key: cord-1046654-ruhk53ob
authors: Gertsman, Shira; Agarwal, Anirudh; O’Hearn, Katharine; Webster, Richard; Tsampalieros, Anne; Barrowman, Nick; Sampson, Margaret; Sikora, Lindsey; Staykov, Emiliyan; Ng, Rhiannon; Gibson, Jess; Dinh, Tri; Agyei, Kwame; Chamberlain, Graham; McNally, James Dayre
title: Microwave- and Heat-Based Decontamination of N95 Filtering Facepiece Respirators: A Systematic Review
date: 2020-08-22
journal: J Hosp Infect
DOI: 10.1016/j.jhin.2020.08.016
sha: 68266bff7ec5cab0f3bf0d5541bcdd108e20c3d1
doc_id: 1046654
cord_uid: ruhk53ob

BACKGROUND: In pandemics such as COVID-19, shortages of personal protective equipment are common. One solution may be to decontaminate equipment such as facemasks for reuse. AIM: To collect and synthesize existing information on decontamination of N95 filtering facepiece respirators (FFRs) using microwave and heat-based treatments, with special attention to impact on mask function (aerosol penetration, airflow resistance), fit, and physical traits. METHODS: A systematic review (PROSPERO CRD42020177036) of literature available from Medline, Embase, Global Health, and other sources was conducted. Records were screened independently by two reviewers, and data was extracted from studies that reported on effects of microwave- or heat-based decontamination on N95 FFR performance, fit, physical traits, and/or reductions in microbial load. FINDINGS: Thirteen studies were included that used dry/moist microwave irradiation, heat, or autoclaving. All treatment types reduced pathogen load by a log10 reduction factor of at least three when applied for sufficient duration (>30s microwave, >60 min dry heat), with most studies assessing viral pathogens. Mask function (aerosol penetration <5% and airflow resistance <25mmH(2)O) was preserved after all treatments except autoclaving. Fit was maintained for most N95 models, though all treatment types caused observable physical damage to at least one model. CONCLUSIONS: Microwave irradiation and heat may be safe and effective viral decontamination options for N95 FFR reuse during critical shortages. The evidence does not support autoclaving or high-heat (>90(o)C) approaches. Physical degradation may be an issue for certain mask models, and more real-world evidence on fit is needed.

During the SARS-CoV-2 pandemic, protecting frontline healthcare workers is of the utmost importance. As SARS-CoV-2 can be transmitted through airborne particles, the U.S.

(PHAC) have recommended the use of N95 filtering facepiece respirators (FFRs) when performing aerosol-generating procedures in suspected COVID-19 patients [1] [2] [3] . N95 FFRs filter out a minimum of 95% of airborne particles and are the personal protective equipment (PPE) preferred by healthcare workers during serious outbreaks of aerosol-borne viruses [4, 5] .

It is widely understood that single-use of FFRs is not sustainable in a pandemic such as COVID-19 [6] [7] [8] . FFR decontamination has been proposed as a safer method than standard "limited reuse" [9] , which involves no disinfection between wears [10, 11] . However, any decontamination method must preserve the structural and functional characteristics of the mask (namely fit, aerosol penetration, and airflow resistance) or it may increase risk to healthcare workers [12] . The lack of clear consensus on how to achieve safe decontamination of single-use FFRs has discouraged manufacturers and public health experts from endorsing decontamination protocols [13] , although several analyses of FFR-decontamination methods have been published and the CDC has provided suggestions for decontamination in critical situations [14] . Previous work has evaluated methods including radiation (UV-C, microwaves), moist heat (autoclaves), and chemical disinfectants (bleach, ethanol, hydrogen peroxide) [15] [16] [17] [18] , which vary in their relative efficacies and feasibilities. For example, not all institutions have access to large UV lamps, and chemical disinfection may cause significant damage to FFRs or leave hazardous J o u r n a l P r e -p r o o f residues [19, 20] . Microwaves and heat are known to inactivate viruses and bacteria [21] [22] [23] [24] [25] , including coronaviruses [26, 27] , and can be accessible and affordable [28] ; however, heat and humidity may impact the electrostatic charges that confer the high filtration efficiency of the polypropylene filter in N95 FFRs [29] [30] [31] .

To help inform FFR-reuse policies and procedures, our team has conducted three systematic reviews to synthesize existing published data regarding the effectiveness of ultraviolet germicidal irradiation (UVGI), heat, microwave irradiation, and chemical disinfectants for N95 FFR decontamination [17, 32] . This review will focus on microwave-and heat-based decontamination with the following objectives: (1) to assess the effects of microwave irradiation and heat on FFR performance, with specific foci on aerosol penetration and airflow resistance;

(2) to determine how effectively microwave irradiation and heat reduce viral or bacterial load on FFRs; and (3) to describe changes in FFR fit or physical traits caused by microwave irradiation or heat.

The study methods were established a priori. The protocol was submitted to PROSPERO on 29 th March 2020 (CRD42020177036) and uploaded to Open Science Framework (OSF) on 30 th March 2020 (https://osf.io/4se6b/) [33] . This systematic review is reported according to PRISMA guidelines (Appendix A) [34] .

Eligible studies met the following criteria: 1) Study was an original article or systematic review;

2) Study investigated decontamination of N95 (including surgical N95) filtering facepiece respirators or their components; 3) Study included a decontamination arm involving microwave irradiation or heat treatment; 4) At least one of the following post-treatment outcomes was reported: i) FFR performance (aerosol penetration, airflow resistance); ii) reduction in viral/bacterial load; iii) mask fit; iv) changes in physical traits. Articles also had to be available in English or French and published after 1972, the first year that an FFR was approved by the National Institute for Occupational Safety and Health (NIOSH) [35] . We excluded abstract-only publications, study protocols, guidelines, commissioned reports, editorials, narrative reviews, book chapters, and patents. 

Titles and abstracts were uploaded to InsightScope (www.insightscope.ca) for title/abstract and full text screening. At both levels of screening, citations were assessed independently in duplicate by a team of six reviewers from CHEO (a pediatric academic hospital in Ottawa, Canada), the University of Ottawa, and McMaster University. To ensure that all reviewers understood the eligibility criteria, the study leads (SG, AA) constructed a test set of 30 citations in which five met all study criteria (true positives) and 25 did not (true negatives). Before gaining access to title/abstract screening, each reviewer was required to complete the test set and achieved a sensitivity of at least 80%. At both title/abstract and full text screening, records were removed only if both reviewers agreed it to exclude; any conflicts were reviewed and resolved J o u r n a l P r e -p r o o f by one of the study leads. Subsequently, the study leads reviewed the eligible citations to eliminate duplicates and confirm eligibility. The study leads developed an extraction tool for demographic and methodology data using REDCap tools hosted at CHEO and piloted the tool on five eligible studies [36, 37] . Based on the data collected on REDCap, the study leads created and piloted spreadsheets (Microsoft Excel) to collect data on post-decontamination aerosol penetration, airflow resistance, germicidal effects, fit, and physical traits. In both phases of data extraction, eligible studies were divided equally among the reviewers for duplicate, independent data extraction, followed by conflict resolution by the study leads. Data from figures were extracted by a reviewer using SourceForge Plot Digitizer (http://plotdigitizer.sourceforge.net/) and cross-verified by a second reviewer. All extracted data and meta-data of all records screened are available on OSF [38] .

All statistical analyses were performed using the R statistical programming language [39] .

Where two or more studies measured the same outcome using the same intervention type, crossstudy data was meta-analyzed using a random effects model with the R package 'meta' [40] .

Variability between point estimates of studies was calculated by taking the standard deviation (aerosol penetration and airflow resistance) or standard error (germicidal effects) across the means. Heterogeneity was assessed using an I 2 statistic; if I 2 ≥ 75%, the pooled estimate was not reported.

Where standard deviation or standard error were not reported and could not be calculated across the means, generic imputation was used. If no arms within a study had a value for uncertainty, the average value between studies was imputed for missing data. For studies that performed the same decontamination intervention on different study arms (e.g. variable mask types, durations of exposure, heat temperatures, transmission modes), within-study data were averaged.

Germicidal data was reported as log 10 pathogen reduction factor calculated from absolute pathogen loads or relative survival if the log 10 reduction factor was not reported directly in the article. For values below minimum detectable limits, we adopted the strategy described by

Heimbuch et al. for imputation of log10 reduction factor: "Based on a US Environmental Protection Agency guideline [41] , half of the limit of detection was used to calculate log reductions for treated samples that had no detectable virus." [15] For Fisher et al.'s 2011 study [42] , the difference in viral load was used without a detection limit correction [41] , as there was inadequate information to perform an adjustment. For studies that reported a final pathogen load of zero and no limit of detection, log10 reduction factor was calculated as the log10 of the control pathogen load (i.e. it was assumed that all virus was inactivated).

Mask performance was evaluated based on percentage aerosol penetration through the mask, equivalent to 100% minus the mask's filtration efficiency, and initial airflow resistance, which is the pressure drop across the mask. Evidence of success for mask performance outcomes was J o u r n a l P r e -p r o o f defined as less than 5% aerosol penetration (i.e. at least 95% filtration efficiency) and airflow resistance under 25 mmH 2 O in accordance with NIOSH certification standards [4, 43] . Pathogen log10 reduction factor of at least three, which is sufficient to fully decontaminate the highest levels of viral contamination that are predicted to occur in hospital settings [44] , was considered a successful germicidal effect. Success thresholds for fit and physical traits were a fit factor (FF) of at least 100 as per Occupational Safety and Health Administration (OSHA) testing guidelines [45] , and no observable changes to the mask, respectively.

Risk of bias was assessed for each outcome in all included studies using criteria that were predetermined by the authors relating to study design, methodological consistency, population heterogeneity, sampling bias, outcome evaluation, and selective reporting (Appendix C). Given the absence of an accepted standard risk of bias assessment tool for laboratory studies, we created a tool with domains applicable to FFR decontamination studies adapted from the Cochrane risk-of-bias tool for randomized trials [46] .

The initial database-and journal-searches identified 466 and three records respectively, and two additional studies were identified via consultation with leaders in the field (Figure 1 ). After duplicate removal, 418 unique records remained for screening. All six reviewers achieved a J o u r n a l P r e -p r o o f sensitivity of 100% on the test set before beginning screening. The review team excluded 397 records at the title/abstract level (κ = 0.79). Three records were excluded after full text review, resulting in 18 reports representing 13 unique studies eligible for inclusion (κ = 0.77). No additional studies were found from checking reference lists of included manuscripts.

Thirteen studies were included in this review ( Table I) . The studies were published between 2007 and 2020, and all were performed in the United States except one from Canada and two from Taiwan. The two studies that investigated the novel coronavirus were published as preprints and not yet peer-reviewed at the time of inclusion [47, 48] . Microwave and heat-based interventions were investigated in nine and 11 studies respectively. Sixteen different mask models were used across the studies, with the 3M 1860 (n = 8), 3M 1870 (n = 7), 3M 8210 (n = 7), and 3M 8000 (n = 5) being the most commonly tested.

Microwave and heat treatments were performed in dry conditions or with the addition of moisture. Two studies used dry microwave treatment [49, 50] , and seven included a reservoir of water or steam bag within the microwave chamber, creating microwave-generated steam (MGS) [15, 42, [51] [52] [53] [54] [55] . Five studies used dry heat (oven or rice cooker) [16, 19, 47, 49, 50] , five employed moist heat incubation (MHI) by adding water reservoirs inside ovens or using laboratory incubators [15, [51] [52] [53] [54] , and four used an autoclave [16, 19, 48, 49] .

J o u r n a l P r e -p r o o f

Almost all studies that measured aerosol penetration [42, [49] [50] [51] 54 ] utilized a neutralized solid polydisperse sodium chloride aerosol (count median diameter (CMD) = 75 ± 20 nm, geometric standard deviation (GSD) ≤ 1.86) and a flow rate of 85 L/min over full masks as per NIOSH certification testing procedures [4] . The exception was Lin et al., who used a lower flow rate (5.95 L/min) to generate equivalent surface velocity on smaller mask segments, and measured penetration of a range of particle sizes using a neutralized potassium sodium tartrate tetrahydrate aerosol (CMD = 101 ± 10 nm, GSD = 2.01 ± 0.08) [19] .

There were five studies that assessed aerosol penetration post-microwave intervention ( Figure 2 , Table II ) [42, [49] [50] [51] 54] , three using moist conditions (MGS) and two using dry. All microwave interventions, which ranged from 90 to 240 seconds in duration, led to small increases in aerosol penetration, but post-treatment values maintained NIOSH certification standards (< 5% penetration). [4] Five studies assessed aerosol penetration after heat treatment ( Figure 2 , Table II ) [19, [49] [50] [51] 54] , four of which had at least one moist condition (MHI or autoclave). MHI was applied for 20 to 30 minutes [51, 54] and, for all mask models, the increase in aerosol penetration was small (< 1%) and remained within NIOSH certification standards (< 5% penetration) [4] . Results in autoclave conditions varied: in one study no increase was noted after 15-minute treatment [19] , another noted increases of over 18% and 34% for 15-and 30-minute treatments respectively J o u r n a l P r e -p r o o f [49] . Three studies examined aerosol penetration post-dry heat treatment and reported small increases with all final values remaining within NIOSH certification standards except the Kimberly-Clark PFR95-270 after 60 minutes at 110 o C (5.4% penetration). [19, 49, 50] Airflow resistance (pressure drop)

Three studies examined airflow resistance simultaneously with aerosol penetration (Figure 3 , Table II ) [19, 50, 51] ; out of these, there were two microwave decontamination arms (one moist and one dry) and three heat arms (one MHI, one dry, and one autoclave). Initial resistance to airflow was reported in millimetres of water column height pressure. Where testing was performed, minimal to no increase in airflow resistance was noted, and all final values were within NIOSH guidelines (< 25 mmH 2 O). [43] 

Seven studies evaluated reduction in pathogen load after microwave or heat interventions ( Figure   4 , Table III ) [15, 16, 42, 47, 48, 54, 55] . One study used a bacterial pathogen (Bacillus subtilis) [16] and all others used viruses: SARS-CoV-2 [47, 48] , Influenza A subtype H1N1 [15] or H5N1 [54] , and Escherichia virus MS2 [42, 55] .

In the four studies that examined the germicidal effect of MGS [15, 42, 54, 55] , all arms demonstrated a log10 viral reduction factor greater than three except the two rapid-treatment arms (30 seconds or less) in Fisher et al.'s 2009 study. [55] All studies using heat treatment J o u r n a l P r e -p r o o f against viral pathogens (dry, MHI, and autoclave) also reported log10 reduction factors in excess of three [15, 47, 48, 54] , although this only occurred after 60 minutes at 70 o C dry heat in Fischer et al.'s SARS-CoV-2 study and not at 10, 20, or 30 minute timepoints [47] . Bacterial decontamination using rapid (three minute) high-temperature dry heat in Lin et al.'s study resulted in a log10 reduction factor of only 2.5, although this was increased to three after a 24hour incubation at "worst case" temperature/humidity (37 o C, 95% relative humidity) [16] . In the same study, no colonies grew post-autoclave treatment.

Four studies assessed FFR fit after microwave and/or heat treatment (Table IV) [47, 46, 52, 53] .

Viscusi et al. abbreviated the standard OSHA fit test [45] from eight exercises to six [53] . An FF, scored from 1 (poor fit) to 200 (best fit), was calculated by measuring the ratio of ambient particle concentration outside the respirator to the particle concentration inside. Each subject donned each mask five times, with two replicates per model-treatment combination, and a multidonning fit factor (MDFF 10 ) was calculated as the harmonic mean of the 10 FFs. MDFF 10 exceeded the passing threshold of 100 for all models after MGS and MHI. Bergman et al. used an abbreviated OSHA fit test similar to Viscusi et al., but performed three cycles of decontamination with a single-donning fit test before the first treatment and after each of the three cycles [52] . The fit test pass rate after three MGS or MHI cycles was 95% for all models.

Fischer et al. incorporated two-hour wear periods before each of three dry heat rounds and performed fit-testing using the official four-exercise modified OSHA protocol between each; deterioration of fit was only seen in two (of six) replicates after the third treatment [47] . Kumar J o u r n a l P r e -p r o o f et al. fit-tested four N95 models after one, three, five, and 10 autoclave cycles using normal and deep breathing exercises only [48] . Across all four studies, most replicates of all tested models maintained adequate fit for all interventions tested, with the exception of the 3M 1860 after multiple (>1) cycles of Kumar et al.'s autoclave treatment.

Nine studies reported on changes in physical traits after treatment, including mask appearance, feel, odor and water retention (Table IV) [15, 19, 42, [48] [49] [50] [51] [52] [53] . Seven used microwave interventions (dry or MGS), and eight used at least one heat intervention (dry, MHI, and/or autoclave).

Physical changes were both treatment-and model-dependent. The 3M 1870 displayed consistent separation of the inner foam nose cushion after MGS and MHI, with this change not observed in any other mask model [15, [51] [52] [53] . Melting of some models occurred after MGS, dry microwaving, or dry heat at temperatures of 100 o C or greater [49] [50] [51] [52] . Autoclaving led to significant mask deformation in two out of three studies [19, 49] .

Changes in odour were assessed in 3 studies [50, 51, 53] ; the only significant increase in odor was noted in the 3M 1860 after MHI in one study [53] . Unacceptable water retention, defined over than one gram of water retained after drying for one hour, was observed in three (3M 1860, 2M 8210, Cardinal Health) out of six models tested [42] .

A full risk of bias assessment for all study outcomes is presented in Appendix D. Overall risks of bias across all studies for aerosol penetration and airflow resistance outcomes were low. Risk of J o u r n a l P r e -p r o o f bias for germicidal outcomes was moderate for most studies, primarily due to risk of population heterogeneity (i.e. masks not from same lot) and the use of unblinded visual assays. Risk of bias for fit was moderate in all studies, due either to high risk of sampling bias or moderate risk for both population heterogeneity and methodology. Risks of bias for physical traits varied between studies, but unblinded/subjective outcome evaluation and potential population heterogeneity were common reasons for increased risk.

A summary of results for all treatments and outcomes can be found in Table V .

In response to PPE shortages during the COVID-19 pandemic, we systematically reviewed the existing literature on N95 FFR-decontamination using microwave irradiation and heat. Our results indicate that moist/dry microwave irradiation and moist/dry heat between 60-90 o C can effectively deactivate viral pathogens on certain N95 FFR models while maintaining mask fit and function within acceptable ranges. General use of high heat (greater than 90 o C) and autoclaving are not supported by the evidence in review as these interventions compromised the integrity of multiple mask models.

Decontamination of N95 masks for reuse is worthwhile only if the masks retain their ability to remove at least 95% of viral particles from the air (i.e. aerosol penetration < 5%) [4] . In the six studies that evaluated aerosol penetration after microwave and/or heat treatment, only two studies showed an increase in penetration above the standard 5% threshold [49, 50] . The J o u r n a l P r e -p r o o f decontamination conditions in these studies (temperature above 100 o C and autoclaving) were also associated with significant physical degradation of the mask. Interestingly, despite also observing physical degradation, Lin et al. reported no significant change to aerosol penetration after autoclaving [19] . This discrepancy may be explained by the non-standard protocol used by Lin et al., which involved mask fragments, modified flow rate, and a different aerosol solution, precluding direct comparison to NIOSH aerosol penetration guidelines.

Mask usability does not only depend on filtration efficiency: N95 FFRs cause breathing resistance and reduce air exchange volume at baseline [56] , so if microwave-or heat-treatment were to increase airflow resistance significantly this could render the masks intolerable, especially when worn for extended periods during PPE rationing [10] . Three studies in this review evaluated airflow resistance in a total of five different decontamination conditions (MGS, dry microwave, MHI, dry heat, and autoclave) [19, 50, 51] . The final average airflow resistance never reached even 50% of the maximum allowable resistance indicated in NIOSH-established guidelines for any mask model [43] , and most models demonstrated slight reductions in resistance after decontamination, making airflow resistance an unlikely obstacle to N95 decontamination using microwave irradiation or heat.

Microwave irradiation and heat both effectively reduced viral load on FFRs with all interventions displaying a log 10 viral reduction factor greater than three when applied for sufficient duration. Although studies used masks that were artificially contaminated in the lab rather than those that had been contaminated during clinical use, viral loading titres that are sufficient for observation of a three log 10 reduction factor meet or exceed the highest levels of J o u r n a l P r e -p r o o f viral contamination modelled to occur in hospital settings [44] . Germicidal impact can be further bolstered by leaving the masks for several days after decontamination before reuse: there is evidence that SARS-CoV-2 naturally decays over time on surfaces [57] , and Lin et al.

demonstrated that bacterial load was further reduced 24 hours after incubation, even in warm, humid conditions [16] . For SARS-CoV-2, a wait time of at least three days is advisable as viable virus is detectable up to 72 hours after application on some surfaces [57] .

N95 FFRs must fit with a tight seal to ensure that air passes directly through the filter.

Data regarding post-decontamination mask fit was promising, but most study protocols did not account for the impacts of repeated donning-wearing-doffing cycles. Previous research indicates that fit failure is associated with extended use and limited reuse of masks even without any decontamination treatment [58, 59] . Thus, applying microwave/heat treatment to unused masks, as three of the four studies did, has limited generalizability. The exception was the protocol used by Fischer et al., which included two-hour wear cycles before each treatment and demonstrated that fit deteriorated after the third decontamination-donning cycle [47] . Kumar et al.'s positive post-autoclave fit results, which did not include wear-periods between cycles, must be interpreted with additional caution as only tested fit using breathing exercises which are not representative of functional movements of healthcare workers [48] . Overall, the results of these studies indicate that a limited number of microwave or heat decontamination cycles may not compromise fit; however, further testing is required using masks that have undergone prolonged wear time and multiple donning-doffing cycles. Regardless, a careful user seal check should be performed by any healthcare worker who dons a decontaminated FFR, just as they should when donning a new one [60] .

Physical degradation of an N95 FFR will almost invariably cause changes in fit, function, and tolerability. Melting of mask components was observed in some microwave and heat arms and depended on mask model, temperature, and treatment duration. Frequent adverse physical changes were observed at temperatures over 90 o C, which corresponds to the maximum operating temperature of polypropylene, the polymer that comprises the N95 filter [61] . High temperature was also the likely cause of melting during microwave treatments: a previous study demonstrated that wet kitchen sponges can exceed 90 o C after one minute of microwave irradiation [25] .

Separation of the inner foam nose cushion was a consistent issue for the 3M 1870 after microwave and heat treatments, but did not lead to a significant reduction in fit and so may not preclude reuse if the mask feels tolerable to the user [53] .

Autoclaving does not appear to be a suitable decontamination option for rigid FFRs as it caused significant physical deformations to the 3M 8000 and 3M 1820 [19, 49] . Although Kumar et al. did not notice any significant physical changes after their autoclave intervention, functional degradation did occur in the one rigid mask model (3M 1860) while the three flexible "pleated" mask models maintained their structural and functional integrity [48] . Thus, it is possible that autoclaving may be effective for pleated N95 varieties, although this needs further study. breathing resistance, hydrophobicity should be a consideration when choosing which mask models to sterilize using moist microwave or heating methods if drying time is limited [62, 63] .

While the results of this review provide a starting point for the development of institutional microwave-or heat-based FFR decontamination protocols, there are several key gaps in the existing evidence. For example, few studies investigated fit; without a tight seal, air will flow through the gaps between the mask and the wearer's face, bypassing the filter altogether and making outcomes of aerosol penetration and airflow resistance irrelevant.

The characteristics of the microorganisms used in several of the germicidal studies must also be taken into account when extrapolating these results to the SARS-CoV-2 coronavirus.

Influenza A viruses, such as H1N1 and H5N1, are enveloped, approximately 120 nm in diameter, and covered in glycoproteins [64] ; coronaviruses share all of these characteristics and may plausibly respond in a similar manner to heat and radiation [65] . Notably, effective 

The moderate risk of bias seen in most studies for germicidal outcomes arises from the fact that all studies quantified pathogens using plaque, colony, or TCID50 assays; while these are widely accepted means of quantifying viral and bacterial load, they involve visual procedures that are not fully objective, and no studies stated that the lab technicians were blinded to treatment and control designations. Two studies that evaluated fit replaced and re-tested masks when their straps broke or melted [52, 53] ; it is possible that this discounted data from the samples that were most vulnerable to physical damage, which could positively skew the fit scores. Similarly, there were two models (3M 1870, 3M 8000) for which aerosol penetration and/or airflow resistance could not be measured after certain treatments due to melting [49, 50] ; these were appropriately accounted for within the articles' conclusions and so did not significantly increase within-study risk of bias, but the absence of measurements from these more-vulnerable masks could positively bias the results of the systematic review. For physical J o u r n a l P r e -p r o o f trait outcomes, several studies reported observations in the results without indicating physical evaluation in their objectives or methods, and/or only commented on changes in some mask models without indicating that unmentioned models were unaffected, making it difficult to rule out methodological inconsistencies and selective reporting.

This is a rigorous systematic review, involving a peer-reviewed search strategy, an a priori registered protocol, training and testing of the screening/extraction team, and adherence to PRISMA reporting guidelines [34] . However, the heterogeneity of the microwave and heat parameters across the 13 studies limits the ability to draw overarching conclusions about any one set of conditions. Temperature, pressure, and moisture all influenced outcomes, especially in heat-decontamination arms where an autoclave provides a vastly different environment than a dry heat rice cooker. There was evidence that different mask models have different physical vulnerabilities, indicating that the response of a given mask model to a particular treatment does not predict how any other model will react. Germicidal outcomes showed consistent viral reduction, but the artificial contamination of samples limits extrapolation to the clinical setting.

The results of this review should therefore be used as a resource for determining which microwave and heat conditions may be most auspicious but cannot guarantee the success of any specific protocol. compromising mask performance or function. The most significant limitation to application of available evidence was observation of differential effects on specific mask models, particularly regarding physical deterioration, and there was a lack of real-world data regarding changes in fit.

Autoclaving is an effective germicidal but caused significant degradation and reduction of filter efficiency in some mask types, and so its use is not supported by the results of this review.

Overall, any hospital implementing these decontamination methods would benefit from monitoring the physical responses of their mask models to determine which, if any, are durable in these treatment conditions, and for how many treatment cycles.

NB: The Association of Home Appliance Manufacturers has emphasized the importance of not using home appliances to microwave/heat facemasks due to risk of damage or injury [67] . Study reported the mean viral recovery post-decontamination as 0 for each model, with no specified limit of detection. Therefore, the log 10 reduction factor is assumed to be equivalent to the Log 10 (TCID50) control h Log 10 reduction calculated from relative survival rate i Control bacterial load not reported, therefore log reduction cannot be calculated No significant degradation of any tested models (n=1) a Green indicates outcome success in all studies where it was measured, yellow indicates mixed results, and grey (hyphenated) indicates outcome not assessed. Outcome successes are defined as aerosol penetration < 5%, airflow resistance < 25 mmH 2 O, log 10 reduction factor ≥ 3, fit factor (or multidonning fit factor) ≥ 100, and no physical changes 

Centers for Disease Control and Prevention. Strategies for optimizing the supply of N95 respirators: Conventional capacity strategies

Infection prevention and control for novel coronavirus (2019-nCoV): Interim guidance for acute healthcare settings

Practical recommendations for critical care and anesthesiology teams caring for novel coronavirus (2019-nCoV) patients

Government Publishing Office. Non-powered air-purifying particulate filter efficiency level determination

Family physicians' experiences, behaviour, and use of personal protection equipment during the SARS outbreak in Singapore: Do they fit the Becker Health Belief Model?

Potential demand for respirators and surgical masks during a hypothetical influenza pandemic in the United States

Personal protective equipment supply chain: Lessons learned from recent public health emergency responses

Hospital preparedness for severe acute respiratory syndrome in the United States: Views from a national survey of infectious diseases consultants

Conserving supply of personal protective equipment-a call for ideas

Centers for Disease Control and Prevention. Strategies for optimizing the supply of N95 respirators: Crisis/alternate strategies

Considerations for recommending extended use and limited reuse of filtering facepiece respirators in health care settings

National Institute for Occupational Safety and Health. NIOSH guide to the selection and use of particulate respirators

Disinfection of filtering facepiece respirators: Considerations for healthcare organizations and occupational health professionals

Centers for Disease Control and Prevention. Decontamination and reuse of filtering facepiece respirators

A pandemic influenza preparedness study: Use of energetic methods to decontaminate filtering facepiece respirators contaminated with H1N1 aerosols and droplets

Relative survival of Bacillus subtilis spores loaded on filtering facepiece respirators after five decontamination methods

Decontaminating N95 masks with ultraviolet germicidal irradiation (UVGI) does not impair mask efficacy and safety: A systematic review

Evaluation of five decontamination methods for filtering facepiece respirators

Filter quality of electret masks in filtering 14.6-594 nm aerosol particles: Effects of five decontamination methods

Analysis of residual chemicals on filtering facepiece respirators after decontamination

The influence of microwave sterilization on the ultrastructure, permeability of cell membrane and expression of proteins of Bacillus cereus

Microwave or autoclave treatments destroy the infectivity of infectious bronchitis virus and avian pneumovirus but allow detection by reverse transcriptase-polymerase chain reaction

Inactivation of HCV and HIV by microwave: a novel approach for prevention of virus transmission among people who inject drugs

In situ airborne virus inactivation by microwave irradiation

Microbial inactivation by microwave radiation in the home environment

Heat inactivation of the Middle East respiratory syndrome coronavirus. Influenza and Other Respir Viruses

Stability and inactivation of SARS coronavirus

Assessment and verification of commercially available pressure cookers for laboratory sterilization

Effect of solvent exposure on the filtration performance of electrostatically charged polypropylene filter media

The effect of temperature and humidity on electric charge amount of polypropylene melt-blown nonwoven fabric

Effect of air humidity on charge decay and lifetime of PP electret nonwovens

Efficacy and safety of disinfectants for decontamination of N95 and SN95 filtering facepiece respirators: A systematic review

Microwave-and heatbased decontamination for facemask personal protective equipment (PPE): Protocol for a systematic review

Preferred Reporting Items for Systematic Reviews and Meta-Analyses: The PRISMA Statement

3M. 3M Disposable Filtering Facepiece Respirators

The REDCap consortium: Building an international community of software platform partners

Research electronic data capture (REDCap)--a metadata-driven methodology and workflow process for providing translational research informatics support

Decontamination of N95 and SN95 Filtering Facepiece Respirators

R Development Core Team. A language and environment for statistical computing: Reference index. Vienna: R Foundation for Statistical Computing

How to perform a meta-analysis with R: A practical tutorial

Robust estimation of mean and variance using environmental data sets with below detection limit observations

Evaluation of microwave steam bags for the decontamination of filtering facepiece respirators

Validation and application of models to predict facemask influenza contamination in healthcare settings

Occupational Safety and Health Administration. Fit testing procedures (mandatory)

Cochrane Handbook for Systematic Reviews of Interventions version 6

Assessment of N95 respirator decontamination and re-use for SARS-CoV-2. medRxiv 2020

N95 mask decontamination using standard hospital sterilization technologies

Effect of decontamination on the filtration efficiency of two filtering facepiece respirator models

Evaluation of five decontamination methods for filtering facepiece respirators

Evaluation of multiple (3-cycle) decontamination processing for filtering facepiece respirators

Impact of three cycles of decontamination treatments on filtering facepiece respirator fit

Impact of three biological decontamination methods on filtering facepiece respirator fit, odor, comfort, and donning ease

Effectiveness of three decontamination treatments against influenza virus applied to filtering facepiece respirators

Development of a test system to apply viruscontaining particles to filtering facepiece respirators for the evaluation of decontamination procedures

Objective assessment of increase in breathing resistance of N95 respirators on human subjects

Aerosol and surface stability of SARS-CoV-2 as compared with SARS-CoV-1

Impact of multiple consecutive donnings on filtering facepiece respirator fit

Correlation between N95 extended use and reuse and fit failure in an emergency department

Filtering out confusion: Frequently asked questions about respiratory protection, User seal check. Pittsburgh: U.S. Department of Health and Human Services

Chapter 3 -Properties of nonwoven filter media

Disrupting the transmission of influenza A: Face masks and ultraviolet light as control measures

Modified N95 mask delivers high inspired oxygen concentrations while effectively filtering aerosolized microparticles

Statement on microwave ovens for sanitizing face masks