Twelve methods were assessed in the 17 papers: hydrogen peroxide, ultraviolet irradiation, ethylene oxide, dry heat, moist heat/pasteurization, ethanol, microwaving, sodium hypochlorite (NaClO), autoclaving, electric rice cooker, cleaning wipes, and isopropanol solutions.
1. Hydrogen peroxide
Hydrogen peroxide was evaluated in its liquid, plasma and gas/vapor forms by six laboratory studies.21–26 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 clips21 and degradation in their filtering performance.23 Treatment with liquid hydrogen peroxide caused oxidation of the metal clips23 but also inactivated the influenza H1N1 virus.24 When used as steam, hydrogen peroxide did not leave residual chemical products,22 the integrity and filtering capacity of the mask were maintained,23,25,26 and the method was effective in eliminating SARS-CoV-225 and spores of Geobacillus stearothermophilus.26
2. Ultraviolet germicidal irradiation (UVGI)
The effect of ultraviolet germicidal irradiation on the functional integrity of the masks was evaluated by 11 studies22,23,25,27−34 and its effect on the elimination of microorganisms by six studies20,25,29,30,33,34, with power calculations with different doses and application times. 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.22,23,25,27−34 The method was effective against the influenza virus H5N1,20 SARS-CoV-2,25 bacteriophages MS2,29 and influenza virus H1N1.30,33 However, even after 20 min of irradiation with 365 nm UVA the relative survival of Bacillus subtilis spores remained above 20%.34
3. Ethylene oxide
Evaluated by three studies,21–23 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 (temperatures between 70 and 85 °C) as a decontamination method did not affect the structural characteristics of the masks under various humidity conditions (≤ 100% RH).25,27 The filtering efficiency remained acceptable up to 50 cycles.27 At 70 ºC the method was effective against the SARS-CoV-2.25
5. Moist heat / pasteurization
Five studies20,23,27,30,31 evaluated the effect of moist heat between 60 and 100 °C. The method did not alter mask fit, odor or comfort.20,23,27,30,31 In one study27 filtering capacity was reduced to 80% after 10 cycles. Moist heat (65±5 °C for 3 h) was also effective in eliminating H1N130 and H5N120 viruses.
6. Ethanol
Different methods of decontamination with ethanol were tested: spray,25 immersion,27,35 and pipette drips.34 Results were divergent between methods. The filtration efficiency of masks was degraded to unacceptable levels when they were immersed in alcohol.27,35 Mask filtration performance was not significantly reduced after single ethanol sprays which were also effective in eliminating SARS-CoV-2.25 Subsequent rounds of spraying caused sharp drops in filtration performance.25 Pipette drips were not effective in eliminating Bacillus subtilis spores.34
7. Isopropanol solution
The filtering capacity of N95 masks was changed after 10 minute immersions in 100% isopropanol solution.35 Effects on microorganisms were not evaluated.
8. Microwave oven
Six studies tested the use of microwave ovens in the disinfection of N95 masks.20,21,23,30,31,36 The type of commercial furnace, maximum temperature, and time protocols varied between the studies (Table 3, Supplementary material 5). When masks were placed directly on the rotating plate of the microwave without protection, two commercial models of the tested masks melted.21 When the masks were placed in containers with water20,23,30,31 or in steam bags specifically marketed for microwave ovens36 no residual odor was observed. 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 H5N120 and H1N1 influenza viruses30 and bacteriophage MS2.36
9. Sodium hypochlorite (NaClO)
Six studies21–23, 27,34,35 evaluated the use of hypochlorite at different concentrations and application methods (Table 3, Supplemental material 5). 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.27 A second found that application of sodium hypochlorite discolored the metallic components of the masks and left a characteristic smell of bleach.21 Finally, one treatment caused the release of low levels of hydrochloric gas.22 On the other hand, two studies23,35 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.34
10. Autoclave
Autoclave disinfection was effective in eliminating Bacillus subtilis spores,34 however, it visibly altered the structural integrity of the masks.35
11. Electric rice cooker
Despite showing 99–100% biocide efficacy against Bacillus subtilis34 spores after using dry heat for 3minutes (149–164 °C, without adding water), the method visibly changed the structural integrity of the mask.35
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.37 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.