Using the included databases and registries yielded a total of 538 abstracts (Figure). Of these, 528 were excluded because they did not meet the eligibility criteria. Ten articles were further investigated with review of their full texts. We captured five papers specifically on the use of N95 masks against SARS-CoV-2.17,18,19,20,21 In addition, our manual search of recommendations from key health agencies, organizations, and Societies identified 16 papers evaluating decontamination methods22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37 and ten papers investigating barrier integrity testing38,39,40,41,42,43,44,45,46,47 that was not specifically related to SARS-CoV or SARS-CoV-2.
For implementation, N95 decontamination methods should achieve virus inactivation at scale, without compromising filtration performance or mask fit, and present no irritation or health concern to the user. These criteria are difficult to achieve in practice and no single method for N95 decontamination and reuse has been uniformly accepted. Characteristics of common decontamination methods are summarized in Table 1 and discussed in detail below.
TABLE 1 N95 decontamination methods Heat, humidity, and autoclaving
The SARS-CoV-2 virus displays variable stability across tested temperature ranges. It remains stable at 4°C for prolonged periods, with only a 0.7 log-unit reduction of the 50% tissue culture infective dose (TCID50, measure of infectious titre) after 14 days.37 At 22°C and 65% relative humidity, virus particles can still be detected on the outside of a surgical mask after seven days (0.1% TCID50 of the original inoculum), indicating potential problems with ambient mask storage.37 Compared with results obtained by reverse transcriptase-polymerase chain reaction (RT-PCR) detection (measuring viral RNA), findings using TCID50 are more applicable to mask reuse as they quantify virus survival. Nevertheless, the complexity of measuring TCID50 limits its implementation.
In contrast to cold or ambient temperatures, SARS-CoV-2 is susceptible to heat inactivation. After five minutes at 70°C on a solid surface, there is a greater than six-fold reduction in TCID50.37 The need for combined humidity and heat in mask decontamination has only been studied for influenza, but indicates that 50–85% relative humidity is beneficial for viral inactivation.22 Similarly, a recent study comparing SARS-CoV (the virus responsible for the 2003 SARS pandemic) to SARS-CoV-2, showed that SARS-CoV is inactivated by a five to 30 min exposure to 70–75°C in liquid media, a susceptibility that likely extends to SARS-CoV-2.23
Few studies have simultaneously evaluated the effect of heat and humidity on viral inactivation, mask filtration, and fit under comparable conditions. Many mask models can undergo at least one cycle of elevated temperature (65–80°C) for 20–30 min without a decrease in their filtration efficacy and fit.24 There is only limited data for multiple cycles. Recent experiments suggest N95 masks can be exposed to up to 50 cycles of 85°C without changes in filtration efficacy and fit, but did not include the elevated humidity that is usually required to more broadly inactivate biological agents.18 In general, data suggest that N95 masks may be able to successfully undergo three 30-min decontamination cycles at 65–80°C with a high relative humidity without losing filtration or fit performance.24,25 The exposure of N95 masks to such a protocol does not alter the filtering capacity after three to five cycles. Recently, it was observed that heat (≤ 85°C) and variable humidity (up to 100% relative humidity) preserved filtration properties in melt-blown fabrics and N95-grade respirators.18 At 85°C and 30% relative humidity, the authors were able to perform 50 cycles of heat treatment without deterioration of filtration efficiency.
Autoclaving is a standard technique used by hospitals and academic labs to sterilize equipment, offering an attractive option because of the widespread availability of the instrumentation. Unlike oven heating (with or without added humidity), autoclaving utilizes an elevated temperature (greater than 120°C) and pressure (greater than 103 kPa, or 15 psi). Nevertheless, this method may significantly impact mask fit and function. Only a few studies have evaluated decontamination efficacy and mask quality after N95 autoclaving. The use of steam sterilization at 125°C for three minutes does not affect the electrostatic charge of the electret in the mask, but there is no evidence that this is sufficient for biological decontamination.26 In a study of heating a contaminated mask for 15 min at 121°C at 103 kPa, there was near 100% sterilization of bacteria (using B. subtilis spores), but virus survival was not evaluated.27 Steam alone is successful at decontaminating from avian coronavirus.20
There is conflicting evidence of the effectiveness of autoclave decontamination on N95 respirator filtration efficiency, with one study showing no increased penetration of 0.075 µm and 0.3 µm particles, but with another study stating significant mask degradation.28,29 Nevertheless, both studies showed that autoclave decontamination induced significant mask deformation. There is no direct evidence for SARS-CoV-2 sterilization using this technique.
Hydrogen peroxide or other chemical decontamination
Hydrogen peroxide, an oxidizer commonly found in cleaning agents, can eradicate a wide range of microorganisms, including nosocomial bacterial spores and viruses. It has been used in high concentrations for medical equipment sterilization for more than 30 years. Specifically, hydrogen peroxide vapour (HPV), hydrogen peroxide gas plasma (HPGP), and ionized hydrogen peroxide (iHP) are the three industry-standard techniques.17,30 Sterilization using hydrogen peroxide is a low-temperature technique preferable for cleaning medical equipment (i.e., endoscopes) that cannot withstand damage from the high temperature and humidity of autoclaving.
Reports on the use of HPV, HPGP, and iHP for decontamination of N95 respirators have shown varying levels of success. All three techniques successfully decontaminated N95 masks with pathogens more resistant than SARS-CoV-2, such as Geobacillus stearothermophilus (6-log spore reduction following treatment), and influenza A virus subtype H1N1 (5-log reduction following treatment).17,21,24,25,29,30 With HPV use, no change in N95 mask filter quality (filter efficiency > 98% and no change in airflow resistance) or fit was shown with three cycles of treatment (further claims of up to 20 cycles exist but are not validated: https://www.fda.gov/media/136386/download).24 With HPGP, no change in N95 respirator filtering quality is observed with one cycle of decontamination, but three or more cycles can impair filtration by greater than 5%.36 With iHP, no reports have evaluated respirator filter quality or fit, although far less investigation has been performed for this method. Of note, N95 respirators containing cellulose materials cannot be decontaminated using HPV and HPGP systems. The cellulose-containing materials absorb the hydrogen peroxide leading to decreased vapour concentrations and a potentially compromised or prematurely terminated decontamination cycle. A list of N95 respirators with and without cellulose is included in Table 2.
TABLE 2 Examples of commercially available N95 respirators with and without cellulose The HPV/HPGP decontamination process consists of five steps: conditioning, pre-gassing, gassing, gassing dwell, and aeration.24,29,31 The latter phase allows for off-gassing and breakdown of HPV into oxygen and water vapour to minimize the risk of chemical contamination to the subsequent user. This process contrasts with other cleaning methods, such as formaldehyde and ethylene oxide, where significant chemical contamination can remain. The mechanism of action by which HPV eradicates microorganisms is primarily via oxidation, with the generation of hydroxy free radicals that can breakdown the cell wall and intracellular structures of microorganisms.31
The main drawback of HPV/HPGP is the lack of availability of the necessary machinery for the procedure. The four most common systems available are the Battelle Critical Care Decontamination System (CCDS, Battelle, Columbus, OH, USA) (HPV), Advanced Sterilized Products STERRAD system (HPV and HPGP, Advanced Sterilization Products, Irvine, California), the STERIS V-Pro sterilizers (HPV, Steris, Mentor, Ohio), and the ClarusR Bioquell system (HPV and HPGP, Bioquell, Andover, UK). Despite limited evidence on the effectiveness of hydrogen peroxide for the decontamination of SARS-CoV-2, the United States Food and Drug Administration (FDA) issued an emergency use authorization on 29 March 2020 for the use of the Battelle CCDS system for the decontamination of single-user N95 masks (up to 20 cleanings) (https://www.fda.gov/media/136386/download). Other hydrogen peroxide sterilization systems, such as models by STERIS and Advanced Sterilization Products, have since received similar emergency use authorizations by the FDA for the decontamination of non-cellulose-containing N95 masks for single-user reuse (https://www.fda.gov/media/136843/download and https://www.fda.gov/media/136882/download), but only for two decontamination cycles. Single-user reuse refers to the return of a specific N95 respirator to the original HCW who used the mask. Both standard and express cycles of the devices have been granted emergency use authorization.
Similar to HPV, ethylene oxide (EtO) gas has a long history of being used as a sterilant for healthcare materials, including heat- or moisture-sensitive medical devices, without deteriorating device elements (including a Centers for Disease Control and Prevention protocol for medical equipment sterilization, https://www.cdc.gov/infectioncontrol/guidelines/disinfection/sterilization/ethylene-oxide.html). Nevertheless, inhaled EtO is a known human carcinogen, and short-term exposure to EtO irritates the eyes and mucous membranes, and can lead to seizures, coma, and potentially death.32
There are a limited number of studies investigating the sterilizing effects of EtO on N95 masks. It has been shown that EtO decontamination does not impact the filter aerosol penetration, filter airflow resistance, or physical appearance of N95 masks.24,29 Nevertheless, these studies did not evaluate the efficiency of the decontamination methods to inactivate microorganisms. While residual EtO was below permissible exposure limits, two potential toxins (diacetone alcohol and ethylene glycol monoacetate) were detected after treatment of N95 rubber straps. To date, no clinical study of EtO decontamination of N95 masks has been conducted.
Ultraviolet light
The use of ultraviolet (UV) light, termed UV germicidal irradiation (UVGI), has also been suggested for N95 decontamination.19,33 No specific studies of UVGI were identified that matched our search criteria though there were at least two studies that might yet appear once they undergo peer review. For example, a method specifically aimed at sterilizing N95 respirators for SARS-CoV-2 that employs UV light between 100 and 280 nm was recently described (but is not yet peer-reviewed, https://www.nebraskamed.com/sites/default/files/documents/covid-19/n-95-decon-process.pdf) and simulated sunlight was shown to be SARS-CoV-2 viricidal.34 Biosafety cabinets with a manufacturer-reported fluence of 100 μWcm−2 were reported to effectively sanitize masks for reuse after approximately 15–20 min per side (pre-print, https://doi.org/10.1101/2020.03.25.20043489). As a proof of concept, light sensors were used to confirm that the entire surface area of the mask received an appropriate dose of radiation without shadow. Nevertheless, none of these studies measured live virus after treatment, and other UV tests illustrated a lack of biological decontamination.19 The use of UVGI may not inactivate viruses that have penetrated into the innermost layers of the N95 where UV transmission is reduced. Furthermore, there is a UV-mediated degradation of polymers and the maximum number of cycles has not been determined. Reassuringly, the elastic straps retain their structural integrity even at high UV doses.35
Post-decontamination barrier integrity testing
Before reuse, cleaned masks should undergo testing to ensure they can still provide an effective barrier for nano-sized particles such as SARS-CoV-2. Testing generally assesses “worst-case scenario” use of N95 masks by simulating prolonged wear in high exposure environments.
Aerosolized particles of differing sizes, characteristics (inert, biological, lipophilicity), and concentrations are flowed (cyclical or constant flows > 85 L·min−1 ± heat ± humidity) across N95 masks.38,39 Humidity is relevant as it reduces the electrostatic barrier and filtration capacity of N95 masks, particularly around the most penetrating particle size (MPPS; 0.05 µm).38,40 Detectors for measuring N95 mask filtration vary in their sensitivity to detect nanoparticles, important given the reported size of SARS-CoV-2 and the known MPPS for N95 masks.40,41
Testing N95 masks specifically for barrier capabilities against small viruses (viral filtration efficiency [VFE]), such as the SARS-CoV-2, is complicated by factors related to the virus, mask, and airflow. As such, VFE is rarely undertaken but is rather implied by successful proof of filtration efficiencies > 95% to inert compounds. In keeping with this concept, published accounts of filtration efficiencies of N95 masks show > 99% FE for the test bacteria and viruses.42 While the use of non-pathogenic viruses as surrogates for SARS-CoV-2 makes intuitive sense for testing N95 barrier function, they may not represent the most rigorous means of testing N95 barrier effectiveness to live particles. Biological agents as test particles are oftentimes inconsistent.43 There are considerable differences within or between bacterial and viral strains, yielding significant variability in size, electrostatic, and hydrophilic properties. Additionally, using biological substrates seldom represents the MPPS for N95 filters. The N95 filtration mechanisms combine to prevent penetration of a wide range of particles. The MPPS depends on filter properties, filtration mechanisms, airflow rates (L·min−1), types of flows (cyclical vs constant), filter fibre charge density, and aerosol particle charge distribution.38,44,45,46 Therefore, most N95 mask tests are not predicated on emulating all the physical properties of the pathogen in question. Rather, they are designed to test particles at the MPPS, where the N95 mask is most vulnerable, and assume that all other particle types will experience superior relative filtration efficiencies. As such, the reported filtration of biological particles that differ from the MPPS should be reported as better than 95%.22,39,47 Evaluated testing methods are summarized in Table 3. The sodium chloride aerosol test involving light photometry detection remains the National Institute for Occupational Safety and Health standard but is not readily available outside of commercial labs. Particle counters are more commonly found in occupational health and safety departments and are readily available for sale or rental from third party vendors.
TABLE 3 N95 retesting strategies