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Technical Report for UV-C-Based N95 Reuse Risk Management
Some of the available literature on decontamination of N95 FFRs reviewed in this document is a result ofrecent efforts to relieve the shortage of N95 FFRs during the SARS-CoV-2 outbreak. Thus, some recent
research papers cited in this document are not yet peer reviewed. ​For clarity, wherever
non-peer-reviewed research results are cited in this report, the citation is preceded by a “*”​.

Summary of Updates in v2.0 Report: ​Added literature review and discussion on efficacy

of UV-C inactivation of other organisms (​Appendix A​) and on efficacy of sunlight on
decontamination (​Appendix B​). Added warnings on inappropriate UV sources (sunlight, tanning
bed lamps, counterfeit UV-C sources, ozone-generating lamps). Emphasized necessity of
UV-C irradiance measurements with a calibrated, NIST-traceable, UV-C-specific sensor.
Distinguished peer-reviewed from non-peer-reviewed references (see note above).

Executive Summary
Ultraviolet germicidal irradiation (UVGI) inactivates pathogens by damaging genomic
material. UVGI has been widely applied for air, water, and surface decontamination. Recently,
UVGI has been identified by the Centers for Disease Control (CDC) as one of the mostpromising methods for N95 filtering facepiece respirator (N95 FFR; also colloquially referred toas ‘N95 masks’) decontamination, and a workflow for UVGI-based decontamination was
successfully implemented at the University of Nebraska Medical Center (UNMC), among otherlocations. Pathogen inactivation depends critically on ultraviolet (UV) wavelength (peakinactivation efficacy with ~260 nm UV-C light) and UV-C dose. It is essential to use sensors(radiometers or sensor strips with sensitivity at 254 nm and appropriate dynamic range) tovalidate that the marginally acceptable dose is reached within the treatment period. UVsources emitting at wavelengths much beyond 260 nm, such as sunlight and tanning bed
lamps, have minimal or no germicidal efficacy.
We find in the literature that a ​UV-C irradiation dose of ≥1.0 J/cm​2​ at the FFR surface
inactivates SARS-CoV-2 analogues (≥3-log reduction) on the majority of tested N95
facepieces​. However, the literature also presents evidence that (i) inner FFR layers and/or
certain FFR models may not receive a high enough dose as light transmittance varies among
FFR models, (ii) FFR straps present a residual contamination risk and thus require a secondary
decontamination method, (iii) it is challenging to ensure that all surfaces/layers are completely
decontaminated due to shadowing effects, and (iv) higher doses may be necessary toinactivate other pathogens (especially bacterial spores). We conclude that UVGI protocols
should be implemented only if there is a dire shortage of N95 FFRs and approval to do so. We
also note that re-use of any N95 FFR may impact FFR fit, and we stress that a user seal checkshould be performed after every re-donning. We also stress that FFRs may be contaminatedwith pathogens other than SARS-CoV-2, and not all of these pathogens may have ≥3-log
inactivation with the suggested workflows present in this report. Any decontamination
approach should be accompanied by an industrial hygiene workflow involving user training andsterile processing as well as compliance with Food & Drug Administration (FDA) andOccupational Safety & Health Administration (OSHA) guidelines.

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1. Overview
Our overarching goal is to expedite access to consolidated information on N95 filtering
facepiece respirator (FFR) decontamination approaches for healthcare workers who are thefrontline against the novel coronavirus (SARS-CoV-2) and essential to maintaining a robust
response to the Coronavirus disease 2019 (COVID-19). In this document, we review ultraviolet
germicidal irradiation (UVGI) N95 FFR treatment, as discussed in the literature. Effective
decontamination requires inactivation of the SARS-CoV-2 virus and maintenance of both the fit
and filtration efficiency of the N95 FFR while minimizing the risk of cross-contamination.
Upper-room and in-duct UVGI has been applied in hospitals to inactivate airborne
pathogens, as a supplement to High Efficiency Particulate Air (HEPA) filtering (​Sehulster et al.,2004​). ​UVGI efficacy is critically dependent on UV wavelength (peak efficacy with UV-C
light ~260 nm) and UV-C dose (J/cm​2​).​ Dose (J/cm​2​) is the product of irradiance (W/cm​2​) and
exposure time (s). Because UV-C irradiance is dependent on the distance and angle from a
UV-C source, characterizing UV-C irradiance at each FFR location using UV-C sensors isneeded (radiometers or sensor strips with sensitivity at 254 nm and appropriate dynamic
range). Measured irradiance can then be used to calculate necessary exposure time to achievea marginally acceptable dose of 1.0 J/cm​2​. Due to limited UV-C transmission through N95
FFRs, both sides of the FFR should be illuminated, and the marginally-acceptable UV-C dose
may not effectively decontaminate all FFR models.
We find in the literature that a UV-C irradiation dose of ≥1.0 J/cm​2​ at 254 nm peakwavelength inactivates SARS-CoV-2 analogues (≥3-log reduction) on the majority of tested
N95 facepieces, although straps require a secondary decontamination method. At this UV-C
dose, N95 FFR fit and filtration performance are not anticipated to be altered for at least 10
cycles ​(​*​Heimbuch & Harnish, 2019​). Repeated donning/doffing may have a larger detrimental
effect on N95 integrity: for some N95 models, fit was found to fall below OSHA standards after
5 don/doff cycles, while others maintained fit for >15 don/doff cycles (​Bergman et al., 2012​).
Based on the results from other enveloped, ssRNA viruses, it is likely that this UV-C
dose inactivates SARS-CoV-2; however, this has not yet been confirmed directly with
SARS-CoV-2 in the peer-reviewed literature as of 4/22/2020. UV-C has been found toinactivate other pathogens (nonenveloped viruses, vegetative bacteria, and bacterial spores) on
FFRs, although in many cases ≥3-log reduction necessitated higher UV-C doses or was notachieved with the doses used in the study. While UVGI treatment is expected to significantly
reduce the risk of contamination, healthcare personnel should continue to handle the respirator
as if contaminated and reuse only their own FFR. Any decontamination approach should beaccompanied by an industrial hygiene workflow involving user training and sterile processing tominimize risk of cross-contamination.
A workflow for UVGI-based decontamination was successfully implemented at the
University of Nebraska Medical Center (UNMC), with a throughput of 90 FFRs/cycle (​Lowe etal., 2020​), and several other medical centers around the United States are developing similar
UV-C N95 decontamination systems.

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2. Status of Federal Guidance
In this unprecedented COVID-19 pandemic, due to a limited supply of N95 FFRs, theCenters for Disease Control and Prevention (CDC) have provided guidance that healthcare
workers can practice extended use or limited reuse of N95 FFRs (​CDC, 2020b​). In addition, the
CDC has provided guidance to hospitals on methods for decontaminating N95 FFRs during acrisis (​CDC, 2020c​).
The Occupational Safety and Health Administration (OSHA) states that cosmetics orother barriers should not be present during respirator use (​OSHA, n.d.​). Emergency use
authorizations (EUAs) that the FDA has granted for N95 FFR decontamination during the
COVID-19 pandemic also stipulate that cosmetics not be present on respirators sent fordecontamination (​Battelle, 2020​).
After decontamination, the CDC recommends that a ‘user seal check’ is performed
when the respirator is donned to ensure adequate seal (​CDC, 2020c​). A user seal check after
every decontamination cycle is especially important because there is evidence that the fit
factor of N95 respirators decreases with numerous don/doffs (​Bergman et al., 2012​).
Per FDA guidelines for N95 FFR decontamination EUAs, virucidal decontamination
requires ≥ 3-log reduction (corresponding to a 99.9% reduction) in viral activity (​FDA, 2020​).
Based on this guideline, we describe a process as sufficiently “decontaminating” or“inactivating” only when it leads to a ≥ 3-log reduction in viral activity. ​Note that our definitionof decontamination in this report, unless otherwise specified, only considers virucidal
activity and does ​not ​consider mycobactericidal or sporicidal activity​, for which the FDA
has other guidelines (​FDA, 2020​).​ N95 FFR decontamination processes for SARS-CoV-2 do
not necessarily result in sterilization (killing of all microorganisms) of the N95 FFR​.
UVGI treatment was identified by the CDC as one of the most promising methods fortreatment of N95 respirators under crisis conditions (​CDC, 2020c​); in this document we offer asummary of the evidence on UV-C decontamination of N95 FFRs. UV-C decontamination is
also in broader use: per the recommendations of the CDC and the Healthcare Infection Control
Practices Advisory Committee (HICPAC), UVGI using UV-C light (254 nm peak) is widely used
in US healthcare facilities for pathogen reduction in air (​Sehulster et al., 2004​). In somesettings, UVGI is also used for surface decontamination (​Marra et al., 2018​). ​The National
Institute for Occupational Safety and Health​ (NIOSH) and the CDC offer guidelines for applying
upper-room UVGI to kill or inactivate airborne tuberculosis bacteria in hospitals (​CDC, 2014​).
Any new methods for decontamination should be verified through organizations’internal processes, which may include FDA clearance, prior to implementation. Please refer tocurrent CDC guidelines that are updated regularly, as well as ​N95DECON’s Full Legal
Disclaimer​.

3. UVGI Mode of Action & Appropriate Dosing in N95 FFRs
UVGI inactivates pathogens primarily by damaging DNA and RNA (max UV absorption
at 260 nm) (​Anderson et al., 2000; Ito & Ito, 1986; Jay, 1995; Kowalski, 2009​). Decontaminationis critically dependent on application of the appropriate UV ​wavelength​ (UV-C, with highefficacy near 260 nm (​EPA, 2006​)) and ​dose (​ ≥ 1.0 J/cm​2​ for inactivation of SARS-CoV-2
analogues in N95 FFRs). UV-C light is attenuated as it passes through the N95 FFR layers,

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resulting in UV-C irradiance values at the internal filtering medium that are ~3-400x lower than
the irradiance at the FFR surface, depending on FFR model (​Fisher & Shaffer, 2011​). A recent
non-peer-reviewed preprint (​*Syphers, 2020​) reports similar levels of UV-C transmission
through N95 FFRs as was measured by Fisher & Shaffer. As a result, the required UV-C doseat the N95 surface for viral inactivation from N95 FFRs is several hundred-fold greater than thedose required for inactivation of these viruses on surfaces or in air (​Table S1​). An ASTM
standard UVGI method for inactivating influenza virus on textile surfaces is being balloted.
Shadowing also reduces the dose that a target receives, and therefore shadows on thetarget N95 FFR(s) should be avoided by: (1) providing UV-C illumination from both sides of theFFR, and/or flipping the N95 FFRs mid-treatment to ensure all surfaces are exposed to themarginally-acceptable UV-C dose, (2) lining walls, ceiling, and other surfaces with
UV-C-reflective materials to increase delivered UV-C dose (​Rutala et al., 2014​), and (3) ensuring
there are no obstructions or materials between the N95 FFRs and the UV-C source that could
block the line-of-sight or attenuate the UV-C before reaching the N95. It is important to notethat glass blocks almost all UV-C light (​International Ultraviolet Association, n.d.​).
In addition to shadowing, materials deposited on the respirator from the skin of theuser, like cosmetics and sunscreen, may also block UV-C light, hindering UV-C
decontamination. Thus, such skin products should not be worn by users. OSHA also statesthat cosmetics or other barriers not be present during regular respirator use (​OSHA, n.d.​). As isadvisable with N95 FFR treatment for reuse, UV-C is viewed as risk mitigation for extraordinary
circumstances rather than complete decontamination. Healthcare personnel are advised toapproach reuse of N95 FFRs as if the treated N95 FFR is contaminated but with mitigated risk.

4. Potential for SARS-CoV-2 Inactivation
Several studies have demonstrated UV-C inactivation of influenza and coronaviruses in
N95 FFRs. Influenza and coronaviruses are hypothesized to be suitable SARS-CoV-2
analogues because they are also enveloped, single-stranded RNA viruses. A
non-peer-reviewed report to the FDA by the contracting research laboratory ARA ​(​*​Heimbuch &Harnish, 2019​) found that UV-C treatment of 1.0 J/cm​2​ at the surface of N95 FFR coupons fromone FFR model yielded no detectable virus (≥3.95-log reduction) for six influenza andcoronavirus strains considered, including MERS-CoV and SARS-CoV. When viral inoculations
were covered with artificial soiling agents (skin oil or saliva), N95 coupons also yielded no
detectable virus after UV-C treatment. Similar UVGI doses were effective for H5N1 and H1N1
in separate, peer-reviewed studies (​Heimbuch et al., 2011; Lore et al., 2012​) (​Table 1​). At a
UV-C dose of 0.5 J/cm​2 ​the viable virus remaining on N95 FFR coupons was 2–3 log lower thanon coupons not exposed to UV-C, but detectable, indicating a UV-C dose of 0.5 J/cm​2​ may beinsufficient for decontamination ​(​*​Heimbuch & Harnish, 2019​).
As of 4/22/2020, UV-C inactivation of SARS-CoV-2 in N95 FFRs has not been
demonstrated in ​peer-reviewed​ studies. Two recent non-peer-reviewed preprint manuscripts
did report SARS-CoV-2 inactivation in some N95 FFR models, however the applied UV-C
doses were not clearly specified as neither manuscript measured UV-C irradiance using a
UV-C-sensitive detector ​(​*​Fischer et al., 2020; ​*​Smith et al., 2020​). Results from studies whichhave not yet been peer reviewed should be interpreted with particular caution.

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In considering different models of N95 FFRs, Heimbuch & Harnish studied the efficacy
of UV-C viral inactivation across 15 different models. In 11 out of the 15 models tested, a UV-C
dose of 1.0 J/cm​2​ at the N95 surface was effective in inactivating H1N1 influenza (≥ 3-log
reduction). The same study found that UVGI treatment was effective for the elastic straps of
only 4 of 15 models; thus, straps may require a secondary decontamination method. N95 FFR
models with a hydrophilic facepiece were less effectively decontaminated with UV-C thanhydrophobic models ​(​*​Heimbuch & Harnish, 2019​). Similarly, related peer-reviewed literature
measured ≥ 3 log reduction in H1N1 viability on the facepieces of 12 of 15 tested models andon the elastic straps of 7 of 15 tested models (​Mills et al., 2018​).
In addition to the N95 FFR model, other factors may influence UV-C inactivation
efficacy. High humidity decreases UV-C efficacy on generic surfaces (​Tseng & Li, 2007​) and on
the surfaces of N95 FFRs ​(​Woo et al., 2012​), suggesting that a drying step prior to N95 FFR
treatment could be beneficial. In contrast to Heimbuch & Harnish, soiling agents have beenfound to reduce UV-C inactivation efficacy of both MS2 bacteriophage from N95 FFRs (​Woo etal., 2012​) and ​C. difficile ​spores from glass and plastic surfaces (​Wallace et al., 2019​). The
effect of soiling agents on UV-C decontamination may depend on the exact concentration andcomposition of the soiling agent, and/or how the soiling agent is applied (e.g., mixed in withpathogens or applied on top of pathogen inoculation). Pathogen transmission mode may also
impact UV-C decontamination efficacy: N95 FFRs inoculated with larger MS2 droplets (9-10μm) generally had lower UV-C decontamination efficiencies as compared to FFRs inoculatedwith smaller MS2 aerosols (1-2 μm) (​Woo et al., 2012​). Given that studies use a variety ofmethods to apply pathogens on an N95 FFR (aerosols, droplets, and/or pipetted solution), thequestion of whether pathogen application method impacts UV-C decontamination efficacy
merits further study.
While a UV-C dose of 1.0 J/cm​2​ at N95 FFR surface inactivates coronavirus analogues
for many models, higher doses may be required to inactivate other classes of pathogens, suchas nonenveloped viruses, bacteria and bacterial spores, and fungi. A meta-analysis
investigating the impact of UVGI on prevention of healthcare-associated infections
demonstrated mixed results depending on the pathogen type (​Marra et al., 2018​). See
Appendix A ​and ​Table S1​ for a summary of UV-C inactivation studies performed on otherpathogens.

5. Integrity of N95 Filtering Facepiece Respirators
Overall, the UV-C doses necessary for SARS-CoV-2 analogue inactivation on N95 FFRs
have been found to have minimal detrimental effects on N95 fit and filtration performance over
10-20 treatment cycles. However, it is possible that the process of donning/doffing may cause
FFR fit to reach unacceptable levels within a shorter number of cycles. One study found N95
FFR fit to decline with each donning and doffing without additional decontamination
processes. For some N95 models, fit was found to fall below OSHA standards after 5 don/doff
cycles, while others maintained fit for >15 don/doff cycles (​Bergman et al., 2012​).
Controlled laboratory studies have subjected ​15 respirator models to 10–20
donning/doffing cycles and UVGI treatment ​(1.0–1.2 J/cm​2​ per cycle), then assessed: strap
elasticity (with Imada force tester), particle penetration and breathing resistance (TSI 8130

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automated filter tester to evaluate respirator function according to the CDC (​CDC, 1997​), and fit
factor (Static Advanced Headform StAH connected to TSI Portacount 8038 automatedbreathing machine, subjected to a 240-s respiration test, testing for a fit factor >100)(​*​Heimbuch & Harnish, 2019​). Although donning and doffing yielded a statistically significant
difference in fit factor for some models, minimal detrimental effects due to UV-C exposure
specifically were observed for respirator fit, air flow resistance, or particle penetration from thisdose (10 cycles, 1.0–1.2 J/cm​2​ per cycle) of UV-C ​(​*​Heimbuch & Harnish, 2019​). Other
evaluation of low doses corroborated good FFR performance after UVGI treatment (​Viscusi etal., 2009​). At 10​2​–10​3​ higher UVGI doses (120–950 J/cm​2​), a substantial effect (>90% in somecases, but highly variable across N95 FFR models) on respirator material breaking strength
was observed (​Lindsley et al., 2015​). As variation in response to UVGI is to be expected fromdifferent N95 FFR models, the respirator must pass the ‘user seal check’ as recommended by
the CDC after decontamination to ensure respirator fit integrity is maintained (​CDC, 2018​).

6. Data Summary Tables
Table 1. Impact of UV-C on enveloped viruses
AuthorOrganism, soiling agent, & methodof application

Material

UV-C dose

Efficacy

Influenza & coronavirus strains: ssRNA enveloped virusA

H5N1 droplets (~5 μm)

N95 FFR (3M
1860, 3M
1870)

1.8 J/cm​2> 4-log reduction

B

H1N1, pipetted on as 1 μL drops.
Artificial saliva or artificial skin oil wereplaced on top of dried virus solution
to study the effects of soiling.

N95 FFR (15models)1.0 J/cm​2≥ 3-log reduction for 12/15
FFR models and 7/15 FFR
straps for all soilingconditions

C

Influenza strains (H1N1, H5N1,
H7N9), MERS-CoV, SARS-CoV, allpipetted as 1 µL drops and dried.
Artificial saliva or artificial skin oil wereplaced on top of dried virus solution
to study the effects of soiling.

N95 FFR (3M1870)

1.0 J/cm​2

No detectable virus (≥3.95-log reduction) for allorganisms for all soilingconditions

C

H1N1, pipetted as 1 µL drops anddried.
Artificial saliva or artificial skin oil wereplaced on top of dried virus solution
to study the effects of soiling.

N95 FFR (15models)1.0 J/cm​2≥ 3-log reduction for 11/15
FFR models and 4/15 FFR
straps for all soilingconditions

D

Murine hepatitis virus (coronavirus)Air

1.83 x 10​-3​ J/cm​23-log reduction**estimated based on
measured viral susceptibility
to UV-C in air

A: (​Lore et al., 2012​), B: (​Mills et al., 2018​), C: ​(*​ ​Heimbuch & Harnish, 2019​), D: (​Walker & Ko, 2007​)

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Table 2. Impact of UV-C on N95 FFRs
Author

FFR
Model

UVGI
dose
(J/cm​2​)

Particle
Penetration

BreathingResistance(mmH​2​O)(max = 25)

Respirator Material
Damage(out of 13 layers)

Strap Damage

E

N95 FFRs(15
models)1.0-1.2

0.18-3.29%(10 cycles)
0.12- 2.74%(20 cycles)

4.53-14.93

No obvious effectfrom UV-C. Some fit
degradation fromdonning/doffing.

No significant difference
from UV-C alone. Somefit degradation fromdonning/doffing.

F

3M 1860

120-950

1-2.5%

10-13

3M 9210120-950

1-2.5%

10-13

General decrease ofstrength

GE1730120-950

3-5%

10

Statistically significant
decrease in breaking
strength for dosage≥590 J/cm​2​ (≥10%decrease of mean
strength)

KC46727120-950

3-5%

15-20

120 J/cm​2 ​dose = 2
layers significantly
impacted
950 J/cm​2​ = 10
layers significantly
impacted

E: ​(​*​Heimbuch & Harnish, 2019​), F: (​Lindsley et al., 2015​)

7. Strategies
The University of Nebraska Medical Center (UNMC) published a procedure (including
N95 FFR handling logistics and treatment), which has been adopted widely during the 2020
SARS-CoV-2 pandemic and ​updated from its original version to indicate 0.6–1.0 J/cm​2​ as
the marginally acceptable UV-C dose for N95 FFRs​ (​Lowe et al., 2020​)​.​ This UNMC Process
Flow is a 51-step process defined by role (healthcare worker, courier, UVGI technician) andcovers the safe handling (intake, transport, processing, return), labeling (N95 FFRs arehealthcare worker specific), and ancillary PPE and hygiene required for the protocol. As withany decontamination strategy, an appropriate industrial hygiene workflow involving user
training ​(Beam & Hayes, 2020​), sterile processing, and other critical considerations must be
implemented to avoid cross-contamination or damage to the N95.
Wavelength appropriate UV-C light sources must be used; sources must be capable ofsupplying sufficient UV-C irradiance to yield the 1.0 J/cm​2​ dose in the UV-C treatment period.
The published UNMC procedure uses a commercial room-scale UVGI system equipped withmultiple low-pressure mercury low-ozone UV-C lamps. In the absence of other sources, theCleveland Clinic has proposed the use of idle biosafety cabinets equipped with UV-C bulbs toprovide the UVGI treatment (​*Card et al., 2020​); however, long exposure times are required toreach the marginally-acceptable dose for viral inactivation due to low UV-C irradiance outputs
from typical biosafety cabinets.
Validation of (1) UV-C decontamination efficacy (e.g., viral inactivity) and (2) subsequent
N95 FFR reuse suitability (e.g., filtration function, fit factor) is widely considered in thepeer-reviewed literature and should be considered for all new processes. ​UV-C dosing design
should meet or exceed a value of 1.0 J/cm​2​ for all surfaces of each N95 FFR and should
ideally be validated with every UVGI cycle, but periodically at a minimum​ (e.g., daily, after
a set number of cycles). Validation should be performed with a NIST-traceable calibrated

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UV-C-specific sensor to measure the UV-C irradiance or dose at each FFR position. Variationin irradiance is likely to be measured across the exposure area; the total exposure time should
be chosen such that all N95 FFRs are exposed to at least the marginally-acceptable dose of
1.0 J/cm​2​.
Use caution, as not all UV sources provide the required UV-C wavelength range,
irradiance, or irradiance uniformity: in particular, sunlight (see ​Appendix B​) and consumer
products (e.g., tanning bed lamps and nail polish curing lamps) do not generate sufficient UV-C
irradiance to decontaminate N95 FFRs (​CDC, 2020a​;​ O’Sullivan & Tait, 2014​). Even moreworrisome, there have been reports of UV sources falsely claiming to be germicidal, withemitted wavelength ranges not consistent with germicidal efficacy. In addition, UV-C sources
emitting wavelengths below 210 nm can produce ozone (​Kowalski, 2009​), which is hazardous
to human health. As a result, it is critical to measure the wavelength and irradiance of UV-C
sources with sensors specific to UV-C to ensure sources emit radiation within the UV-C
germicidal range (peak efficacy at ~260 nm, with ~10X lowered viral inactivation efficacy at 300
nm compared to 254 nm ​(EPA, 2006​; ​Lytle & Sagripanti, 2005​). The measured UV-C specificirradiance values should then be used to calculate the time required to reach a minimum UV-C
dose of 1.0 J/cm​2​ across all N95 FFR surfaces.

8. Primary Risks and Unknowns
We anticipate the following to be the primary risks and unknowns from UVGI
decontamination of N95 FFRs:
1. Direct exposure to UV-C light is harmful to humans. Proper engineering controls mustbe established prior to using UV-C systems to ensure that all users are protected fromthe UV-C light source before the light is turned on.
2. UV wavelengths of 175–210 nm can generate ozone, which is hazardous to humanhealth. Some low pressure UV lamps and most medium pressure UV lamps emit some
185 nm UV and thus will generate ozone (​Kowalski, 2009​). UV-C sources with minimal
or no ozone generation should be selected, and/or adequate ventilation should be
confirmed to minimize ozone risk.
3. UV-C only inactivates viruses subjected to the necessary UV-C dose. There remain
open questions about UV-C penetration into N95 FFR materials, and the amount ofpenetration likely varies widely across N95 FFR models (​Fisher & Shaffer, 2011​).
Although the ARA report ​(​*​Heimbuch & Harnish, 2019​) and related peer-reviewed
literature (​Mills et al., 2018​) demonstrate >3-log viral reduction (measured from fluid
extraction from the N95 FFR materials), live virus could persist inside the N95 FFR. As
such, UV-C and other deactivation approaches should be viewed as risk mitigation for
extraordinary circumstances rather than complete decontamination.
4. UV-C light sources may generate shadows (as any light source would), and theconfiguration of N95 FFRs should be designed to avoid or mitigate shadow generationon the FFR surface. For instance, UV-reflective materials may be used and/or N95 FFRs
may be rotated and/or flipped to ensure that the adequate dose is applied across theentire surface area of the FFR (and this dose should be validated with a UV-C-specificsensor).

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5. Reports have demonstrated residual virus on N95 FFR straps post UV-C exposure(likely due to the ability of N95 FFR attachment straps to twist and be shielded from the
UV-C light), suggesting a need for supplementary decontamination of the straps(​*​Heimbuch & Harnish, 2019​; ​Mills et al., 2018​). Mills ​et al.​ suggest wiping N95 FFR
straps with a compatible disinfectant (​Mills et al., 2018​). If this additional step is
employed, extra caution should be used to avoid touching the N95 FFR facepiece ascommon disinfectant chemicals can degrade N95 FFR function ​(Price & Chu, 2020​).
6. Although ≥ 1.0 J/cm​2​ dose of UV-C resulted in ≥ 3-log reduction in viral activity of
SARS-CoV-2 analogues, such an observation does not imply full decontamination of
the N95 FFR, as the N95 may still be contaminated with other pathogens that might not
be similarly susceptible to UV-C irradiation.

9. Conclusions
UVGI protocols should be implemented only if there is a dire shortage of N95 FFRs andapproved to do so. If implemented properly, with ​validation of the delivered UV-C dose to
the FFR​, it is likely that UVGI inactivates SARS-CoV-2 on the outer layers of non-shadowed
regions of the N95, based on results from similar viruses, but not confirmed directly for
SARS-CoV-2 by peer-reviewed studies as of 4/22/2020. UVGI has shown promise as aneffective method for inactivation of viruses and bacterial spores on N95 respirator material;
however, UVGI cannot inactivate pathogens that it does not illuminate. For that reason, UVGI
may not effectively decontaminate inner layers of the FFR and an auxiliary method of
decontamination may be necessary for FFR straps. Furthermore, to avoid user-to-user crosscontamination, N95 FFRs should be returned to their original user as not all pathogens may beeffectively inactivated by UVGI treatment. N95 FFR model-dependent decontamination efficacy
has been reported. We once again stress that (i) after each round of decontamination, a user
seal check should be performed, (ii) extended cycles of doffing and re-donning may affect FFR
fit, and (iii) that the FFR should not be considered fully decontaminated, as there may be otherpathogens contaminating the FFR whose activity may not be fully reduced by UVGI. ​Thus,UVGI treatment should be viewed as risk management rather than complete
decontamination. Healthcare personnel should continue to handle the respirator as if it iscontaminated and reuse only their own N95 FFR.
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Appendix A: Effect of UV-C on other pathogens
UV-C susceptibility of different pathogens in air, water, and on surfaces
The UV-C dose required to inactivate pathogens in air, water, and on surfaces isorganism-dependent, due to organism-to-organism differences in nucleic acid structure andnucleotide content, as well as varying amounts of UV-absorbing proteins and otherphotoprotective components. Higher UV-C doses are generally required to inactivate bacterial
and fungal spores, as compared to viruses and vegetative bacteria (​Kowalski, 2009​). Among
viruses, ~3x higher UV-C doses are required to inactivate viruses with double-stranded RNA or
DNA on surfaces, as compared to single-stranded viruses; higher dosage requirements are
attributable to damage of one strand being able to be repaired using the second strand as atemplate (​Tseng & Li, 2007​). While enveloped viruses are generally more susceptible toinactivation by mechanical and chemical agents (​World Health Organization, 2004​), it is unclear
whether the UV-C susceptibility of enveloped and non-enveloped viruses differ. Blazquez et al.
found that in water, enveloped viruses were inactivated with lower UV-C doses thannon-enveloped viruses (​Blázquez et al., 2019​); however, it is unclear what the mechanism of
the observed difference is, as well as whether similar trends exist for viruses in air or on othermaterials.
UV-C susceptibility of different pathogens on N95 FFRs and textiles
UV-C irradiation​ has been shown to yield ​≥ 3-log reduction of several pathogens fromN95 FFRs. A higher UV-C dose is required for decontamination of N95 FFRs, due to reduced
UV-C transmittance through the layers of the FFR material (​Fisher & Shaffer, 2011​). The
required UV-C dose to inactivate both enveloped and nonenveloped viruses from N95 FFRs is
several hundred-fold greater than the dose required for inactivation of these viruses on
surfaces (​Table S1​). MS2, a nonenveloped virus, has generally been reported to require higher
UV-C doses to achieve 3-log reduction from N95 FFRs (​Fisher & Shaffer, 2011​; ​Vo et al., 2009​)
as compared to enveloped influenza and coronaviruses ​(​*​Heimbuch & Harnish, 2019​; ​Mills etal., 2018​)​; however, it is unclear whether other differences in study design (e.g., FFR model andmethod of virus application to the FFR) also contribute to the difference in required UV-C dose.
While UV-C has been demonstrated to inactivate several species of vegetative bacteria
and bacterial spores on N95 FFRs and other textiles (​Bentley et al., 2016; Fu et al., 2020; Kenaret al., 2007; Lin et al., 2018; Smolle et al., 2018; Tomas et al., 2015​), 3-log reduction was notalways demonstrated and it is unclear how many bacterial pathogens would ​be​ inactivated by
the 1.0 J/cm​2​ UV-C dose required for coronavirus inactivation on N95 FFRs. For example,
UV-C inactivation of ​C. difficile o​ n N95 FFRs has not been studied. However, much higher
UV-C doses are required to inactivate ​C. difficile s​ pores on surfaces (~0.17-0.63 J/cm​2​;(​Wallace et al., 2019​) as compared to MS2 on surfaces (~0.006-0.010 J/cm​2​; (​Tseng & Li,
2007​). It is unclear whether the same trend (higher UV-C doses required to inactivate ​C. difficile
spores as compared to MS2 on surfaces) would hold true in the case where these organisms
are on N95 FFRs. Additionally, ​E. faecium​ in polycotton swatches was inactivated to a lower
degree (<1.97-log reduction) by UV-C​ ​(​Smolle et al., 2018​) as compared to laundering (3- to
4-log reduction) (​Tano & Melhus, 2014​).

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Table S1. Impact of UV-C on microorganisms
AuthorOrganism, soiling agent, &method of application

Material

UV-C dose

Efficacy

Influenza & coronavirus strains: ssRNA enveloped virus(​Lore et al.,2012​)

H5N1 droplets

N95 FFR (3M
1860, 3M
1870)

1.8 J/cm​2> 4-log reduction(​Mills et al.,2018​)

H1N1. 1 μL drops ofsuspension pipetted on.
Artificial saliva or artificial skinoil were placed on top of driedvirus solution to study theeffects of soiling.

N95 FFR (15models)1.0 J/cm​2≥ 3-log reduction for 12/15
FFR models and 7/15 FFR
straps for all soilingconditions(​*​Heimbuch& Harnish,
2019​) Option Task
B

Influenza strains (H1N1, H5N1,
H7N9), MERS-CoV, SARS-CoV,
all pipetted as 1 µL drops anddried.
Artificial saliva or artificial skinoil were placed on top of driedvirus solution to study theeffects of soiling.

N95 FFR (3M1870)

1.0 J/cm​2

No detectable virus (≥3.95-log reduction) for allorganisms for all soilingconditions(*Heimbuch& Harnish,
2019​) - Base
Task 4

H1N1, pipetted as 1 µL drops
and dried.
Artificial saliva or artificial skinoil were placed on top of driedvirus solution to study theeffects of soiling.

N95 FFR (15models)1.0 J/cm​2≥ 3-log reduction for 11/15
FFR models and 4/15 FFR
straps for all soilingconditions(​Walker &Ko, 2007​)

Murine hepatitis virus(coronavirus)Air

1.83 x 10​-3​ J/cm​23-log reduction**estimated based measured
viral susceptibility to UV-C inair

MS2: ssRNA nonenveloped virus(​Vo et al.,2009​)

MS2 droplets

N95 FFR(Willson
N1105)

4.32 J/cm​23-log reduction(​Fisher &Shaffer,
2011​)

MS2 aerosol

N95 FFR (6models)0.32-40 J/cm​2(equates to 0.1
J/cm​2 ​at theinternal filtering
medium)≥ 2.9-log reduction(​Woo et al.,2012​)

MS2 droplets (9-10 μm) andaerosol (1-2 μm), in water, beef
extract (BE), or artificial saliva(AS)

N95 FFR (3M1870)

3.6 J/cm​2

Droplets: 4.8-, 2.7-, 2.5-log
reduction in water, BE, AS
Aerosols: 5.2-, 3.0-, 2.7-log
reduction in water, BE, AS

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(​Tseng & Li,
2007​)

MS2

Surfaces~0.006-0.010
J/cm​2> 3-log reduction

Vegetative bacteria & bacterial spores(​Lin et al.​,2018​)

Bacillus subtilis​ spores,
aerosolized

N95 FFR (3M8210)

2.27 J/cm​2​, 5.7
J/cm​22.27 J/cm​2 ​→ ~2.7-log
reduction
5.7 J/cm​2 ​→ No detectable
spores(​Bentley etal., 2016​)

E. coli, P. aeruginosa, S. aureus(drug-sensitive anddrug-resistant), ​S.
pseudointermedius(drug-sensitive anddrug-resistant). 1-2 mL
suspension pipetted on.

Microfiber,polyester,and cotton
fabricswatches

0.27 J/cm​2>2.5-log reduction for all
bacteria on all fabrics. No
detectable bacteria in 20/24
conditions.

(​Wallace etal., 2019​)

C. difficile s​ pores (with andwithout tri-part soiling agent)
MRSA and MS2 (with andwithout 5% FBS)

Glass &
plastic0.17-0.63 J/cm​2

C. diff:​ mean 2.1-log
reduction with soiling agent
across all UV-C doses; mean
3.2-log reduction without
soiling agent across upper 3
doses.
MRSA: mean 2.9-log
reduction with FBS, mean
3.4-log reduction without FBS
MS2: mean 3.7-log reduction
with FBS, mean 2.9-log
reduction without FBS

Vegetative fungi(​Fu et al.,2020​)

5 ​Candida s​ trains

Bed sheets

0.075 J/cm​2>3-log reduction in all strains

Appendix B: Sunlight is not an effective decontamination approach for N95 FFRs
As of 4/22/2020, the CDC does not list sunlight as an appropriate method of N95 FFR
decontamination (​CDC, 2020c​). UV-C radiation with a peak wavelength of 254 nm, at a dose of≥1.0 J/cm​2​, has been found to inactivate viral particles from N95 FFRs ​(​*​Heimbuch & Harnish,
2019​). However, UV-C radiation from sunlight is absorbed by the top layer of the atmosphere,
so negligible UV-C radiation reaches the surface of the earth ​(CDC, 2019​). Sunlight at theearth’s surface consists of UV-A (320-400 nm) and UV-B (280-320 nm) radiation. UV-A
radiation is considered non-germicidal, while UV-B radiation has germicidal effects which aremuch weaker than UV-C (​Kowalski, 2009​). Theoretical calculations for the necessary sunlight
exposure time needed to achieve UV-B germicidal effects in US cities (equivalent to a 1.0
J/cm​2​ UV-C dose) suggest timescales of 57 - 5000 days, depending on season and geographic
location (​Sagripanti & Lytle, 2007​). Furthermore, studies with simulated sunlight showedN95Decon Research Document. Not Peer Reviewed.
Version 2.0, 4/23/2020.

minimal to no effect in inactivating MS2 and human adenovirus on the surface of fresh produce(​Carratalà et al., 2013​).
UV-B radiation has some germicidal effects; studies of UV-B irradiation on MS2
bacteriophage and murine noroviruses (MSV) in suspension (​not​ on surfaces) demonstrated a4-log reduction with UV-B doses of 0.909 J/cm​2​ and 0.367 J/cm​2​, respectively (​Lee & Ko,
2013​). To reach these doses, 0.34-4.2 hours of sunlight exposure would be required, assuming
UV-B irradiance from sunlight of ~60-300 μW/cm​2​ (though UV irradiance from sunlight varies
significantly depending on geographic location, season, and time of day) (​Heisler et al., 2007​).
However, the UV-C dose required for viral inactivation in N95 FFRs is ~1000x higher than forviral inactivation in water, air, or on hard nonporous surfaces (​Table S1​) (​Kowalski, 2009​). Thus,
many days of sunlight exposure would be required to achieve a sufficient virucidal dose on
N95 FFRs, in agreement with theoretical estimates (​Lytle & Sagripanti, 2005; Sagripanti & Lytle,
2007​).
As of 4/22/2020, to our knowledge, in the peer reviewed literature, there is no evidenceof viral inactivation of SARS-CoV-2 on N95 FFRs by sunlight. As of 4/22/2020, we have notfound any studies in the peer-reviewed literature assessing N95 respirator integrity after
exposure to sunlight. As a result, we conclude that there is no evidence in the peer-reviewed
literature that supports sunlight-assisted disinfection and decontamination of N95 FFRs,
specifically. Extensive experimental verification and validation must be performed before
considering sunlight as a disinfection method for N95 FFRs, and evidence from thepeer-reviewed literature on viral inactivation by the wavelengths present in sunlight (UV-A and
UV-B, not UV-C) suggest that sunlight-assisted N95 decontamination will not be effective.

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