N95Decon Research Document. Not Peer Reviewed.
Version 1.0, 4/24/2020.

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 Because of this, some of the research cited inthis document is not yet peer reviewed.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

Executive Summary
Time—simply waiting for a minimum period before re-using an N95 FFR that is stored ina clean, breathable environment at moderately humid, room temperature conditions—ispotentially the simplest and lowest cost viral inactivation method. In addressing the global
shortage of N95 Filtering Facepiece Respirators (N95 FFRs), time between uses is a firstrecommendation for decontaminating N95 FFRs between re-use in a healthcare setting ​(​CDC,
2020b​)​. However, published data on the lifetime of SARS-CoV-2 on surfaces are sparse,
making it difficult to draw conclusions about decontamination on N95 FFRs.
In this report, we analyze recent literature, comparing results on a common quantitativebasis. ​This is an area where new experimentation is urgently needed to provide moreclear, actionable advice.
For an N95 FFR that is stored individually in a clean and breathable container atroom temperature, a 7 day waiting period before reuse is expected to significantly
decrease risk of exposure to SARS-CoV-2 via the N95 FFR. ​With additional precautions,such as individual storage in a clean, breathable container, user seal checks, hand hygiene,
and proper donning and doffing, this waiting time can significantly reduce SARS-CoV-2
infection risk with re-use of N95 FFRs. This method will not protect against bacterial or fungal
infection.
Viral inactivation is sensitive to temperature and humidity.​ Storage at temperatures
below 22°C or at very low or very high humidities is expected to significantly increase theacceptable waiting period. More data are needed to quantify these effects.

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 (N95 FFRs, also referred toas “N95 masks”). In this document, we review the use of a waiting time between uses as amethod to decontaminate N95 FFRs with the goal of increasing the useful lifetime of N95 FFRs
worn by healthcare providers during 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.
Time—simply waiting for a minimum period before re-using an N95 FFR that is stored
in a clean, breathable environment at room temperature conditions—is potentially the simplest
and lowest cost viral inactivation method. Enveloped RNA viruses, such as the coronavirus
SARS-CoV-2, eventually lose their infectious capacity at room temperature. The precise timing
and variability of this process are addressed experimentally in a number of studies that we
review in this document.

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Here, we evaluate studies based on the time required to reach a 3-log level of viralinactivation. This is the time for the viral load to be reduced by a factor of 1000, which is thetarget level of reduction identified in FDA guidance documents for decontamination ​(​FDA,
2020​)​. The reduction in viral load may not be identical to the reduction in probability of
infection. With a probabilistic dose-response model, e.g. ​(​Watanabe et al., 2010​)​, the viral
infection risk decreases more slowly than the viral load decreases. For example, if the viral loaddecreases by 90%, the viral infection risk decreases by <90%.
In this report, we survey the existing literature, highlighting assumptions that are
required to interpret the data and clear qualitative conclusions that can be drawn from multiple
studies. A waiting period of 7 days (168 hr) encompasses the available experimental data on
decontamination (as defined by 3-log decay on the sample mean) of a single N95 FFR and of asurgical mask.​ This is an area where new experimentation is urgently needed to provide
more clear, actionable advice.
Reusing the same mask within a day (i.e. at next shift)​ ​is not expected to, in
general, allow sufficient time for viral inactivation and ​is not recommended​ if the N95 is
not also decontaminated via another effective method.​ Given the sensitivity of the virus tomaterial and local environment, we do not have enough data to make a precise
recommendation that encompasses all N95 FFR models in reasonable room temperature
conditions.​ For an N95 FFR that is stored individually in a clean and breathable containerat room temperature, a 7 day waiting period before reuse is expected to significantly
decrease risk of exposure to SARS-CoV-2 via the N95 FFR.
The time needed to reduce infection risk of an enveloped RNA virus to an acceptablelevel depends on the amount that is originally deposited, the threshold for infectiousness, andthe environmental conditions including temperature, humidity, surface type, and the presenceof other agents including proteins and salt.​ Critically, cooler temperatures will extend thelife of SARS-CoV-2. For example, storage at temperatures colder than tested (e.g. in an
unheated cabinet, basement or vehicle where temperature falls below 22°C) could
substantially extend the life of the virus beyond what is described.

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 ​(​Instructions for Healthcare Personnel: Preparation of Compatible N95
Respirators for Decontamination by the Battelle Memorial Institute Using the Battelle
Decontamination System,​ 2020​)​.

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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​.
A first recommendation from the CDC’s recent guidance on reuse ​(​CDC, 2020b​)​ is foreach healthcare worker to be issued at least five N95 FFRs, to wear one per day, and to store
each in a breathable paper bag between uses. The healthcare worker is to rotate through thefive N95 FFRs so that there is a waiting period of at least 5 days before reuse.
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. Mechanism
For N95 FFRs that are stored at room temperature, environmental conditions caneventually lead to disruption of the virus envelope, proteins, or RNA. Viral inactivation can
additionally be affected by the surface material, protein content, pH, chemicals, and the
medium in which the virus is prepared (e.g. ​Coulliette et al., 2013​; ​Firquet et al., 2015​). Detailsof how this inactivation happens are beyond the scope of this report.
Virus inactivation with time is often assumed to follow first-order kinetics (e.g. ​Seo et al.,2012​)​, which means that the number of active organisms decreases at a rate proportional to
the number that exist at that moment in time. The assumption of first-order kinetics implies 1)
an exponential decrease with time in the number of infectious organisms, characterized by atime constant (also called a rate constant), and 2) that the fold-decay in a given time-interval
does not depend on the size of the initial viral inoculum, i.e. the time to get from 1000 active
particles to 1 active particle is the same as the time to get from 5000 to 5.
On a real mask, different virus particles may experience dramatically different local
environments, leading to a broad distribution of decay rates and a deviation from the idealized
exponential decay at the population level. Many experiments observe non-exponential decay, acharacteristic feature of which is that the decay rate becomes slower over time. One should
thus be cautious in extrapolating measured decays beyond the measurement interval.
Thus, 3-log decay time is influenced by a number of environmental factors and ideallyshould be assessed via direct experiment.

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4. Potential for SARS-CoV-2 Inactivation
Experiments that test persistence and inactivation of a virus on a surface all share thesame high-level steps:(1) Re-suspend virus in a medium(2) Inoculate this suspension onto the material being tested(3) Wait a specific amount of time(4) Recover the virus from the material(5) Quantify amount of infectious virus recovered(6) Repeat in parallel steps (2)–(5) for different wait times(7) Fit data to a model(8) From the model fit, report a number that characterizes how the virus is inactivated overtime.
Different choices in each of these steps can lead to different reported results for how
virus inactivation changes with time.
In this section, we summarize the recent papers that study SARS-CoV-2 and two earlier
papers that study other coronaviruses applied to materials relevant to a hospital setting. In a
more extensive review of earlier literature without any experiments on SARS-CoV-2, (​Kampf etal., 2020​)​ focus on the survival of coronaviruses on surfaces and coronavirus inactivation withbiocidal agents. Across experiments reviewed there, there are large variations in the reported“persistence” time for viruses even on nominally the same material in similar conditions. For
example, across the cited experiments, the reported times for the un-defined “persistence”
span more than an order of magnitude from 8 ​hours​ to 9 ​days.​ Part of this variation is likely due
to what “persistence” time means; it is undefined in ​(​Kampf et al., 2020​)​ and the papers thatare cited in that review report varying metrics.
In contrast, here we attempt to compare all papers using the same metric ofinactivation: the 3-log decay time. We focus on medical materials and SARS-CoV-2
experiments. Even when making the comparison of the 3-log time across different studies,varying experimental and mathematical choices in the published literature lead to substantial
variations in, for example, the reported 3-log decay time on stainless steel.
Studies are summarized in this section and the resulting 3-log decay times arequantified in the data tables below. Methods are highlighted in this section to show thevariations in experimental choices.
In a non-peer-reviewed preprint,​ ​(​*Fischer et al., 2020​)​ ​evaluate the stability of
SARS-CoV-2 on samples of stainless steel and N95 filter material from AOSafety N9504C
respirators. The virus is recovered from the material by adding 1 mL of medium. The data are fit
using Bayesian regression assuming exponential decay. The 3-log decay time from this study
is the reported median “time to one thousandth” from Bayesian regression. For the N95 FFR,
that time is 13 hr, with a 95% confidence interval spanning 11–15 hr.
In the table below, the maximum measured time is where the estimated mean titre
across replicates is shown in plotted data to reach the measurement threshold (​Morris,

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personal communication, April 22, 2020​). The data at
https://github.com/dylanhmorris/n95-decontamination​ show the time intervals used.
(​van Doremalen et al., 2020​)​ ​test both SARS-CoV-1 and SARS-CoV-2, choosing an
inoculum at a level relevant to samples from the human respiratory tract. For the test on
cardboard, the virus is recovered by swabbing the surface and adding 1 mL DMEM. For othersurfaces, the virus is recovered by adding 1 mL DMEM. Data show that the viruses persist
longer on plastic and stainless steel than on cardboard or copper. Virus persistence as anaerosol is also significantly less than on stainless steel or plastic.
It is noted in this paper that data for copper and cardboard do not show exponential (oreven monotonic) decreases in viral load with time and thus fits are to be interpreted withcaution.
For the data tables below, the maximum time at which data were measured is the timeat which the estimated titre from all three replicates first reaches the threshold for detectability.
In almost all cases this time is shown on the plots and the raw data are on Github athttps://github.com/dylanhmorris/sars-cov-2-stability​.
Though the raw data in ​(​van Doremalen et al., 2020​)​ for stainless steel are the same asin ​(​*Fischer et al., 2020​)​, the two papers use different titre inference methods (​Morris, personalcommunication, 4/19/20​). To estimate the 3-log decay time, we multiply the median reported
half-life by 9.966.
(​Chin et al., 2020​)​ ​tests SARS-CoV-2 on various surfaces, including the inner and outerlayer of a surgical mask. While N95 FFRs differ from surgical masks, the N95 FFRs commonlyused in medical settings, sometimes referred to as “surgical N95 FFRs,” are FDA-certified forfunctionality as surgical masks. Without further information about the material used by ​(​Chin etal., 2020​)​, we cannot judge the relevance of this experiment to N95 FFRs. ​The decay timereported by (​Chin et al., 2020​) is substantially longer than what is reported in the othermeasurements reviewed here. In the interest of being conservative, we have included
this result in our overall assessment.
The stability of the virus in the viral transport medium at varying temperature is also
tested in ​(​Chin et al., 2020​).​ Using a simple linear fit to the reported log data yields 3-log decay
times of 2070 hr at 4℃ (the temperature of a household refrigerator), 167 hr at 22℃ (‘room
temperature’), and 20 hr at 37℃. This implies that for a temperature change of 10℃, the 3-log
decay time could change by a factor of 4 or 5. Though these data were taken for virus in
solution, not on a surface, they show that virus stability is highly sensitive to temperature.
In this study, the virus was recovered from each material by soaking in 200 uL of viraltransport medium for 30 min. SARS-CoV-2 was found to remain infectious longer on
non-porous materials (glass, stainless steel, plastic) than on porous materials (paper, tissue
paper, wood, cloth).
Data were fit to a bi-phasic model, instead of to the simple exponential model. Data
were fit assuming that the kinetics follow exponential decay with one time constant at the
beginning and a longer time constant for much longer times. Thus, the reported model fits

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cannot be directly compared with model fits from other papers that assume exponential decay
with one time constant.
Instead, the 3-log decay time given in the tables below has been deduced directly fromthe reported raw data instead of from the model fit. Raw data are reported at 0 min, 30 min, 3
hr, 6 hr, 1 day, 2 days, 4 days, 7 days. In the table below, the 3-log decay time is the time atwhich the mean measurement shows a 3-log reduction from the mean measurement at 0 min.
If the detection threshold is reached before the 3-log reduction, the 3-log decay time isreported here as greater than the time at which the threshold was reached. The maximum
measured time in the table below is the time at which the reported data are first at themeasurement threshold (undetectable) or the last time at which data were reported (even if still
above threshold).
For the surgical mask on the outer layer, it is notable that there is a relatively large
standard deviation (0.46) on the final measurement (mean of 2.79), which is both themeasurement that defines the 3-log decay time and the longest measurement made.
(​Lai et al., 2005​)​ tested the earlier SARS-CoV-1 virus, measuring its lifetime in stool andrespiratory specimens as well as on paper (from a laboratory request form), a disposable gown
made of impervious material, and a cotton gown. To recover the virus, material was inoculated
into cell culture tubes and incubated.
In the stool samples, the virus persisted longer at higher pH. In respiratory specimens,
the virus persisted above the 3-log level for about a week at room temperature and 3 weeks at
4℃.
The raw data were not shown; instead the times to inactivation (at the measurement
threshold) for three different starting titres were reported for each material. In the table below,
the maximum measured time is the reported “time taken to inactivate” for the largest titre of
10​6​ TCID​50​/mL (where TCID​50​ is the median tissue culture infectious dose).
Estimating the 3-log decay time from such minimal data requires many assumptions.
Two possible approaches, both assuming that the system follows first-order kinetics, are:1) If the system follows first-order kinetics, the time to inactivation should be a
linear function of the log of the titre. With the three data points given, this fit isgood only for the case of the disposable gown. For all three materials, thismethod yields a 3-log decay time that is on the order of twice the longest
measurement time.
2) Alternatively, if it is assumed that the threshold measurement for inactivation is 1
PFU (plaque-forming unit), a first-order kinetics model would yield a
3.5-log-decay from the inoculation using the titre 10​6​ TCID​50​/mL to thethreshold. This method of estimation yields a number that is less than the total
measurement time.
Both methods were applied to the data from this publication. These two methods yield
different results for each material case and the reported one in the table below is the larger of
the two (to be conservative). These numbers should be considered to be very rough estimates;
the difference in the estimates produced by these methods further illustrates the need for moredata and evaluation of models used for fitting.

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(​Sizun et al., 2000​)​ ​evaluates the lifetime of human coronaviruses HCoV-229E and
HCoV-OC43 when dried on surfaces and in various aqueous solutions. The difference insurvival times in the different aqueous suspensions points to a challenge in doing experiments
on surfaces: if there are persistent droplets, the liquid in which the virus is suspended can
impact the survival time.
To recover the virus from the materials, the material was incubated in a sonicatingwater bath and eluate is analyzed. The 3-log decay time cannot be extracted from the data inthis paper because the data are only presented as a plot on a linear (not logarithmic) scale andthus this paper is not summarized in the data tables below. For both viruses, the infectivity inthe first 3 hr drops the slowest for aluminum, compared to latex gloves and sterile sponges.

5. Data Summary Tables
The table below summarizes the above-mentioned tests of virus lifetime on surfaces.
Many of these papers include other experiments (such as lifetime in stool and respiratory
specimens or responsiveness to disinfectants); those additional results are not summarized
here. Cited numbers below are mean (for frequentist analysis) or median (for Bayesian analysis)
unless otherwise specified. All log values are assumed to be base 10.
Material

Virus
Starting(medium*) Titre/
Inoculum

Environmental Maximum Estimated Ref
Conditions
measured 3-log
time (hr)** decay time(hr)***

Personal Protective Equipment Materials
N95 FFR
material

SARS-CoV-2 10​5​ TCID​50​/mL;
apply 50 μL

Surgicalmask, inner
layer

SARS-CoV-2 10​7.8​ TCID​50​/mL; 22℃, ~65% RH(VTM)
apply 5 μL

168

96

Surgicalmask, outerlayer

SARS-CoV-2 10​7.8​ TCID​50​/mL; 22℃, ~65% RH(VTM)
apply 5 μL

168

168

Disposable
gown

SARS-CoV-1 10​4​–10​6(PBS)
TCID​50​/mL;
apply 5 μL

Not Reported

48

70.5

Cotton gown

SARS-CoV-1 10​4​–10​6(PBS)
TCID​50​/mL;
apply 5 μL

Not Reported

24

46.1

21–23°C, 40% RH 48

48.2

21–23°C, 40% RH 24

13(​*Fischer et al.,2020​)(​Chin et al.,2020​)(​Chin et al.,2020​)(​Lai et al., 2005​)(​Lai et al., 2005​)

MetalsStainless steel SARS-CoV-2 10​5​ TCID​50​/mL;
apply 50 μL(​*Fischer et al.,2020​)

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Copper

SARS-CoV-2 10​7.8​ TCID​50​/mL; 22℃, ~65% RH(VTM)
apply 5 μL

168

168

SARS-CoV-2 10​5​ TCID​50​/mL;
apply 50 μL

21–23℃, 40% RH

96

56.1

SARS-CoV-1 10​5​ TCID​50​/mL;
apply 50 μL

21–23℃, 40% RH

72

41.5

SARS-CoV-2 10​5​ TCID​50​/mL;
apply 50 μL

21–23℃, 40% RH

87.7

SARS-CoV-1 10​5​ TCID​50​/mL;
apply 50 μL

21–23℃, 40% RH

24

14.9

(​Chin et al.,2020​)(​van Doremalen
et al., 2020​)(​van Doremalen
et al., 2020​)(​van Doremalen
et al., 2020​)(​van Doremalen
et al., 2020​)

Organic Materials
Wood

SARS-CoV-2 10​7.8​ TCID​50​/mL; 22℃, ~65% RH(VTM)
apply 5 μL

48

6

Cloth

SARS-CoV-2 10​7.8​ TCID​50​/mL; 22℃, ~65% RH(VTM)
apply 5 μL

48

>24

Glass

SARS-CoV-2 10​7.8​ TCID​50​/mL; 22℃, ~65% RH(VTM)
apply 5 μL

96

48

Banknote

SARS-CoV-2 10​7.8​ TCID​50​/mL; 22℃, ~65% RH(VTM)
apply 5 μL

96

48(​Chin et al.,2020​)(​Chin et al.,2020​)

Other(​Chin et al.,2020​)(​Chin et al.,2020​)

Paper-based Materials
Cardboard

SARS-CoV-2 10​5​ TCID​50​/mL;
apply 50 μL

21–23℃, 40% RH

48

34.5

SARS-CoV-1 10​5​ TCID​50​/mL;
apply 50 μL

21–23℃, 40% RH

24

5.9

Tissue paper

SARS-CoV-2 10​7.8​ TCID​50​/mL; 22℃, ~65% RH(VTM)
apply 5 μL

3

0.5

Paper

SARS-CoV-2 10​7.8​ TCID​50​/mL; 22℃, ~65% RH(VTM)
apply 5 μL

3

>0.5

SARS-CoV-1 10​4​–10​6(PBS)
TCID​50​/mL;
apply 5 μL

24

42.7

Plastic

Not Reported(​van Doremalen
et al., 2020​)(​van Doremalen
et al., 2020​)(​Chin et al.,2020​)(​Chin et al.,2020​)(​Lai et al., 2005​)

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Plastic (typeSARS-CoV-2 10​7.8​ TCID​50​/mL; 22℃, ~65% RH
not specified) (VTM)
apply 5 μL

168

48

SARS-CoV-2 10​5​ TCID​50​/mL;
apply 50 μL

21–23℃, 40% RH

96

67.9

SARS-CoV-1 10​5​ TCID​50​/mL;
apply 50 μL

21–23℃, 40% RH

96

75.2

(​Chin et al.,2020​)(​van Doremalen
et al., 2020​)(​van Doremalen
et al., 2020​)*VTM=viral transport medium. PBS=phosphate buffered serum. If not named, the medium is not specified in thegiven study.
**The maximum measured time is estimated differently for each reference. See details in the text above.
***The method by which the 3-log decay time is estimated from the published data varies by reference. See details
in the text above.

6. Integrity of N95 Filtering Facepiece Respirators
If N95 FFRs are stored in a clean, vented container at ​room temperature​ between uses,
a primary risk to integrity is the degradation in fit over multiple donnings and doffings. For
some N95 FFR models, fit was found to be unacceptable after 5 don/doff cycles, while others
maintained fit for >15 don/doff cycles ​(​Bergman et al., 2012​)​. Storage conditions should notdeform or crush the N95 FFR.
Furthermore, risk with reuse can be reduced by discarding N95 FFRs with visible blood,
hair, soiling with facial cosmetics, or damage ​(​CDC, 2020a; OSHA, n.d.​)​.

7. Strategies
NIOSH and the CDC give recommendations for N95 FFR reuse, including storage in aclean, breathable container (e.g., paper bag) or hanging N95 FFRs between reuse ​(​CDC,
2020a​)​. N95 FFRs contaminated with blood, respiratory or nasal secretions, or other bodily
fluids should be discarded ​(​CDC, 2020a​)​. N95 FFRs should be re-used only by the originaluser.
Using time as a method for virus inactivation is viewed as risk mitigation for
extraordinary circumstances rather than complete decontamination or sterilization. Risk isfurther mitigated if a re-used N95 FFR is treated, in terms of hand hygiene for example ​(​CDC,
2020a​)​, as if the N95 FFR might still be contaminated.
Given the unknowns in using time for decontamination, proper donning, doffing, andhand hygiene are critical for reducing risk. ​(​Brady et al., 2017​)​ shows that improper doffing ofan N95 FFR can lead to higher contamination from N95 FFR to hands than proper doffing andreuse (without decontamination). Hand hygiene is another part of the recent NIOSH / CDC
recommendations ​(​CDC, 2020a​)​.
As summarized above, virus persistence is expected to be much greater at lower
temperatures. The presented studies all use moderate humidity; higher or lower humidity may
increase virus inactivation time ​(​Lin & Marr, 2020;​ Coulliette et al. 2013​)​.

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After the waiting period and before re-use, physical inspection and a ‘user seal check’,as recommended by the CDC, should be performed to ensure N95 FFR integrity and adequate
seal ​(​CDC, 2020b​)​.

8. Primary Risks and Unknowns
Enveloped RNA viruses such as SARS-CoV-2 tend to be more rapidly inactivated bytime than other clinically relevant pathogens that could co-inoculate an N95 such as
mycobacterium, antibiotic resistant bacteria, bacterial spores, or other pathogens. An
adequate wait time that inactivates SARS-CoV-2 may not inactivate other common pathogens.
Analysis of an appropriate waiting time for inactivation of other common pathogens is currently
beyond the scope of this report.
The existing literature leaves several gaps that necessitate judgment in choosing a time
period for decontamination of SARS-CoV-2. The range of values across papers for stainlesssteel illustrates this: ​(​Chin et al., 2020​)​ shows 168 hr, ​(​van Doremalen et al., 2020​)​ shows 56 hr
and ​from the same data​ ​(​*Fischer et al., 2020​)​ reports 48.2 hr. The discrepancy between ​(​Chin
et al., 2020​)​ and ​(​van Doremalen et al., 2020​)​ cannot be reconciled from the available data andit presents a large uncertainty on enumerating an appropriate waiting period fordecontamination.
Here, we enumerate some of the experimental and modeling choices and assumptions
that leave uncertainty.
(1) The assumption of first-order kinetics is a mathematically-convenient assumption thathas proven to be effective ​(​Peleg & Cole, 1998​)​. However, it is not always the bestmodel for fitting the data. Indeed, in ​(​Chin et al., 2020​)​, the data for survival of CoV-2 ondifferent surfaces are fit to a model where the time constant in the first-order kinetics isdifferent in the first hour than over the rest of a day. None of the studied papers giveassessments of the quality of one model vs another. As an example of a different model
for virus kinetics, in ​(​Seo et al., 2012​)​, a Weibull model is found to fit the data better formurine norovirus.
(2) If the first-order kinetics model is not the right model, then the time for each additionallog of decay might be longer than for the previous log of decay.
(3) Model fits may not be appropriate for 3-log decay time if the assessed time is much
longer than the time of the experiment. In ​(​Chin et al., 2020​)​, 7 days is the longest
measurement time used. In ​(​van Doremalen et al., 2020​)​, the experiments are less than
100 hr at maximum and extrapolation to the 3-log decay time may be longer than what
was measured.
(4) It is expected that the time constant, even in a first-order model, will depend on
environmental conditions. In ​(​Vejerano & Marr, 2018​)​, for example, it is shown that therelative humidity determines the evaporation rate of a droplet and it is argued that virus
survival is impacted by the micro-environment of this evaporating droplet. Other viruseshas been shown to persist longer at extreme values of humidity than at moderatevalues ​(​Lin & Marr, 2020​)​. Temperature has a dramatic impact on virus survival times asshown, for example, in ​(S​ eo et al., 2012​)​ for the norovirus and in ​(​Chin et al., 2020​)​ for
SARS-CoV-2. User conditions may be highly variable and different from the controlled

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environment of a lab. ​For example, storage at temperatures colder than tested (e.g.
in an unheated cabinet, basement or vehicle where temperature falls below 22°C)
could substantially extend the life of the virus beyond what is described.
(5) Even with the same model, ​how​ the data are analyzed can matter. ​(​Peleg & Cole, 1998​)
give one example for how choices in fitting data to a model can matter. Incidentally, thedata for stainless steel in ​(​van Doremalen et al., 2020​)​ and in ​(​*Fischer et al., 2020​)​ arereported to be the same data, with a difference in titer inference methods (​Morris,
personal communication, 4/19/20​). That yields a difference of almost 10 hr in the 3-log
decay time.
(6) Experiments in the literature use different viruses, different media, and different
methods for recovering the virus from the surface and all of these can impact theresults. ​(​Chin et al., 2020​)​ uses viral medium and the other two SARS-CoV-2 studies do
not report what medium is used. This may mean that the studies presented here
underestimate the inactivation time for a virus protected by mucus, for example.
(7) Virus inactivation times vary widely across different materials, as shown in the reviewed
papers. These data are presented without a fundamental understanding of why acertain material might promote longer or shorter virus survival times. This creates achallenge in extrapolating from data on one material to inactivation times for anothermaterial. For example, the material of the N95 FFR in ​(​*Fischer et al., 2020​)​ may bedifferent, from the point of view of virus inactivation, from the material used in the N95
FFRs that are marketed for use in medical settings. The surgical mask used in ​(​Chin etal., 2020​)​ is not specified and it is unknown how this material compares to a given N95
FFR. In general, N95 FFRs are fabricated from layers of differently-textured
polypropylene, and layers sometimes include other materials like polyester. They havehydrophobic and hydrophilic layers varying by model. Surgical N95 FFRs (which are
FDA-certified for additional functionality as surgical masks, and which are typical in
healthcare settings) commonly have an additional hydrophobic outer layer, while
non-surgical N95 FFRs may have a hydrophilic outer layer ​(​Viscusi et al., 2009​)​.
Common models including the 3M 1860 additionally feature an external aluminum
noseclip ​(​3M Technical Data Sheet: Disposable respirator, 1860, 1860S, N95,​ 2018​)​.
These material differences are another source of uncertainty in the data.
In comparing the two peer-reviewed studies on SARS-CoV-2 ​(​Chin et al., 2020; vanDoremalen et al., 2020​)​, the environmental conditions (in terms of temperature and humidity)
are similar. There are two surfaces tested in both of these papers: stainless steel and plastic,
without further details on the materials. When the results from these two references arecompared ​using the same gauge of persistence—​ a 3-log decay in virus level—then the results
are closer to each other than the large ranges given in ​(​Kampf et al., 2020​)​. From ​(​Chin et al.,2020​)​, the 3-log decay time of the mean SARS-CoV-2 on stainless steel is between 4 and 7
days. From ​(​van Doremalen et al., 2020​)​, it is ~2.3 days. ​(​Chin et al., 2020​)​ shows a 3-log decay
time on plastic of 2 days and ​(​van Doremalen et al., 2020​)​ shows 68 hr (almost 3 days).​ With
the limited number of studies on SARS-CoV-2 and coarse data, the variation between
different studies can be as much as 3–5 days.

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(​van Doremalen et al., 2020​)​ compares SARS-CoV-1 and SARS-CoV-2, which gives asense of the uncertainty that may be incurred from extrapolating from the results of one virus
to another virus. Across the 5 tested cases, these differences (median, extrapolated to 3-log
decay) range from ~1 hr (for the short life of aerosols) to ~1 day (for cardboard).
(​van Doremalen et al., 2020​)​ gives 95% confidence intervals for each result.​ These
confidence intervals for the SARS-CoV-2, again extrapolating to the 3-log decay time, areapproximately 1 day in all cases.
Together, these results point to the high uncertainty in the published data and the needfor more experiments.
In this report, we have used 3-log decay time as a standard way of comparing acrossdifferent experiments. This choice is in the absence of a clear specification on the amount ofviral load on an N95 FFR that constitutes “decontamination”. All N95 FFRs used in a clinicalsetting should be considered contaminated. The initial infectious viral load will greatly impact
what the infection risk is after waiting a given period of time. N95 FFRs directly exposed todroplet or aerosol-generating procedures with infected patients would be more likely to requirea longer duration of treatment.

9. Conclusions
SARS-CoV-2 and other enveloped viruses survive for a limited time on surfaces at room
temperature; the precise time period needed for satisfactory inactivation depends on a numberof environmental variables.
Though there are many modeling assumptions that go into the experiments as well as
variability in the tested environments, across the literature surveyed here, there are clear
qualitative conclusions that can be drawn:● Coronaviruses survive longer at colder temperatures than at warmer temperatures,
which makes storage temperatures a critical consideration in using time as a method of
decontamination.
● Coronaviruses, including SARS-CoV-2, generally live longer on surfaces that are
qualitatively described as smooth or non-porous, than on surfaces described as roughor porous. A notable exception from experiments is copper, which yields a very short
lifetime for SARS-CoV-2 ​(​van Doremalen et al., 2020​)​. There is a need for a better
understanding of which material properties determine virus lifetime on a surface.
● There is not a fundamental understanding of why SARS-CoV-2 might live longer one
one surface than another and without that it is a challenge to extrapolate experimental
results from one material to another.
● Proper donning, doffing, and hand hygiene are critical, irrespective of decontamination
procedures.
● The risk of exposure to SARS-CoV-2 virus from an N95 FFR stored individually in aclean, breathable, room-temperature environment goes down the longer one waits
before re-using the N95 FFR.

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From the reviewed literature, a 5-day waiting period encompasses the 3-log decay time
for tested coronaviruses on a variety of surfaces at room temperature conditions for most of
the published results, including the most recent non-peer-reviewed experiment on an N95 FFR.
The exceptions are: 4–7 days for stainless steel and 7 days for the outer layer of a surgicalmask tested with SARS-CoV-2 ​(​Chin et al., 2020​)​.​ ​As explained above, the experimental resultson the surgical mask are conservatively judged to be relevant for this discussion of N95 FFR
decontamination.
An N95 FFR stored in a moderately-humid room-temperature environment (22°C,
40–65% relative humidity) will eventually achieve 99.9% reduction in viral load after somewaiting period, thereby meeting the threshold for viral reduction from the FDA (​FDA, 2020​). The
two most relevant studies show significant variation and yield ​estimates between less than 1
day and about 1 week required for this decontamination time for SARS-CoV-2 onmaterials that could be relevant for an N95 FFR.​ ​This is an area where newexperimentation is urgently needed to provide more clear, actionable advice.
Because many factors regulate viral decay, users should understand that small changes
in temperature, humidity, or initial high viral loads on a N95 FFR could reduce the margin ofsafety. More studies are needed to have higher confidence in recommendations, especiallyconsidering the range of room temperature conditions that exist in health care situations andthe range of materials used for different models of N95 FFRs. It would be especially useful tohave further studies that encompass measurement times well beyond the 3-log decay time andprepare the virus in a suspension of a medium that is similar to human mucus.
Irrespective of the waiting time that is chosen, proper donning and doffing of the N95
FFR and hand hygiene are critical. Moreover, this waiting time is only an estimate and it is notexpected to decontaminate the N95 FFR against other pathogens or infectious agents.
The Content provided by N95DECON is for INFORMATIONAL PURPOSES ONLY, DOESNOT CONSTITUTE THE PROVIDING OF MEDICAL ADVICE and IS NOT INTENDED TO BE
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