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Technical Report for Heat-Humidity-Based N95 Reuse Risk
Management
Much 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. Because of this,many of the research papers cited in this document are not yet peer reviewed. ​For clarity, wherever
non-peer-reviewed results are cited in this report, the citation is preceded by a “*”​.

Summary of Updates in v2.0 Report​: Updated to reflect recent additions to the literature,including new studies of the impact of heat on N95 FFR integrity, inactivation of SARS-CoV-2
in liquid media, and inactivation of SARS-CoV-2 on N95 FFRs using 1) dry heat, 2) autoclaving
and 3) microwave-generated steam. An important recent finding indicates that 30 minutes ofdry heat at 70 o​​ C is not sufficient to obtain a greater than one thousand-fold (≥3-log) reduction
in viral activity, which is suggested by the FDA as the necessary level of inactivation for
SARS-CoV-2 on N95 FFRs (*​Fischer et al., 2020​;​ FDA, 2020)​. Also included is a short
discussion of possible validation mechanisms to ensure stability and homogeneity of heat andhumidity application.

Executive Summary
A combination of heat and humidity, often referred to as moist heat, is a known methodof inactivating some pathogens on surfaces and in bulk media. Recently, moist heat has been
identified by the Centers for Disease Control (CDC) as one of the most promising methods for
N95 filtering facepiece respirator (N95 FFR; also colloquially referred to as ‘N95 mask’)decontamination. In this report, we review the available literature on decontamination of N95
FFRs containing SARS-CoV-2, the novel coronavirus that causes coronavirus disease 2019
(COVID-19), as well as related viruses. Additionally, we review literature on the impact of
repeated heat and humidity cycles on N95 FFR filtration and fit integrity. Based on currently
available data it is clear that the effectiveness of moist heat viral inactivation is highly sensitiveto 1) temperature, 2) humidity, 3) duration of exposure, and 4) the local environment (surface,mask fiber, soiling medium—e.g. mucus or saliva). These data point towards a set of promisingconditions for inactivation of SARS-CoV-2 on N95 FFRs, which include temperatures between
70​–​85​o​C, humidities between 50–85%, and process times of at least 60 minutes​. Temperatureand humidity should be calibrated and monitored, as heating devices can be highly variable.
We additionally examine N95 decontamination by autoclave and by microwave-generated
steam. While these methods show some promise, more data is required to make definite
conclusions for either method.

1.

Overview
The novel coronavirus (SARS-CoV-2) that causes coronavirus disease 2019 (COVID-19)
has led to a global shortage of N95 Filtering Facepiece Respirators (FFRs). In this document,
we review the use of heat and humidity to decontaminate N95 FFRs (colloquially: ‘N95 masks’)with the goal of increasing the useful lifetime of N95 FFRs worn by healthcare providers during

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the COVID-19 pandemic. 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. We examined reports on ​inactivation of SARS-CoV-2 in
response to heat. Since few studies have extensively tested the susceptibility of coronaviruses
to heat and humidity on N95 FFRs, we also examined procedures developed in anticipation of
influenza pandemics. Influenza, although not closely related to SARS-CoV-2, is anotherrespiratory virus containing a segmented single-stranded RNA genome and a lipid envelope.
It is increasingly clear from this growing body of literature that while SARS-CoV-2 andrelated pathogens are likely to be susceptible to heat-based inactivation, the conditions
required for this inactivation are critically sensitive to 1) temperature, 2) humidity, 3) duration of
exposure, and 4) the local environment (surface, mask fiber, media used under experimental
conditions). Data indicate that for process times of 60 minutes and a temperature of 70​o​C​ withno added humidity SARS-CoV-2 will be inactivated by at least 3.3-log, although the local
environment for the virus in that non-peer-reviewed report may not adequately resemble thatfound on N95 FFRs in a clinical setting ​(*​Fischer et al., 2020​)​. Existing data from influenza
H1N1 suggests that higher humidity will better inactivate similar (enveloped) viruses (​McDevitt
et al., 2010​). It has also been found in recent non-peer-reviewed reports that several ​N95 FFRs
can withstand 5 cycles at up to 75–85​o​C with 60–90% relative humidity for 30 minutes, while
maintaining adequate performance (*​Anderegg et al., 2020​;​ *Massey et al., 2020​). ​Thesefindings together suggest that the most promising conditions for SARS-CoV-2 inactivation on
N95 FFRs are likely to be ​temperatures​ ​between ​70–85​o​C at a relative humidity greaterthan 50%, for 60 minutes or more​. It is accepted that higher temperatures and longer heatingtimes lead to more effective decontamination and could be used provided they preserve N95
FFR fit and filtration integrity. ​A significant parameter space including ​higher temperatures atlower humidity and/or shorter times​, and similarly, ​lower temperatures at higher humidity
and/or longer times,​ may allow for sufficient inactivation and should be investigated. ​Further
studies with media that most closely match saliva and/or mucus are warranted, especiallybecause the presence of such substances can protect the virus from inactivation (​Darnell et al.,2004​; ​Darnell & Taylor, 2006​; ​Rabenau et al., 2005​). As studies with ​SARS-CoV-2 inactivation
on metal surfaces at 70 o​​ C​ indicate inadequate inactivation for 60 minutes in dry heat (*​Fischer
et al., 2020)​, additional decontamination of the metal nose piece on N95 FFR using liquid
disinfectant (on the metal only) may be desired.
Further SARS-CoV-2 research will be required to determine the actual minimum
sufficient conditions for inactivation on N95 FFRs.
Additionally, we reviewed literature on the damage caused to FFRs subjected torepeated applications of heat. As these studies show, different makes and models of N95 FFRs
exhibit different levels of robustness under various sets of inactivation conditions. Therefore, inall cases only make- and model-appropriate inactivation protocols should be considered for
implementation (see summaries in ​Section 4​ and ​Table 2​). This summary is intended to inform
healthcare professionals and decision makers in this time-critical period of the ​SARS-CoV-2
pandemic​.

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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, 2020a​). In addition, the
CDC has provided guidance to hospitals on methods for decontaminating N95 FFRs during acrisis (​CDC, 2020b​).
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, 2020b​). 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​.
Currently, we are not aware of any FDA EUA for heat-humidity treatments of N95 FFRs.
CDC released guidance on the decontamination and reuse of N95s on March 31, 2020, which
identifies the use of moist heat as one of the most promising methods for treatment of N95
respirators under crisis conditions (​CDC, 2020b​).
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.

Mode of Action
In droplets, inactivation of enveloped viruses has been shown to be enhanced atintermediate humidity values, due to increasing solute concentrations as droplets shrink but
are not fully dried (​Lin & Marr, 2020​;​ Vejerano & Marr, 2018​). There is currently little data on
methods for disinfection of SARS-CoV-2; no definitive best practice can be identified at thistime.
Further work is needed to refine this protocol for implementation. Importantly, heat andhumidity ​may not sterilize ​the FFR of all pathogens, and bacterial spores, including
Clostridium difficile​, may remain. In particular, it was found that while 15 minutes of exposure
to 85​o​C did lead to a >4.5-log reduction of viable ​C. difficile​ in a liquid medium, the bacterial

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spores exhibited an ability to recover during later incubation; meanwhile, exposure to 63​o​C for
30 minutes led to a 50% reduction in viable spores (​Rodriguez-Palacios & LeJeune, 2011​).

4.

SARS-CoV-2 Inactivation and N95 FFR Durability
Sections 4.1​ ​and 4.2​ below give summaries and discuss the available literature on theeffects of heat-humidity treatments on SARS-CoV-2 inactivation and the functional integrity of
N95 FFRs. ​Sections 4.3 and 4.4​ are specifically dedicated to autoclave and microwave-ovengenerated steam (MGS) treatments.
4.1.

Potential for SARS-CoV-2 Inactivation
While there is evidence that SARS-CoV-1 and SARS-CoV-2 can be inactivated by heat
when in solution (30 min at >56​o​C, ​Appendix A​), this ​does not appear to translate to their
sensitivity on N95 FFR surfaces​, particularly when the viral population is in mucus or saliva
droplets. A recent, non-peer-reviewed report indicates that 70​o​C dry heat for 30 min was NOT
sufficient to inactivate SARS-CoV-2 in DMEM media on N95 FFR fabric (*​Fischer et al., 2020​).
Only a 1.9-log reduction was observed, which is below the minimum inactivation guidance of
3-log reduction set by the FDA for viruses (​FDA, 2020​). While 70​o​C dry heat for 60 minutes did
sufficiently inactivate SARS-CoV-2 (>3-log reduction) on N95 FFR fabric, details of the media
used for inoculation were not provided, and may not have included mucin or other proteins thathave been shown to stabilize viral particles in a real-world scenario (​Darnell et al., 2004​; ​Darnell& Taylor, 2006​; ​Rabenau et al., 2005​). DMEM, which does not contain these proteins, waslisted as the viral growth media. In that same study, 60 minutes of 70​o​C dry heat showed only a2-log reduction of viral concentration on a stainless steel surface (*​Fischer et al., 2020​), further
indicating that 70​o​C dry heat may not sufficiently decontaminate N95 FFRs (which often
contain metallic components). Therefore, ​further studies with viruses in different media arenecessary to find a safe working range of temperature, time, and humidity that willinactivate SARS-CoV-2.​ Other recent work suggests that N95 FFRs contaminated with
SARS-CoV-2 can be sufficiently decontaminated via a standard autoclave cycle at 121​o​C for
15 minutes, though some N95 FFR models were found to fail fit tests after more than one cycle
under these conditions (*​Kumar et al., 2020​). Autoclave treatment is further discussed in
section 4.3​, while the literature on viral inactivation on surfaces is summarized in ​Table 1​,
below.
Few other studies have simultaneously evaluated the effect of heat and humidity on
viral inactivation and mask fit and filtration under comparable conditions, though results
evaluating FFR performance alone under these conditions are discussed in ​section 4.2​.
Heat and humidity have been used to inactivate other enveloped viruses (H1N1 andH5N1 influenza) on various N95 FFRs (​Heimbuch et al., ​2011​;​ Lore et al., 2012​) and surfaces(​McDevitt et al., 2010​). N95 FFRs contaminated with influenza can be adequately
decontaminated at temperatures over 60​o​C with sufficient humidity and exposure times (seeTable 1​). One study, using a dried solution of H1N1 on stainless steel, found inactivation was
more effective when either temperature or relative humidity was increased (​McDevitt et al.,2010​). ​Multiple studies using various viral samples have shown a correlation between mid toN95Decon Research Document. Not Peer Reviewed.
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high relative humidity and increased viral inactivation, but 100% humidity may be less effective(​Casanova et al., 2010​;​ Prussin et al., 2018​;​ Guan et al., 2017​;​ Lin & Marr, 2020​). It ishypothesized that heat inactivation of SARS-CoV-2 will also be more efficient at intermediate
to high humidity levels, though there is currently insufficient data to support this hypothesis.
4.2.

Integrity of N95 Filtering Facepiece Respirators
N95 FFRs are intended as single use respirators. There is, however, literature on theperformance of N95 FFRs after multiple heat decontamination cycles, summarized in ​Table 2​.
This table lists, for each specific N95 FFR model, the filtration and quantitative fit tests for themost relevant studies on N95 FFR durability under heat-humidity treatments. A more extensive
version of this summary table with all the literature surveyed can be found in ​Appendix B​.
Many common N95 FFR models are able to undergo 1-3 cycles of 30 minutes at 60​o​Cand 80% relative humidity while maintaining both fit and filtration performance (​Bergman et al.,2010​;​ Bergman et al., 2011​;​ Viscusi et al., 2011​). Several models (3M 8200, 3M 8511, andmore) have recently been shown to pass fit tests for at least 5 cycles of 30 minute dry heat at
75​o​C (*​Price et al., 2020​). A recent study indicates that the filtration efficiency of the fabric usedas the filtering material in N95 FFRs may remain unaffected under high temperature (75​o​C) andhumidity (up to 100%) conditions for up to 20 30-minute cycles (*​Liao et al., 2020​). In general,
the trend in the available data suggests that many common models may be able to withstandseveral heat-humidity treatments at temperatures up to 85​o​C and high relative humidity. Data
for multiple treatment cycles and higher temperatures with both filtration and fit tests is still
limited, however. Only a few models (3M 1860, 3M 8210, and 3M 8210+) have been confirmed
to pass quantitative fit and filtration tests after >3 cycles with temperatures >70​o​C and relative
humidities >50% (*​Anderegg et al., 2020​;​ ​*​Massey et al., 2020​;​ ​*​3M, 2020​). ​Different N95 FFR
models have varying susceptibilities to elevated temperatures, and durability results forone model do not necessarily extend to other models. Higher temperature and humidity
will likely lead to more effective inactivation of SARS-CoV-2, which warrants exploring
N95 FFR durability at temperatures >70​o​C and humidities >50% for other common N95
FFR models.​ ​For the specific data on various N95 FFR models, see Table 2. ​Any protocol
implemented should be tested with the specific N95 FFR models used locally.​ ​For healthcare
personnel utilizing any kind of FFR, a user seal check is crucial before reuse to ensure therespirator still seals properly to the face (​Price & Chu, 2020​). Finally, as an important additional
consideration for N95 FFR reuse, repeated donning/doffing has been shown to have an impact
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​)​.
When considering the integrity of N95 FFRs after decontamination, an importantdistinction should be made between surgical N95 FFRs (also abbreviated as SN95 FFR) andnon-surgical N95 FFRs. While both surgical and non-surgical N95 FFRs are NIOSH-certified fortheir filtration efficiency, surgical N95 FFRs are additionally FDA-certified for their functionality
as surgical masks, e.g. protecting against fluid penetration. It is not well-studied whether
surgical N95 FFRs maintain their functionality as surgical masks after treatment. Because of
the different materials used in the construction of surgical and non-surgical N95 FFRs, theintegrity of these respirators under heat treatment may differ​ ​(​Viscusi et al., 2009​).

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4.3.

SARS-CoV-2 Inactivation and N95 FFR Integrity under Autoclave Treatment
Autoclave treatment is a readily-accessible hospital sterilization procedure that has thepotential to be employed for decontamination and reuse of N95 FFRs. While there are few
studies specifically examining the inactivation of SARS-CoV-2 on these respirators under
autoclave treatment, there is at least one piece of recent evidence, from a non-peer-reviewed
report, suggesting that a 15 minute autoclave cycle at 121​o​C can effectively decontaminate
N95 FFRs of the virus (*​Kumar et al., 2020​; see ​Table 1​). Furthermore, autoclave treatment at
121​o​C for 30 minutes is considered a general sterilization process in medical settings (​CDC,
2008​).
There exist a handful of studies on N95 FFR durability under autoclave treatment. The
available data, included in ​Table 2​, shows that the impact of autoclave treatment depends on
the specific N95 FFR model. Studies indicate that three molded models, 3M 1860, and 3M
8000, and 3M 8210, fail after only one or two cycles of autoclave treatment while some layered
fabric, pleated models such as the 3M 1870 and 3M 1862+ may keep their functional integrity
for up to 10 cycles of autoclave treatment (​Viscusi et al., 2007​;​ ​*​Kumar et al., 2020​; ​van Straten
et al., 2020​). Additional autoclave studies that include filtration tests are required to supplement
these findings. More generally, given the limited amount of data, ​additional studies areneeded in order to fully understand the effects of autoclave treatment on N95 FFR
durability for different models. ​We are aware of ongoing experimental work in this area.

4.4.

SARS-CoV-2 Inactivation and N95 FFR Integrity under Microwave-Oven Generated
Steam Treatment
While there is limited literature on the deactivation of SARS-coronaviruses via
microwave-oven generated steam (MGS) treatment, studies examining the decontamination of
N95 FFRs containing influenzas (H5N1 and H1N1) or bacteriophage MS2 suggest that MGS
treatment can be an effective means of decontaminating FFRs of some viruses. A summary ofthese studies is given in ​Table 1​. S​ pecific studies of SARS-CoV-2 are limited, and theeffectiveness of MGS for decontamination of SARS-CoV-2 contaminated N95 FFRs
cannot currently be evaluated.​ Additionally, it is important to note that MGS treatment may
not fully inactivate bacterial spores, or may require additional time. It was found in one study
that ​Bacillus cereus​ spores required at least four minutes of microwave radiation to be fully
destroyed on a wet sponge (​Park et al., 2006​).
The literature on the durability of N95 FFRs under MGS treatment, included in ​Table 2​,
suggests little to no impact on functional integrity in up to three decontamination cycles, albeit
with some respirator damage on the inner foam nose cushion and head straps (​Bergman et al.,2010​; ​Bergman et al., 2011​; ​Viscusi et al., 2011​). The available studies are currently limited to asmall number of repeated decontamination cycles, however, and recent tests on the meltblown
fabric used as the filtering material in N95 FFRs suggest that steam treatment can haveadverse effects on filtration efficiency beyond three decontamination cycles (*​Liao et al., 2020​).
Additionally, there is insufficient data on N95 integrity after MGS treatment in the high-power,
1250 W microwaves used for several viral inactivation studies described above. ​Extending
these studies to test N95 FFRs beyond three decontamination cycles would be beneficial
to our understanding of the effects of MGS on N95 FFR durability.

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When evaluating MGS as a method of N95 decontamination, it is also important toconsider variations in power and geometry between different microwave models. In particular,
the impact of powers higher than 1250 W on mask integrity is relatively unknown, and merits
caution. The metallic components of many N95 FFR models (e.g. nosepieces) may present
additional risks due to extreme heat or sparking, though no such effects have been observed instudies to date (​Bergman et al., 2010​;​ Bergman et al., 201​;​ Viscusi et al., 2011​;​ Heimbuch et al.,2011​). N95 FFRs have been shown to melt in microwave ovens in the absence of steam(​Viscusi et al., 2009​), and care should be taken to introduce steam in an appropriate manner.
Finally, MGS treatment may be sensitive to the specific protocol employed (e.g. placed above
a water reservoir vs. contained in a steam bag), and the references in ​Tables 1 and 2​ should
be consulted for details on their specific implementation.

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5.

Data Summary Tables
Table 1. Impact of heat & humidity on SARS-CoV-2 and other viruses on N95 FFRS/surfaces
Strain(s)(medium, if known)

Surface

Temp & RH(Method)

Time
(min)

Effectiveness (log reduction)

SARS-CoV-2

AO Safety
N9504C (N95
fabric)70ºC, dry heat(oven)

30
60

1.9 (Insufficient)
>3.3

60

2.0 (Insufficient)

Stainless steel 304

Refs.
A

SARS-CoV-2(BSA​b​, tryptone,
mucin)

3M 1860 & 1870
3M Vflex 1804
AO Safety 1054

121°C, steam(autoclave)

15

≥4.6
≥5.3
≥5.6

B

Influenza H1N1
(mucin, aerosol
and/or droplets)

3M 1860
3M 1870
KC PFR95-270
3M 8210
3M 8000
Moldex 2200

65 ± 5ºC,
85% RH(oven)

30>3.0-7.0 (FFR-dependent)C(1250W MGS​a​,water reservoir)

2>3.3-6.3 (FFR-dependent)

Influenza H1N1

Stainless steel

60°C, 25% RH
60°C, 50% RH
60°C, 75% RH
65°C, 25% RH
65°C, 50% RH
65°C, 75% RH(oven)

30

1.5 (Insufficient)
>5.0
>5.2
2.2 (Insufficient)
>5.1
>5.1

D

Influenza H5N1
(aerosolized allantoic
fluid)

3M 1860
3M 1870

65°C, moist
heat (oven)

30

>4.62
>4.65

E

3M 1860s
3M 1870(1250 W MGS​a​,water reservoir)

2

>4.81
>4.79

Bacteriophage MS2​c(ATCC medium 271)

3M 1870
KC PFR95-270
Moldex 2200

(1100 W MGS​a​,in steam bag)

1.5

3.1
3.45
≥3.1

A: (*​Fischer et al., 2020​), B: (*​Kumar et al., 2020​), C: (​Heimbuch et al., 2​ 011​), D: (​McDevitt et al., 2010​), E: (​Lore et al., 2012​),
F: (​Fisher et al., 2011​)
a. Microwave-oven generated steam. Listed power is microwave specification; actual power may be somewhat lower
b. Bovine serum albuminc. Non-enveloped virus; may be more resistant than SARS-CoV-2 or influenza to certain treatments

F

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Table 2. Impact of heat-humidity treatment on N95 FFR durability
Model

Temp. &
Rel. Humidity(oven-based,
30 min cycles)# cyclesfiltration
tested# cyclesfit tested

Autoclave
121°C steam, 15 min

MGS
1100 W​e​, 2 min

Refs.

3M 1860

85°C, 60–85%

Passed​a​ 5

Passed​b​ 5

Failed after 1-2
cycles

Passed 3 cycles

A, B, C,
D, E

3M 8210+85°C, 60–85%

Passed 5

Passed 5--

3M 1870

60°C, 80%

Passed 3

Passed 3​c

Fit passed 10 cycles(no filtration tests)

Passed 3 cycles

B, C, D

3M 8000

60°C, 80%

Passed 3

Passed 1

Failed after 1 cycle

Passed 1 cycle

C, E, L

Moldex 2200

60°C, 80%

Passed 3

Passed 1-

Passed 1 cycle

C, L

KC
PFR95-270

60°C, 80%

Passed 3

Passed 3​c-Passed 3 cycles

C, D

3M 821075°C, 90%

Passed 10​dPassed 10

Failed after 1 cycle

Passed 1 cycle

B, C, F,
L, M

3M 8200

75°C, dry-

Passed 5--G

3M 851175°C, dry-

Passed 5--G

4C Air75°C, dry-

Passed 5

-

-

G, H

Jackson 20

75°C, dry-

Passed 5--G

3M 9211+

70°C, dry-

Failed after
3 cycles--I

3M 9210---

Fit passed 10 cycles(no filtration tests)-

B

3M 1804S---

Fit passed 10 cycles(no filtration tests)-

B

3M 1862+---

Filtration passed 5cycles (no fit tests)-

J

Aearo 1054S

-

-

-

Fit passed 10 cycles(no filtration tests)-

B

Cardinal
Health----Filtration passed
1 cycle (1.5 min)(no fit tests)

K

A

A: (*​Anderegg et al., 2020​), B: (*​Kumar et al., 2020​), C: (​Bergman et al., 2010​), D: (​Bergman et al., 2011​), E: (​Viscusi et al., 2007​),
F: (*​Massey et al., 2020​), G: (*​Price et al., 2020​), H: (*​Liao et al., 2020​), I: (*​Fischer et al., 2020​), J: (​van Straten et al., 2020​),
K: (​Fisher et al., 2011​), L: (​Viscusi et al., 2011​), M: (*​3M, 2020​)
a. “Passed” implies that filtration efficiency was >95% after the specified number of cycles.
b. “Passed” implies that quantitative fit tests resulted in fit factors > 100.
c.​ ​Fit tests were performed with 15-minute cycles, rather than 30-minute cycles used in most literature.
d. Filtration tests were performed at lower temperature and humidity (65°C and 50-80%); see ref. L for details.
e. Studies cited for MGS all used 1100 W rated microwaves. The authors note that the actual power might have been lower.

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6.

Strategies
Many hospitals are currently equipped with or can readily buy devices that canachieve the 70–85​o​C temperatures and >50% humidities mentioned above,​ including
warming cabinets, circulating water baths, autoclaves, convection ovens, or microbial
incubators. Devices with direct heating elements should not be used, as they create local
temperatures that are higher than the target, therefore risking damage to the respirator. Target
humidities could be accomplished in these devices, for example, by temporarily placing N95
FFRs in impermeable heat-stable plastic boxes (e.g., ziploc containers) with a source ofmoisture inside each box, or by isolating N95 FFRs in permeable containers and increasing thehumidity of the heating device. Individual containment of N95 FFRs is recommended as it
ensures that N95 FFRs are kept physically separated (reducing possible cross contamination)and enables decontaminated N95 FFRs to be returned to their original users. We emphasizethat airing of N95 FFRs ​immediately​ after a thermal cycle is recommended and could reducerisk of pathogen growth.
For any given device and method, the critical process parameters should be validated
to ensure proper control and performance. It is important to determine that any chosen method
is able to achieve and remain at the target temperature and relative humidity for the target time,with maximal spatial homogeneity across the device. This validation should be performed
under conditions as close to regular process conditions as possible with sufficient monitoringby electronic temperature and humidity sensors. Care should be used when choosing anappropriately rated sensor. This validation should be repeated periodically at a frequency
determined by the facility’s established quality control (QC) practices and the party responsible
for oversight and implementation of the procedure.
In donning an N95 FFR that has been through any decontamination process, the user
should perform the locally recommended steps to ensure N95 FFR fit, so as to ensure that theseal is not compromised.

7.

Primary Risks and Unknowns
Only two studies described in this report, neither of which has yet completed peer
review, directly examined the efficacy of decontamination of N95 FFRs contaminated with
SARS-CoV-2. Data regarding the role of relative humidity in SARS-CoV-2 inactivation via heatis forthcoming, as well as data related to the effect of inoculation media (mucus, culture media,
aerosolized droplets, etc.) on N95 FFR viral inactivation.
In this review we have only examined conditions that would likely result in theinactivation of SARS-CoV-2; ​the​ ​risk of other pathogens remains.​ Since the current practiceof many hospitals is to keep N95 FFRs at room temperature between uses, it is crucial toevaluate whether the microbial load on an N95 increases after incubation in moist heat relative
to incubation at room temperature.
In testing heat as a possible method for viral inactivation, N95 FFRs should stayphysically separated from each other and should only be reused by the same clinician.

N95Decon Research Document. Not Peer Reviewed.
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Because data suggest relative humidity is an important factor in viral inactivation,
quality assurance measures are critical to achieving decontamination. Process variability inheating elements or humidity sources could result in cycles with inadequate virucidal activity.

8.

Conclusions
When possible, unused N95 FFRs and other personal protective equipment should be
provided; however, in crisis situations this is not always feasible. We are sharing this review to
aid in the development of real-world processes to protect clinical staff by employing equipment
and supplies that may be readily available or easily obtained. We hope to guide healthcare
institutions that face the need to decontaminate and reuse N95 FFRs during this COVID-19
pandemic. For heat-humidity-based decontamination, we stress that (i) after each round of
decontamination, a user seal check should be performed, (ii) extended cycles of doffing andre-donning may affect FFR fit, and (iii) that the FFR should not be considered fully sterilized, asbacterial and mold spores may remain even after viral inactivation.
Our review of the available literature revealed that the conditions required forinactivation by heat and humidity are pathogen-specific. Therefore, studies to determine
appropriate conditions for SARS-CoV-2 inactivation on N95 FFRs are urgently needed.
Preliminary inactivation data for SARS-CoV-2 on N95 FFRs, considered alongside data forother pathogens that are likely to exhibit similar stability to SARS-CoV-2 (e.g., influenza H1N1
and H5N1 on N95 FFRs), ​suggests that conditions of moist heat at 70°C to 85°C with >50%
relative humidity for 60 minutes might provide a good basis for further studies on
decontamination of N95 FFRs contaminated with SARS-CoV-2.​ Experiments are underway
to evaluate the efficacy of heat-humidity inactivation of SARS-CoV-2 on N95 FFRs.
The literature on autoclave treatment indicates that it may be an effective SARS-CoV-2
decontamination method for certain N95 FFR models (namely layered, pleated models such as
the 3M 1870), while molded FFRs such as the 3M 1860 appear to fail after only 1–2 cycles.
Several N95 FFR models are able to endure up to 3 cycles of MGS treatments, but the efficacy
of MGS treatment in inactivating SARS-CoV-2 is unknown given the current literature.
This document will be updated as more information becomes available. The strategies
considered here are potentially compatible with implementation in numerous clinical settings
with different heating appliances (e.g. warming cabinets, water baths, autoclaves, microbial
incubators, industrial convection ovens). These strategies focus only on inactivation of the
SARS-CoV-2 virus and its surrogates, and ​do not serve as a means of complete N95
sterilization​. This document does not evaluate the efficacy of heat or humidity on inactivationof other pathogens of concern in hospital settings. Ultimately, we hope that our summary can
aid hospitals in formalizing their own N95 FFR decontamination strategies for approval with the
FDA to better protect the health of essential healthcare workers and front-line personnel.

N95Decon Research Document. Not Peer Reviewed.
Version 2.0, 4/23/2020.

9.

Appendix A: Impact of Heat on SARS-Coronaviruses in Liquid Media
Though there is little data examining inactivation of SARS-CoV-2 on N95 FFRs andother surfaces, there is evidence supporting heat inactivation of viruses suspended in liquid
solution. While these data are included for completeness, there is evidence to suggest thatinactivation in solution does not imply decontamination on N95 FFRs under similar
conditions. ​Very recent non-peer-reviewed work indicates a >6-log decrease in infectivity of
SARS-CoV-2 in liquid media after 15 minutes of exposure to 92​o​C, while 30–60 minutes at
56–60​o​C is slightly less effective (*​Pastorino et al, 2020​). Additionally, recent data indicate that
SARS-CoV-2 in liquid media can be inactivated by exposure to 70​o​C for 5 minutes (​Chin et al.,2020​). Considering that SARS-CoV-2 and SARS-CoV-1 share 79% genome identity (​Lu et al.,2020​) and have similar stability of infective particles in aerosol and on fomite surfaces including
plastic, stainless steel, and copper (*​van Doremalen et al., 2020​), we also examined thetemperature sensitivity of SARS-CoV-1. When held at 60–75​o​C for 5–30 minutes in various
liquid media, SARS-CoV-1 infectivity is reduced (see ​Table S1​). It is important to note that thetime and temperature for viral inactivation is dependent upon the media the virus is in,including blood or mucus (​Darnell et al., 2004​; ​Darnell & Taylor, 2006​; ​Rabenau et al., 2005​).
CDC guidelines state that FFRs with visible blood, mucus or other soils should not be reused(​CDC, 2020a​). ​These results are specific to liquid media, and do not quantitatively reflect
the conditions necessary for decontamination of N95 FFRs.​ A summary of the effect of heaton SARS-coronaviruses in liquid media is given in ​Table S1​.

N95Decon Research Document. Not Peer Reviewed.
Version 2.0, 4/23/2020.
Table S1. Impact of heat on SARS-coronaviruses in liquid media
Note: Heat inactivation in liquid media DOES NOT imply similar behavior on N95 FFR surfaces
AuthorMedia

Temperature

Time (min)Strain(s)Effectiveness (log
reduction)

A

VTM​b

56​o​C
70°C

30
5

SARS-CoV-2≥4.6​a≥3.0​aB

MEM + 5% FBS+/- BSA56​o​C60​o​C92​o​C

30
60
15

C

0, 10% & 16% BSA 60°C

20D

MEM +/- 20% FBS

30

CHuman plasma

E

DMEM

F

EM + 10% FBS

67°C

60

SARS-CoV-1

No detectable CPE​c

E

DMEM

75°C

15

SARS-CoV-1≥4F

EM + 10% FBS

G

4% FBS

58°C
68°C

30
10

SARS-CoV-1

4.9
>4.3

H

MEM + 10% FBS,
25% human serum

60°C

30

SARS-CoV-1

>4.5

65°C

20

>5
>5
>6
SARS-CoV-1

≥3.5
≥5.01

SARS-CoV-1

5

≥4.25
≥4.5

No detectable CPE​c30

A: (​Chin et al., 2020​), B: (*​Pastorino et al., 2020​), C: (​Darnell & Taylor,​ 2006​), D: (​Rabenau et al.,​ ​2005​), E: (​Darnell et al.,​ ​2004​),

F: ​(Duan et al.,​ ​2003​), G: ​(​Pagat et al., ​2007​), H: (​Yunoki et al., 2004​)
a. Calculated by N95DECON as minimum fold change from 5.3 +/- 0.17 log TCID50 to below the 100 TCID50 limit of detection at
95% confidence.
b. Viral Transport Medium (​buffered salt solution, fetal bovine serum, antibiotics, and fungicides)c. CPE (cytopathic effect)

N95Decon Research Document. Not Peer Reviewed.
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10.

Appendix B: Extended Data Summary Table for N95 FFR Durability

Table S2. Impact of heat-humidity treatment on N95 FFR durability (extended version)
Model

MethodTemp. &
Relative Humidity

Time
(min)

# cyclestestedFiltration
efficiency

Fit
testing​a

3M 1860

Oven
Oven
Oven
Autoclave
MGS

85°C, 60–85%
80 to 120°C, dry
60°C, 50–80%
121°C, steam
100°C, steam (1100​e​ W)

30
60
30
15
1.5–2

5
1
10
10
3

100%
99 to 98%
>95%
N/A
99%

PassedN/A
PassedFailedPassed3M 1870

Oven
Oven
Oven
Autoclave
MGS

80 to 120°C, dry
60°C, 80%
60°C, 80%
121°C, steam
100°C, steam (1100​e​ W)

60
15 / 30
30
15
1.5–2

1
3/1
3
10
3

99 to 98%
N/A
99%
N/A
99%

N/A
PassedN/A
PassedPassedNose foam partly delaminated for all methods

N
D/L
C
B
C, D, K

3M 1862+

Autoclave

121°C, steam

15

5

97%

N/A

Blind test indicated 3-times cycled FFRs
visually indistinguishable from untreated FFRs

J

3M 8000

Oven
Oven
Oven
Autoclave
MGS

80 to 120°C, dry
80°C, dry
60°C, 80%
121°C, steam
100°C, steam (1100​e​ W)

60
60
30
15, 30
2

1
1
3
1
3

99 to 98%99%
99%
81%, 66%
99%

N/A
N/A
Passed​b
N/A
Passed​b

Oven
Oven
Oven
Oven
Autoclave
MGS

80 to 120°C, dry
60°C, 50–80%
75°C, dry
75°C, 90%
121°C, steam
100°C, steam (1100​e​ W)

60
30
30
30
15
1.5–2

1
10
10
10
10
3

100 to 99%
>95%
N/A
N/A
N/A
100%

N/A
Passed​b
PassedPassedFailedPassed​b

3M 8200

Oven

75°C, dry

30

5

N/A

PassedG

3M 8210+

Oven
Oven

85°C, 60–85%
75°C, dry

30
30

5
5

100%
N/A

PassedPassedAG

3M 8511Oven

75°C, dry

30

5

N/A

PassedG

3M 1804S

Autoclave

121°C, steam

15

10

N/A

PassedB

3M 9210

Autoclave

121°C, steam

15

10

N/A

Passed3M 9211+Oven

70°C, dry

30

3

N/A

FailedAearo 1054S

Autoclave

121°C, steam

15

10

N/A

PassedMoldex 2200

Oven
Oven
MGS

80 to 120°C, dry
60°C, 80%
100°C, steam (1100​e​ W)

60
3
1.5–2

1
3
3

99 to 95%99%
98–99%

N/A
Passed​b
Passed​b<95% filtration efficiency for 2/3 samples at
120 °C (ref. O)NC​d​, Ld​C​ , L, K

KC PFR95-270 Oven
Oven
Oven
MGS

80 to 120°C, dry
60°C, 80%
60°C, 80%
100°C, steam (1100​e​ W)

60
15 / 30
30
1.5–2

1
3/1
3
3

99 to 95%
N/A98%
96–98%

N/A
PassedN/A
Passed4 of 9 samples melted at oven temperatures
above 90 °C and were not tested (ref. O)

N
D/L
C​d
C, D, K

Cardinal Health MGS

100°C, steam (1100​e​ W)

1.5

1

100%

N/A

K

4C AirOven
Oven

75°C, dry
75°C, dry

30
30

20
5

99%
N/A

N/A
PassedH
G

Jackson 20

Oven

75°C, dry

30

5

N/A

PassedG

N95 fabric​c

Oven
Oven
Oven
Steam

75°C, dry to 100%
100°C, <30%
125°C, <30%
100°C, steam

30
10
10
10

20
20
20
10

95%
95%
91%
80%

N/A
N/A
N/A
N/A

H
H
H
H

3M 8210Notes on respirator damage

Failed after first cycle

No visible changes to the FFR
FFRs “deformed, stiff, and mottled”

Fit factor decreased from ~150 to ~125 after 1
cycle (ref. R)
Failed after first cycle

Refs.
A
N
C, D, L, M
B
C, D, K

N
E
C, L
E
C, L
N
C, D, L, M
FF
B
C, L, KB
Fit decreased factors from ~200 to ~90IB

A: (*​Anderegg et al., 2020​), B: (*​Kumar et al., 2020​), C: (​Bergman et al., 2010​), D: (​Bergman et al., 2011​), E: (​Viscusi et al., 2007​), F: (*​Massey et al., 2020​),
G: (*​Price et al., 2020​), H: (*​Liao et al., 2020​), I: (*​Fischer et al., 2020​), J: (​van Straten et al., 2020​), K: (​Fisher et al., 2011​), L: (​Viscusi et al., 2011​),
M: (*​3M, 2020​), N: (​Viscusi et al., 2011​)
a. “Passed” implies that quantitative fit tests resulted in fit factors > 100.
b. Fit tests were performed after a single cycle, see ref. R for details.
c. This study was performed on pre-cut meltblown fabric material used as the filtration fabric in N95 FFRs.
d. This study was performed on the “small” size versions of the models Moldex 2200 and KC PFR95-270 (named Moldex 2201 and KC PFR95-174).
e. Studies cited for MGS all used 1100 W rated microwaves. The authors note that the actual power might have been lower.

N95Decon Research Document. Not Peer Reviewed.
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