope images of the four layers, with lower row at fourfold higher magnification. To see this figure in color, go online.
Figure 2
Figure 2
Densitometric UV-C radiometry and transmittance of shell material. (A) Color change of UV intensity label stamps upon exposure with increasing UV-C dosage. (B) Exponential fit of the density of the stamps quantified as the mean inverted red channel intensity of the flatbed scanner image. (C) UV intensity labels exposed through the shell material of the 3M 8210 N95 mask for 60- and 180-min exposures in the UVP CL-1000 cross-linker. The incident exposure was quantified by integration of the UV-C meter reading sampled in 1-s intervals. The transmitted exposure is quantified using the calibration curve from (B). To see this figure in color, go online.
Figure 3
Figure 3
Optical transmittance of N95 mask materials and spectral sensitivity of viral RNA. (A) Colors of the visible spectrum. Wavelength ranges corresponding to different colors (R, red; O, orange; Y, yellow; G, green; B, blue; V, violet) and ultraviolet A, B, and C (UV-A, UV-B, UV-C) are shown. (B) Optical transmittance of the four layers of the 3M 8210 N95 mask (coverweb, filter 1, filter 2, and shell). The dotted line indicates the location of the strong 254-nm line of the spectrum of a low-pressure mercury-vapor lamp. (C) Germicidal action spectrum shows the wavelength-specific sensitivity of single-stranded RNA viruses. The sensitivity is calculated as the inactivation rate constants for viral infectivity normalized to the value at 254 nm, as determined for the MS2 bacteriophage (19). (D) Illustration of the five different experimental setups for transmittance measurement. Sample refers to the different mask materials. (E) Transmittance estimates for the four layers of the 3M 8210 mask using the five setups. The solid symbols correspond to our preferred transmittance values used in the models. Error bars correspond to the 95% CIs. To see this figure in color, go online.
Figure 4
Figure 4
UV-C dose-dependent inactivation of SARS-CoV. We fitted the original experimental data in Kariwa et al. (22,24) to a double exponential model.
Figure 5
Figure 5
Modeling of the UV-C dosage and virus viability in different layers of the N95 mask. (A) Local UV-C dosage as a function of the relative position inside the four layers of the N95 mask. The three different curves show the local UV dosage for three cases of illumination: illumination from the inside only (orange), illumination from the outside only (green), and illumination from both sides (black) with 1000 mJ/cm2 surface UV-C dosage. (B) Predicted local virus viability calculated from the local UV-C dosage distributions for the three cases. To see this figure in color, go online.
Figure 6
Figure 6
Modeling of the virus viability for three different models of UV-C sensitivity. (A) Predicted local viability calculated from the local UV-C dosage as a function of the relative position inside the four layers of the N95 mask illuminated from each side together with 1000 mJ/cm2 surface dosage. Model 1 (black) uses the empirical double-exponential model to describe the UV-C dose-dependent inactivation shown in Fig. 3. Model 2 (cyan) uses a UV-C sensitivity of 0.522 cm2/mJ, corresponding to the fast, initial virus inactivation process from Fig. 3. Model 3 (red) uses a UV-C sensitivity of 3.77 cm2/mJ, which describes the UV-C inactivation of MHV coronavirus aerosols. (B) Predicted total virus viability from integration of the local viability over all mask layers as a function of total surface dosage equally divided across both sides for the three models. To see this figure in color, go online.
Figure 7
Figure 7
Modeling of the UV-C dosage in different layers of the N95 mask including the effects of reflectivity. Local UV-C dosage is shown as a function of the position inside the four layers of the N95 mask. The three different curves show the local UV dosage for three cases of illumination: illumination from the inside only (orange), illumination from the outside only (green), and illumination from both sides (black) with 1000 mJ/cm2 surface UV-C dosage. The dotted lines indicate the UV-C dosage necessary to achieve log-3 and log-6 level inactivation of SARS-CoV-2 using recently published data (77). To see this figure in color, go online.
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