Background. The COVID-19 pandemic has presented an unprecedented strain on healthcare supplies. Currently there is a global shortage of personal protective equipment (PPE), especially N95 masks. In order to safeguard healthcare personnel in this critical time and to mitigate shortages of N95 respirators, reuse of N95 respirators has to be considered. Methods. Using PubMed and Science Direct, a literature search was conducted to find and synthesize relevant literature on decontamination of N95 respirators for their subsequent reuse. Peer-reviewed publications related to methods of decontamination from January 2007 to April 2020 in the English language are included in this narrative review. Bibliographies of articles for relevant literature were also scrutinized. Findings. A total of 19 studies are included in this narrative review. The appraised methods include ultraviolet germicidal irradiation (UVGI), moist heat incubation (MHI), ethylene oxide (EtO), hydrogen peroxide vapor (HPV), microwave steam bags (MSB), microwave-generated steam (MGS), dry microwave oven irradiation, hydrogen peroxide gas plasma (HPGP), dry heat, liquid hydrogen peroxide, and bleach and alcohol. Conclusion. In light of the COVID-19 pandemic, reuse of N95 respirators, although suboptimal, can be considered. Evidence reveals that UVGI, MHI, and HPV are amongst the safest and efficacious methods for decontamination of N95 masks. More research is needed to establish the safety and effectiveness of MGS, MSB, dry heat, EtO, liquid hydrogen peroxide, and HPGP. Alcohol, microwave irradiation, and bleach are not recommended because they damage N95 respirators.

1. Background

The COVID-19 pandemic caused by SARS-CoV-2 virus has created a shortage of PPE globally. Keeping in view the nature of transmission of the virus and its contagiousness, the demand of N95 respirators has increased drastically. Nosocomial transmission of the virus has also been reported, and substantial numbers of healthcare workers have been affected worldwide [1]. N95 respirators provide essential protection against the virus owing to of their tight fit and efficient filtration capacity [2].

In general, N95 respirators are designed for single use prior to disposal. They must be discarded after performing aerosol generating procedures, when damaged, deformed, visibly dirty or when they stop forming an effective seal on the face [3]. The rationale for these recommendations may be due to the fact that respiratory viruses can remain viable on respirators for an extended period of time [4, 5]. SARS-CoV-2, although more stable on plastic and stainless steel than on copper and cardboard, can remain viable for up to 72 hours [6]. Extended use of N95 respirators implies using the same respirator for an extended period of time with donning and doffing carried out only once. This was previously endorsed during pandemic situations with respiratory pathogens to mitigate shortage of respirators [7, 8]. Although extended use of respirators may bring about physical discomfort to the healthcare workers, it is endurable without any additional health risks [9].

In order to deal with the gap in the demand and supply of N95 respirators, guidelines proposed by the Infectious Diseases Society of America now recommend optimization of personal protective equipment [10]. In order to mitigate shortage of respirators, reuse after decontamination has been recommended which is based on laboratory evidence as there is lack of clinical experience [10, 11].

A range of decontamination methods for N95 respirators have been studied. Various characteristics of the respirator have to be taken into account including modifications in their physical appearance/odor, structural integrity, filtration efficiency, fit, seal and airflow resistance, and degradation of the material and chemical residues that may be irritant to the skin. The equipment needed for decontamination, its ease of use, cost, and the allowed number of decontamination cycles must also be considered [3]. This article aims to present a review of different methods used for decontamination of N95 respirators for their subsequent reuse.

2. Methods

A literature search was conducted using PubMed and Science Direct. The following search terms were used: “Decontamination”(Mesh) OR “Reuse”(Mesh) OR “Disinfection”(Mesh) AND “Respiratory Protective Devices”(Mesh) OR ″”N95 Respirator (Mesh)” OR “Filtering Facepiece Respirator (Mesh).” Peer-reviewed publications related to methods of decontamination of N95 respirators and their reuse from January 2007 to April 2020 in the English language are included in this review. Bibliographies of articles for relevant literature were also scrutinized. Because this is a narrative review, all relevant studies were included regardless of the sample size. Non-English language publications and those for which full text was not available were not eligible for inclusion in the review.

3. Results

A total of 192 studies were found via electronic search. 168 studies were not deemed relevant after scrutinizing the title, abstract, and then the full text and hence were excluded from the review. Five additional studies were identified after examining the reference lists of relevant articles. A total of 19 studies are presented in the review classified by the method of decontamination. The appraised methods include ultraviolet germicidal irradiation (UVGI), moist heat incubation (MHI), ethylene oxide (EtO), hydrogen peroxide vapor (HPV), microwave steam bags (MSB), microwave-generated steam (MGS), dry microwave oven irradiation, hydrogen peroxide gas plasma (HPGP), dry heat/hot air oven, liquid hydrogen peroxide, and bleach and alcohol. Some studies assessed multiple methods of decontamination, but they are counted as one publication although the results are presented separately in Table 1.

4. Discussion

4.1. Ultraviolet Germicidal Irradiation (UVGI)

Ultraviolet germicidal irradiation (UVGI) is a disinfection method that employs short-wavelength ultraviolet C or UV-C light to destroy microorganisms. [31]. UVGI has been successfully used to disinfect medical equipment [32] and high touch surfaces in hospital [33]. Several studies have studied the effects of decontamination of N95 respirators using UVGI, and the majority of them evaluated the performance of a single cycle (15 minutes) of UVGI on N95 respirators. However, two studies evaluated the performance of three cycles of UVGI [19, 23]. A varying range of one to fifteen N95 respirators were employed in these studies. Five studies assessed particle penetration posttreatment with UVGI [14, 15, 17, 19, 23]. Apart from Lindsley et al. [14] whose study took into account five UVGI intervention arms, the rest evaluated a single UVGI protocol. Particle penetration for masks ranged from 0.7% to 2.48%. For the control group, particle penetration ranged from 0.57% to 2.63%. Fisher et al. [22] described aerosol penetration for the individual layers of six N95 FFR models and found that the majority of filter layers had an efficiency of >94%. In studies that assessed airflow resistance, average values ranged from 7.98 mm to 11.44 mm H2O for the control and post-UVGI treatment arms [14, 15, 19]. However, Fisher et al. reported the airflow resistance of the individual filtering layers between 2.5 mm and 7.6 mm H2O [22].

Evidence demonstrates that the germicidal impact of UVGI is good [12, 16, 2022, 24] with majority of studies examining its effect on H1N1 and MS2. In only one study, a Bacillus subtilis prototype strain was used [21]. Multiple studies assessed physical changes in respirators following UVGI treatment by visual inspection [15, 1719, 23]. No significant changes in physical appearance have been noted. No significant changes in odor were observed in the studies that evaluated this aspect [18, 19]. In two studies assessing fit of the respirator, no significant reduction in fit was found [18, 23]. In view of the available evidence, UVGI is an effective method of decontamination of N95 respirators and is endorsed by the Food and Drug Administration (FDA) [3].

4.2. Ethylene Oxide (EtO)

Ethylene oxide (EtO) is a commonly used sterilizing agent for medical equipment [34]. However, CDC advocates using ethylene oxide with caution for sterilizing N95 masks, as it is carcinogenic and teratogenic and can cause neurologic dysfunction thus harming the wearer [3]. Salter et al. [25] measured the amount of residual chemicals created or deposited on N95 masks with EtO treatment and reported that it resulted in detectable toxins.

A study utilizing two EtO sterilization protocols revealed that ethylene oxide did not negatively impact filtration performance for one tested N95 model [15]. Another study showed that EtO did not affect physical appearance, filter aerosol penetration, and filter airflow resistance of six models of N95 respirators [17]. Bergman et al. evaluated the physical changes, odor, and laboratory filtration performance of six models of N95 respirators after EtO treatment and reported no significant changes [19]. However, these studies [15, 17, 19] did not evaluate FFR fit testing after EtO treatment.

4.3. Hydrogen Peroxide Vapor (HPV)

HPV is a vapor form of hydrogen peroxide (H2O2) with cidal action against a wide range of microorganisms and nontoxic to human health [35]. Evidence delineates that HPV does not compromise the efficacy and performance of N95 respirators and can be used for simultaneous sterilization of a large number of used but intact respirators. However, it can only be utilized for certain types of N95 models, e.g., 1860 which do not contain cellulose [36].

A study carried out in 2007 by Viscusi et al. [15] utilized T1, T7, and Pseudomonas phage phi-6 as proxies for SARS-CoV-2 and established that HPV decontamination resulted in complete eradication of phage from one model of the N95 respirator without causing any physical damage or affecting filtration performance. In another study [27], the virucidal activity of HPV was evaluated using a BQ-50 system. In this experiment, a single HPV cycle resulted in complete eradication of phage from masks, and no physical deformity was observed after 5 cycles [27]. Half of the respirators underwent steam sterilization at 275°F for 5 min which degraded the respirators and resulted in no additional virucidal action [27].

Two studies established the efficacy of HPV against Geobacillus stearothermophilus spores [26, 28]. Battelle Memorial Institute reported a 6 log reduction without affecting filtration performance and noted that the performance of N95 FFRs exceeded 95% efficiency after 50 cycles and minor degradation in elastic respirator straps occurred after 30 cycles [26]. T Schwartz et al. [28] performed decontamination successfully with no physical or performance degradation of FFRs using commercially available equipment, and fit testing was conducted on humans instead of mannequins.

4.4. Liquid Hydrogen Peroxide

Hydrogen peroxide is a colorless liquid which can be used for disinfection of medical devices and surfaces [37]. Decontamination of N95 respirators with liquid hydrogen peroxide is an encouraging method but it has limitations. This method has been assessed by only two studies. Viscusi et al. [15] submerged samples of one model of N95 in 3% and 6% hydrogen peroxide solutions for 30 min and reported no change in filtration performance; however, 6% hydrogen peroxide slightly faded label ink on the respirator fabric. Bergman et al. [19] demonstrated that soaking six FFR models in 6% solution of hydrogen peroxide for 30 min did not affect filtration performance. However, in this study, the fit integrity was not tested, and staples oxidized to varying degrees in N95 models with staples [19].

4.5. Hydrogen Peroxide Gas Plasma (HPGP)

HPGP is the nontoxic method of sterilization which inactivates wide range of microorganisms by the combined use of hydrogen peroxide gas and generation of free radicals and is useful for materials that cannot endure high temperatures and humidity [38]. Conflicting results were found for the use of hydrogen peroxide gas plasma for decontamination of N95 respirators. Viscusi et al. [17] decontaminated six models of N95 FFRs using the STERRAD100S H2O2 Gas Plasma Sterilizer and found that one treatment cycle had filter aerosol penetration and filter airflow resistance levels similar to untreated models. However, Bergman et al. [19] reported that three treatment cycles using the same machine negatively affected the filtration performance of N95 respirators. Although Bergman et al. [19] reported no physical changes in respirators, Viscusi et al. [17] found that HPGP treatment tarnished metallic nosebands of respirators with loss of shine.

4.6. Hot Air Oven/Dry Heat

Hot air oven is a form of dry heat sterilization in which microorganisms are destroyed by exposing them to extremely high temperatures over several hours [39]. Limited evidence is available for dry heat decontamination, and the optimal parameters for temperature and duration are ambiguous. A few studies have shown encouraging results. Liao et al. [13] kept respirators inside a hot air oven at 70°C for 30 min, demonstrating favorable results in terms of fit and filtration efficiency. Fischer et al. [29] exposed contaminated respirators to 70°C heat in an oven. Quantitative fit tests showed that the filtration performance of the N95 respirator was retained after decontamination and respirators may be utilized up to two times [29].

4.7. Moist Heat Incubation (MHI)

Moist heat incubation is usually performed between 60% and 85% relative humidity (RH) in laboratory incubators where warm moisture causes biocidal action. Moist heat is more effective than dry heat in destroying microorganisms and less likely to negatively impact filter performance [15, 17]. Data from multiple studies revealed encouraging results for the decontamination and reuse of N95 respirators using MHI. However, there is limited available evidence on the ability of MHI to disinfect different pathogens [3]. Heimbuch et al. [20] used moist heat to sterilize H1N1-contaminated N95 masks and demonstrated 99.99% reduction in H1N1 influenza virus. Lore et al. [16] found that moist heat effectively decontaminated two N95 models contaminated with H5N1. Bergman et al. [19] performed a 30-minute incubation of six models of N95 at 60°C with 80% relative humidity and ascertained minimal changes in physical appearance, odor, and filtration performance of the masks. Both Lore et al. [16] and Bergman et al. [19] found the post-decontamination filtration efficiency to be greater than 97.5% [16, 19]. Viscusi et al. [18] reported that decontamination of N95 respirators with MHI did not cause significant reduction in fit, increase in odor, increase in discomfort, or increased difficulty in donning, thereby rendering it a safe and effective method. The exposure time per decontamination cycle was 20 minutes in one study [16] and 30 minutes in three studies [1820]. Another study [23] utilized a shorter decontamination cycle of 15 min and reported that MHI did not negatively impact fit and filtration efficiency.

4.8. Microwave Steam Bags (MSB)

Commercially available microwave steam bags (MSBs) are available in retail stores and are commonly used to decontaminate breast infant feeding accessories [30]. A study by Fischer et al. [30] employed two steam bags for decontamination of N95 respirators and tested them for water absorption and filtration efficiency. Steam exposure was given thrice. Efficacy was evaluated using bacteriophage MS2 as a surrogate for the pathogenic virus. The tested steam bags were found to be 99.9% effective for inactivating MS2 on FFRs and filtration efficiency remained above 95% [30]. However, further research is needed to establish the effectiveness of this method.

4.9. Microwave-Generated Steam (MGS)

Microwaves are commonly available electrical appliances, and several studies have revealed beneficial decontamination effects for microwave-generated steam. However, inadequate controls for uniform steam distribution, the unknown effect of microwave power capacity on respirators, and danger of combustion of FFR metal nosebands must be taken into account [34]. Two studies reported nonsignificant effects on filtration and fit performance of N95 respirators employing one decontamination cycle [16, 20]. Using MGS, Heimbuch et al. [20] revealed a minimum of 99.9% reduction in viable H1N1 virus, and Lore et al. [16] reported 99.9% reduction in bacteriophage MS2. In a study performed by Bergman et al. [19], the filtration efficiency remained intact after three cycles of exposure to microwave-generated steam, and no sparking was observed. However, slight physical changes were seen in two N95 models [19].

4.10. Other Methods—Dry Microwave Oven Irradiation

This method is not recommended for decontamination of FFRs due to significant filter degradation and damage to masks [40]. Viscusi et al. [17] exposed six models of N95 FFRs for 2 minutes at maximum power setting in a commercially available 2450 MHz microwave oven. After treatment, the SN95-E model melted in areas adjacent to the metallic nosebands and therefore was not assessed further for filter aerosol penetration or filter airflow resistance [17]. In other models, filter aerosol penetration and filter airflow resistance were unaffected [17].

4.11. Other Liquid Disinfectants—Bleach and Alcohol

Bleach (sodium hypochlorite) and alcohol are not recommended for decontamination of N95 respirators. According to the World Health Organization and Centres for Disease Control and Prevention, bleach and alcohol cause damage to N95 masks, loss of filtration efficiency, and have potential for toxicity [3, 40]. A recent study [13] reported that alcohol and chlorine-based solutions are detrimental to the static charge in the microfibers of N95 masks and result in reduced efficiency. In addition, chlorine-based solutions generate odor and harmful fumes after decontamination [13, 15, 17].

5. Conclusion

Due to the existent shortage of PPE in the current COVID-19 pandemic, reuse of N95 respirators may be considered, although it has limitations since respirators are inherently designed to be single use items. In light of the above evidence, ultraviolet germicidal irradiation, moist heat incubation, and vaporous hydrogen peroxide are the most efficacious and safe methods for decontamination of N95 respirators. Microwave-generated steam, microwave steam bags, dry heat, ethylene oxide, liquid hydrogen peroxide, and hydrogen peroxide gas plasma are promising methods but the amount of available evidence is limited to recommend them as first line methods. Alcohol, microwave irradiation, and bleach are not recommended because they can damage N95 respirators and cause filter degradation.

The safety and effectiveness of N95 respirators after decontamination in clinical settings still remains a question. It is suggested that studies proposing methods of decontamination must ensure maximal decontamination (reduction in viral load) as well as preservation of the integrity, and filtering function and proper seal of the mask. It is envisioned that high quality experiments designed to address the decontamination of N95 respirators with COVID-19 in real clinical settings will shed light on these issues in the future.

Conflicts of Interest

The authors declare that they have no conflicts of interest.

Authors’ Contributions

Aamer Ikram conceived idea for the article. Muhammad Amjad Khan conducted the literature search. Muhammad Amjad Khan, Saba Savul, and Farida Khurram Lalani drafted the manuscript. Aamer Ikram, Mumtaz Ali Khan, and Munnaza Sarfraz critically revised the manuscript. All authors read and approved the final manuscript.