Search results and study selection process
The complete search strategies can be found in Table 1 of Supplementary material 2. Initial searches retrieved 552 articles (Medline: 381, Cochrane: 52 and EMBASE: 119). Manual searches detected 11 additional publications. Mendeley citation management software was used for automatic removal of duplicate articles, remaining 301 studies. Two reviewers (LFP and AIQC) independently screened the 301 studies using the Rayyan systematic review application to screen abstracts and titles.23 Of these, 240 were excluded for not meeting the inclusion criteria. The full texts of 61 studies were screened by two reviewers (LFP and AIQC) and thirteen additional studies were excluded. Excluded full text studies and the reasons for exclusion are listed in Table 2 of Supplementary material 3. At the end, 48 studies were selected for the full review process (Figure 1).
Data extraction
Two reviewers (LFP and AIQC) independently used a pre-specified data extraction sheet form designed to obtain the specific data required for this review. The data extracted from the primary studies were: data related to the author, year, study objective, intervention, comparator, commercial mask model, target microorganism, results and conclusions of the study authors, limitations and detailed description of the decontamination process making its reproducibility in other scenarios possible.
Characterization of the included studies
The specifics of each study (cycles, temperatures, protocols, densities, exposure time, technology used and results) are described in Table 3 of Supplementary material 4 and the differences before and after decontamination are shown in Table 4 of Supplementary Material 5 for outcomes: filter aerosol penetration, filter airflow resistance and filtration efficiency. Results of the studies were summarized based on decontamination method and results for the two issues:24
- Whether the device maintains its structural characteristics and provides an adequate level of protection after the decontamination method, without any risk of exposure for the health professional to exposure to inhalable chemical residues that may have remained after the method used. The penetration of 0.3 μm (aerodynamic mass mean diameter) of sodium chloride aerosols aerosol particles through a certified N95 respirator cannot exceed 5%.25
- Whether the decontamination method used was effective in reducing or completely eliminating the infectious capacity of the target organism without any risk of exposure for the health professional to contamination.
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).24,26–72
1. Hydrogen peroxide
Hydrogen peroxide was evaluated in its liquid, plasma and gas/vapor forms by nineteen laboratory studies.28,35–39,43,47,53–57,60,61,64,68,70,71 The effect of hydrogen peroxide on the filtering capacity varied according to the method used. Hydrogen peroxide plasma led to changes in the masks’ metallic nasal clips61 and degradation in their filtering performance.37 Treatment with liquid hydrogen peroxide caused oxidation of the metal clips37 but also inactivated the influenza H1N1 virus.70 When used as steam, hydrogen peroxide did not leave residual chemical products,57 the integrity and filtering capacity of the mask were maintained,28,37,73 and the method was effective in eliminating SARS-CoV-228 and spores of Geobacillus stearothermophilus.57
2. Ultraviolet germicidal irradiation (UVGI)
The effect of ultraviolet germicidal irradiation on N95 respirators masks was evaluated by 21 studies24,28,30,33,37,42,45–48,50,51,53,56,58–63,65 and there was a difference between studies in relation to UVGI doses and application of 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.24,28,30,37,42,45,46,48,53,56,60–62 However, different commercial brands of N95 models resisted differently in terms of performance penetration after multiple cycles and doses applied.46 The method was effective against the influenza virus H5N1,24 SARS-CoV-2,28 bacteriophages MS2,30 and influenza virus H1N1.33,50 At the same time, even after 20 min of irradiation with 365 nm UVA the relative survival of Bacillus subtilis spores remained above 20%.45
3. Ethylene oxide
Evaluated by four studies,37,56,60,61 the effectiveness of ethylene oxide (EtO) depended on the type of sterilization equipment used, whether there was a hot cycle, and exposure to EtO. 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 and by-products. None of the studies reported the effectiveness of EtO treatments on microorganisms.
4. Dry heat
The use of dry heat was evaluated for by eleven different experimental studies.27,28,40–42,47,52,59–61,66 Temperatures between 70 and 85 °C did not affect the structural characteristics of the masks under various humidity conditions (≤100% RH).28,42 The filtering efficiency remained acceptable (≥95%) up to 50 cycles at 85 °C and 30% of RH.42 Dry heat at 70ºC was effective against the SARS-CoV-2 for 1–2 rounds of decontamination but should not be used for 3 rounds.28
5. Moist heat / pasteurization
Nine studies24,26,27,33,37,42,48,49,62 evaluated the effect of moist heat between 60 and 100°C. The method did not alter mask fit, odor or comfort.24,33,37,42,62 In one study42 filtration efficiency has a significant drop after cycle 5.
Moist heat (65±5 °C for 3h) was also effective in eliminating H1N133 and H5N124 viruses.
6. Ethanol
Different methods of decontamination with ethanol were tested: spray,28 immersion,31,42,44 and pipette drips.45 Results were divergent between methods. The filtration efficiency of masks was degraded to unacceptable levels when they were immersed in alcohol.31,42,44 Mask filtration performance was not significantly reduced after single ethanol sprays which were also effective in eliminating SARS-CoV-2.28 Subsequent rounds of spraying caused sharp drops in filtration performance.28 Pipette drips were not effective in eliminating Bacillus subtilis spores.45
7. Isopropanol solution
The filtering capacity of N95 masks was changed and the particle penetration through the N95 mask exceed 5% after they had been submerged in isopropanol solution.44,60 Effects on microorganisms were not evaluated.
8. Microwave oven
Eleven studies tested the use of microwave ovens in the disinfection of N95 masks.24,29,30,33,37,48,52,60–62,67. 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.61 When the masks were placed in containers with water24,33,37,62 or in steam bags specifically marketed for microwave ovens no residual odor was observed.30 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 H5N161 and H1N1 influenza viruses62 and bacteriophage MS2.30
9. Sodium hypochlorite (NaClO)
Ten studies29,37,42,44,45,53,56,60,61,63 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 between studies. One study found that one round of disinfection drastically degraded filtration efficiency to unacceptable levels.42 A second found that application of sodium hypochlorite discolored the metallic components of the masks and left a characteristic smell of bleach.61 Finally, one treatment caused the release of low levels of hydrochloric gas.56 On the other hand, two studies37,44 reported that the method did not affect the filtration or airflow resistance performance of the masks. Only one of the studies tested sodium hypochlorite as a disinfectant, reporting that it was effective in eliminating Bacillus subtilis spores.45
10. Autoclave
As it is an equipment present in all hospital environments, the autoclave was evaluated by 7 studies with the perspective of being an accessible method.31,32,44,45,60,69,72 Autoclave disinfection was effective in eliminating Bacillus subtilis spores,45 however, had little altered the structural integrity of the masks.44
11. Electric rice cooker
Despite showing 99–100% biocide efficacy against Bacillus subtilis45 spores after using dry heat for 3minutes (149–164 °C, without adding water) and the treatment (for 13-15 minutes, including 8-10 minutes of heating and 5 minutes of steam) also resulted in a greater than 5 log10 reduction in bacteriophage MS2 and methicillin-resistant S aureus.40 However, the method visibly changes the mask's integrity.44
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.34 The three mask models withstood handling and abrasion during the disinfection process. All were successfully disinfected against atypically high microbe levels by wipes containing antimicrobial agents but the inert wipes did not produce adequate disinfection.
13. Bar soap and water
Average penetration had markedly increased for N95 respirators after being submerged and the authors hypothesized the soap could have removed the charge on the fibers similar to the effect observed with isopropanol solution exposure.60
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.59
15. Chlorine dioxide (ClO2).
The method significantly changed the filtering efficiency of the tested masks.68