Results of the search.
We identified 552 records through searches on databases (Medline: 381, Cochrane: 52 and EMBASE: 119) and 11 records detected through other resources. The complete search strategies with the results for each database can be found in Table 1 of Supplementary material 2. Mendeley citation management software was used for automatic removal of duplicate articles, leaving 301 remaining studies. Two reviewers (LFP and AIQC) independently screened the 301 studies using the Rayyan systematic review application to screen abstracts and titles.30 Of these, 240 were excluded for not meeting the inclusion criteria. 61 studies full texts were screened by two reviewers (LFP and AIQC). At this stage, we excluded 13 full-text reports and provided the reasons for exclusions and their references in Table 2 of Supplementary material 3. Disagreements during the selection process were resolved by discussion with a third review (SMLVO). Finally, 48 studies were selected for the full review process. The results of these 48 included studies are summarized in the following narrative results and in details in Table 3 of Supplementary material 4 and Table 4 of Supplementary material 5. We provided a diagram of the search and study selection process in a PRISMA flow-chart (Figure 1).
Fifteen methods were assessed in the 48 papers: hydrogen peroxide, ultraviolet irradiation, ethylene oxide, dry heat, moist heat/pasteurization, ethanol, isopropanol solution, microwaving, sodium hypochlorite (NaClO), autoclaving, electric rice cooker, cleaning wipes, bar soap and water, Multi-Purpose High-Level Disinfection Cabinet (Altapure, Mequon, WI) and chlorine dioxide (ClO2).27,31–77 Each method will be briefly analyzed bellow:
1. Hydrogen peroxide
Hydrogen peroxide was evaluated in its liquid, plasma and gas/vapor forms by nineteen laboratory studies.33,40–44,48,52,58–62,65,66,69,73,75,76. The effect of hydrogen peroxide on the filtering capacity varied according to the method used. The average penetration was not significantly changed when the masks were submersed in 3% or 6% hydrogen peroxide liquid or treated with Vaporized Hydrogen Peroxide (STERRAD®) for one cycle65. However, it resulted in mean penetration levels > 5% after 3-Cycles.42 The integrity and filtering capacity of the mask were preserved when Hydrogen Peroxide was used as steam.33,42,78
Additionally, studies have shown Hydrogen Peroxide led to changes in the masks’ metallic nasal clips.42,65,66 As for the ability to eliminate microorganisms, SteraMist™ Binary Ionization Technology® (BIT™) was effective against Influenza A virus subtype H1N175 and Geobacillus stearothermophilus spores76 and STERRAD 100NX sterilization system eliminated SARS-CoV-2, Staphylococcus aureus and Acinetobacter baumannii.40
On the other hand, vaporized hydrogen peroxide (VHP) was effective in eliminating SARS-CoV-233, Geobacillus stearothermophilus spores62, Porcine respiratory coronavirus (PRCV)52, Escherichia coli, Mycobacterium smegmatis and spores of Bacillus stearothermophilus60 and 3 aerosolized bacteriophages: T1, T7, and Pseudomonas phage phi-6.43
2. Ultraviolet germicidal irradiation (UVGI)
The effect of ultraviolet germicidal irradiation on N95 respirator masks was evaluated by 21 studies27,33,35,38,42,47,50–53,55,56,58,61,63–68,70 and there was a difference among studies in relation to UVGI doses and application time periods. In general, UVGI did not affect the integrity and ability of the masks to filter aerosols or adapt to the face, nor did it leave a smell, irritating/toxic residues, or create important changes in appearance even when multiple cycles were performed.27,33,35,42,47,50,51,53,58,61,65–67 However, different commercial brands of N95 models resisted differently in terms of performance penetration after multiple cycles and doses applied.51 The most efficient N95 brand in terms of long standing penetration performance was 3M 9210, but the mean penetration values for all brands were 5% or less both before and after exposure.51 The most efficient N95 brand in terms of long standing flow resistance performance was 3M 9210, but the mean flow resistance values was less than 1% of the initial value for all brands.51
Ultraviolet germicidal irradiation was effective against MS2 coliphage (ATCC 15597-B1)68, the influenza virus H5N127 SARS-CoV-233,56,63, bacteriophages MS235,70, influenza virus H1N138,55, Porcine respiratory coronavirus (PRCV).52 However, at the same time, one study reported that even after 20 min of irradiation with 365 nm UVA the relative survival of Bacillus subtilis spores remained above 20%50 and another one highlighted the UV-C technologies tested did not meet pre-established criteria for decontamination against Methicillin-resistantStaphylococcus aureus (MRSA), Bacteriophage Phi6 and Bacteriophage MS2.64, which is worrisome.
3. Ethylene oxide
Evaluated by four studies, the effectiveness of ethylene oxide (EtO) depended on the type of sterilization equipment used, whether there was a hot cycle, and exposure to EtO.42,61,65,66 The process did not affect the filtration, resistance, odor or appearance of the masks. The main limitations of the method were the processing time and the presence of toxic residues. It is also important to note that none of the studies tested the effectiveness of EtO treatments against microorganisms. Thus, there is no evidence that EtO can eliminate any microorganism from N95 masks.
4. Dry heat
The use of dry heat was evaluated for by eleven different experimental studies.32,33,45–47,52,57,64–66,71 Temperatures between 70 and 85 °C did not affect the structural characteristics of the masks under various humidity conditions (≤100% RH).33,47 Also, filtering efficiency remained acceptable (≥95%) up to 50 cycles at 85 °C and 30% of RH.47 When an oven was used at 70ºC, filtration performance was maintained if only one cycle was performed.33 However, there were no detectable changes in aerosol filtration efficiency, even after three reprocessing cycles when the masks were subjected to the same temperature of 70ºC when laboratory MINI/6 incubator (Genlab Ltd) was used to provide dry heat.32
These results concerning dry heat reinstates that decontamination attempts must be conducted in identical manners and conditions to the ones performed in the laboratory. Not only regarding the temperature, but also regarding the device used. Nevertheless, the effect against different microorganisms varied greatly between different studies due to the decontamination cycle time.
Dry heat treatment (70°C for 60 min) was effective against SARS-CoV-232,33, however, dry heat (70°C for 30 min) had limited effectiveness against bacteriophages MS2 and Phi6 versus methicillin-resistant Staphylococcus.64 Also, dry heat at 100°C for 15 minutes did not eliminate Methicillin-resistant Staphylococcus aureus (MRSA) and Nonenveloped single-stranded RNA virus bacteriophage MS2.45
5. Moist heat / pasteurization
Nine studies27,31,32,38,42,47,53,54,67 evaluated the effect of moist heat between 60 and 100°C. The method did not alter mask fit, odor or comfort.27,38,42,47,67 In one study47 filtration efficiency presented a significant drop after cycle 5 when stacked on top of a beaker with boiling water inside (around 15 cm above the water), although, filtration efficiency was not affected when masks were subjected to 5 cycles with a lower temperature – at 85ºC + 60-85% humidity.31 Moist heat (65±5 °C for 3h) was effective in eliminating H1N138 and H5N127 viruses, in addition, when masks were steamed in boiling water the treatment was effective against the Avian infectious bronchitis virus H120 without changing.54 Another study also has shown that a single heat treatment for thermal disinfection in cycles of 60 minutes at 70°C at 50% eliminated SARS-CoV-2 and Escherichia coli32.
Different methods of decontamination with ethanol were tested: spray,33 immersion,36,47,49 and pipette drips.50 Results were divergent between methods. The filtration efficiency of masks was degraded to unacceptable levels when they were immersed in alcohol.36,47,49 Mask filtration performance was not significantly reduced after single ethanol sprays which were also effective in eliminating SARS-CoV-2.33 Subsequent rounds of spraying caused significant decrease in efficacy in filtration performance.33 Nonetheless, pipette drips were not effective in eliminating Bacillus subtilis spores.50
7. Isopropanol solution
The filtering capacity of N95 masks was changed and the particle penetration through N95 mask exceeded 5% after they had been submerged in isopropanol solution.49,65 Although effects on microorganisms were not evaluated, this method could be further studied in order to check these effects.
8. Microwave oven
Eleven studies tested the use of microwave ovens in the disinfection of N95 masks.27,34,35,38,42,53,57,65–67,72. The type of commercial furnace, maximum temperature, and time protocols varied between the studies (Table 3, Supplementary material 4). When masks were placed directly on the rotating plate of the microwave without protection, two commercial models of the tested masks melted.66 When the masks were placed in containers with water27,38,42,67,72 or in steam bags specifically marketed for microwave ovens no residual odor was observed.35 In addition, there were no structural changes affecting adjustment on the face, filtration capacity, or resistance to airflow and none of the metal components melted or combusted. Microwaving the masks was effective in eliminating H5N166 and H1N1 influenza viruses67, bacteriophage MS235,72 and Staphylococcus aureus.57
9. Sodium hypochlorite (NaClO)
Ten studies34,42,47,49,50,58,61,65,66,68 evaluated the use of hypochlorite at different concentrations and application methods (Table 3, Supplementary material 4). The maintenance of mask integrity and filtering capacity varied among studies. For instance, one round of disinfection drastically degraded filtration efficiency to unacceptable levels when samples were left to air dry and off-gas completely, hanging47 or submersed in 0.5% sodium hypochlorite for 10 min.49 Application of sodium hypochlorite discolored the metallic components of the masks which, unfortunately, caused a characteristic smell of bleach.66
One of the study treatment caused the release of low levels of hydrochloric gas.61 Hypochlorite concentrations of 0.006%, 0.06%, and of 0.6% was not effective against Coliphage MS234. On the other hand, when higher sodium hypochlorite doses (>8.25 mg/liter) were used for the dilute solutions containing 2.75 to 5.50 mg/liter during 10-min decontamination period MS2 coliphage was inactivated.68 When 5.4%, 2.7% or 0.54% NaOCl was used the method was effective in eliminating Bacillus subtilis spores.50
Considered an accessible method, as it is an equipment present in all hospital environments, autoclave decontamination was evaluated by 7 studies.36,37,49,50,65,74,77 Autoclave disinfection was effective in eliminating Bacillus subtilis spores,50 however, the integrity of the mask was altered.37,49,65 Two studies pointed that even though masks were able to retain their structural integrity and efficacy, no results were provided regarding the filtering capacity and only the fittest was performed.74,77 In addition, a survey conducted in 2020 showed the degree of integrity of the mask can be preserved depending on the model tested (Table 3, Supplementary material 4).36
11. Electric rice cooker
The method showed 99–100% biocide efficacy against Bacillus subtilis50 spores after using dry heat for 3 minutes (149–164 °C, without adding water). Also, the treatment for 13-15 minutes, including 8-10 minutes of heating and 5 minutes of steam resulted in a greater than 5 log10 reduction in bacteriophage MS2 and methicillin-resistant S aureus.45 However, the method visibly changes the mask's integrity.49
12. Cleaning wipes
The effectiveness of commercial wipes containing 0.9% hypochlorite, benzalkonium chloride or no active antimicrobial ingredients was evaluated in masks contaminated with Staphylococcus aureus and mucin.39 The three mask models, 3M-1860S, 3M-1870 and Kimberly-Clark-46727-PFR, withstood handling and abrasion during the disinfection process. All were successfully decontaminated against atypically high microbe levels by wipes containing antimicrobial agents, however, inert wipes did not produce adequate decontamination.
13. Bar soap and water
Average penetration has markedly increased for N95 respirators after being submerged. Authors have hypothesized the soap could have removed the charge on the fibers similar to the effect observed with isopropanol solution exposure.65
14. Multi-Purpose High-Level Disinfection Cabinet (Altapure, Mequon, WI)
The treatment was effective against microorganisms and the researchers reported no visible changes in the masks. However, the efficiency of the filtration has not been confirmed.64
15. Chlorine dioxide (ClO2).
The method significantly changed the filtering efficiency of the tested masks which makes this method not worth using or testing in further studies.73