Factors affecting decontamination of N95 masks for reuse: Feasibility & practicality of various methods

Heat-based methods

N95 masks are made of multiple layers of polypropylene non-woven fabrics and the most critical part among these layers is the one that is produced through the melt-blown process. Since the maximum operating temperature for polypropylenes is 80-100°C and the melting point is 165°C, temperatures >80° C can soften and melt the filter, hence compromising its performance4.

Heat is the most basic, easy, and cheapest method of decontamination. The main concern with heat-based methods is the loss of structural integrity and fit due to overheating, which could render the mask non-wearable or could impact fit. The identification of a temperature that provides the user a good fit along with maintenance of structural integrity while fulfilling other NIOSH (The National Institute for Occupational Safety and Health) criteria is required567.

Heat-based methods (Table I) included dry heat (3 studies)51314, moist heat (3 studies)7910, steam (2 studies)1114, autoclave (2 studies)58 and MGH (6 studies)56791012.

1. Structural integrity: Viscusi et al56 assessed the filtration performance and structural integrity following decontamination by the method of dry heat at two temperatures (80°C and 160°C). At 160°C the N95 masks melted and were unfit for use but tolerated the heat up to the 80°C. Bergman et al7 demonstrated that moist heat at 60°C was able to maintain structural integrity up to one cycle. Beyond three cycles, slight melting of the head straps with partial separation of the inner foam nose cushion was observed. Ma et al11 studied the efficacy of steam (100°C) at four different durations of 0, 20, 60, and 120 min. The masks were able to tolerate steam at 100°C up to one cycle in contrast to autoclave and MGH where the masks were deformed, shrunken, stiff, and mottled. Also, there was a loss of elasticity of the rubber straps of the masks with each autoclave treatment8. The use of MGH/irradiation (MGH/MGI) resulted in loss of structural integrity of the mask and caused sparking due to the metallic noseband5. A study by Lore et al9 also concluded that rather than the structural make of the mask, the type of heat based method along with the time of exposure was a critical factor affecting the end result of decontamination.

2. Biocidal efficacy (BE): is critical for assessing the efficacy of a decontamination procedure in controlling the microbial burden. The BE has been assessed by using bacteriophages and the H1N1 virus as a surrogate. Cadnum et al13 demonstrated a good BE for surrogate phage Phi 6 and MS2 by the dry heating method. Liao et al14 and Heimbuch et al10, demonstrated a reduction of 8-5.5 log₁₀ TCID₅₀/ml of H1N1 by moist heat and microwave generated irradiation (MGI). All methods of heating provided a good BE.

3. Filtration efficacy and reduction in pressure drop: Filtration efficiency was best maintained by heat and steam compared to autoclaving and MSH/MGI58. Liao et al14 loaded three models of N95 masks into a preheated 5-sided heating chamber with dry heat (≤85°C) under various relative humidity (≤100% RH) conditions. They found this as the most promising, non-destructive method for the preservation of filtration properties in N95-grade respirators. Heating could be applied up to 50 cycles (85°C, 30% RH) without observation in the degradation of the filtration efficacy14.

Thus, among the various methods of decontamination by heat, steam at 100°C best maintains the structural integrity, along with a good fit and filtration efficacy. Heat is effective in providing BE provided the temperature and contact time are sufficient. Installation of heat-based decontamination methods in a small setting is a feasible option. This method is cost-effective and does not require a large infrastructure. The disadvantage is that heat-based methods allow the decontamination for a single decontamination cycle only.

Ultraviolet germicidal irradiation (UVGI) based decontamination of N95 masks

UVGI is a highly energetic short-wave (254 nm) ultraviolet light shown to be a useful sterilization technique in a variety of applications. The virucidal mechanism of UVGI is derived from the energy contained within the electromagnetic wave and its effect on the viral genome. Single-stranded RNA viruses are especially susceptible to this type of radiation. This method was evaluated for its efficacy as a decontamination method in 10 studies (Table II)567891013141516.

1. Structural integrity: A major disadvantage of UV radiation is that it tends to degrade polymers, which presents the possibility that UVGI exposure while decontaminating, may also reduce the efficacy of the mask and decrease the level of protection. Therefore, the energy of exposure needs to be standardized for the UVGI exposure. Varying amounts of energy ranging from 1 to 1000 J/cm² were administered in different studies. Lindsley et al15 gauged the energy and noticed that the structural integrity was lost when the masks were exposed to energy >500 J/cm². Liao et al14 evaluated the performance of the masks after 10 cycles, and even after multiple exposures the structural integrity was maintained if exposed to energy <500 J/cm². The time exposure varied in all studies ranging from 1 to 30 min. Lindsley et al15 exposed both sides of material coupons and mask straps from four models of N95 masks to UVGI doses ranging from 120-950 J/cm². They found that at the higher UVGI doses, the strength of the layers of mask material was substantially reduced (in some cases, by >90%).

2. BE: Mills et al16 evaluated the decontamination efficacy by UVGI on N95 masks contaminated with influenza virus after soiling them with artificial saliva or artificial skin oil. Significant reductions in influenza virus viability (≥3 logs) were observed for both soiling conditions (artificial saliva and artificial skin oil) on UVGI-treated face pieces from 12 of 15 filtering facepiece respirator (FFR) models and UVGI-treated straps from 7 of 15 FFR models. Lore et al9 examined all FFRs post decontamination by viral culture. UVGI was successful in reducing the viral load by >4 logs median tissue culture infective dose (TCID₅₀).

3. Filtration efficiency: Viscusi et al5 evaluated the filtration performance of the N95 and N100 masks after using UVGI through the poly-dispersed sodium chloride aerosol method. UVGI did not significantly affect the average penetration results. Lore et al9 found that there was practically no reduction (<5%) in the penetration of 300 nm particles of sodium chloride (1%) after UV decontamination

4. Bergman et al7: found that the filter aerosol penetration and filter airflow resistance were maintained for three consecutive UVGI decontamination cycles. Thus, in all the above-mentioned studies, UVGI provided a good method for the maintenance of filtration efficacy and pressure resistance.

Even though there are many advantages of UVGI as a decontamination method, the logistical issues such as the need for the installation of the specific equipment, and their availability at the different levels of healthcare delivery in many low- and middle-income countries (LMICs) may be an issue to be considered when considering implementation. Also, any factor that contributes to the non-uniform distribution of UV irradiation across the face of the mask can alter its effectiveness. UVGI can only be effective if the radiation reaches the surfaces. The efficacy varies with different mask types and in different locations on the mask. In N95 masks the edge can receive less UV, thus can lead to decreased efficacy of the method. The upper limit for the UVGI exposure during repeated disinfection cycles would be set by the physical degradation of the respirator material and not by the loss of infiltration capacity14. UVGI is not associated with any residual toxicity. Thus, this method can be considered as a potential method of decontamination.

H₂O₂-based methods

H₂O₂ can be used in various forms for mask decontamination; vaporized, ionized, liquid, and gas plasma. Many commercial platforms are available for decontamination including STERRAD, SteraMist, and Bioquell. The H₂O₂ concentration and the time of exposure vary with each form of H₂O₂ application used (Table IIIA)2181920212223.

1. Structural integrity: H₂O₂ causes the elastic straps to degrade and also causes tarnishing of the aluminium nosebands56. Schwartz et al19 found the method satisfactory in both the structural integrity and fit for up to 30 cycles. Cramer et al3 found that the fit was retained for five cycles in all conditions. Similarly, Saini et al22, showed maintenance of N95 fabric for up to 10 cycles.

2. BE: H₂O₂ treatment provided good BE on various surrogate viruses such as H1N1, and phage such as Phi 6 and T1, T71821.

3. Filtration efficiency: Bergman et al7 compared three forms of H₂O₂ treatment along with other methods. When H₂O₂ was applied in plasma form, after three cycles it led to a decrease in filtration efficacy beyond recommended levels of >5 per cent. Vaporous H₂O₂ could be provided safely up to 30 cycles19. N95 masks still met performance requirements after decontamination with vaporous H₂O₂ in the laboratory setting for over 50 cycles22. Others reported that filtration efficacy and pressure resistance were within recommended limits1821.

Ethylene oxide (EtO)-based methods

1. Structural integrity: Two studies by Viscusi et al56 and one by Bergman et al7 noted that masks were able to preserve their structural integrity after EtO reprocessing (Table IIIB).

2. BE: The three studies567 did not evaluate the ability of the ETO based decontamination to inactivate infectious biological organisms from the contaminated N 95 masks..

3. Filtration efficiency: The three studies567 found that after EtO reprocessing the filter aerosol penetration, as well as the appearance and filter airflow resistance, were unaffected. Furthermore, changes in filter penetration following three cycles of treatment were <5 per cent. EtO was able to maintain filtration efficacy below four per cent and pressure resistance <17.6 mm H₂O. These criteria are acceptable for mask reuse according to the NIOSH guidelines567.

The limiting factor with the EtO method is that it requires a long duration in comparison to the other decontamination methods. There are always concerns regarding the carcinogenic potential of EtO, which hampers its usage for any application. There was a minor cosmetic effect of darkening of the straps. If a longer duration is given for aeration (4-5 h), the EtO evaporates making masks safe for usage6.

Decontamination using other methods

Alcohols, in concentrations between 70-75 per cent have been used in a few studies5614 (Table IIIC). It was found that small molecules such as solvents can permeate into the fabric and liberate the charge traps or frozen charges of the electret, thereby decreasing the filtration efficiency24. Bleach is another method of decontamination56714. Bleach was shown to oxidize the aluminium nose-bands and staples and to create a peculiar smell persisting even after drying (Table IIID). Though lesser than alcohol-based methods, the use of chlorine-based solutions may also degrade the efficiency of the N95 masks due to their higher water content. As the material of the mask (i.e. the polypropylene) is hydrophobic, the chlorine-based solutions may have more difficulty penetrating the fabric, and the static charge of fibres deeper within the melt-blown may be less affected. Chlorine-based solutions, or soaps used to clean the mask, lead to degradation in the static charge that is necessary for the mask to meet the N95 standard25. These chemical methods are not recommended to use for decontamination of N95 masks.

Among other methods, HLD (aerosolized peracetic acid and H₂O₂) and decontamination with soap and water has also been discussed. In the few studies on these methods, structural integrity and charge on the electret mask were not maintained (Tables IIIE and F)513. Hence these methods would not be useful for mask decontamination.

Limitations

All these studies have been performed on the surrogate virus like H1N1 and bacteriophage. The actual performance of decontamination methods with COVID-19 virus will have greater impact in the current pandemic. Fischer et al27 demonstrated the inactivation of SARS-CoV-2 on the surface of N95 masks in the preliminary results. Another important limitation is the lack of clinical studies. There are no studies that have evaluated the efficacy of recycling methods in a clinical setting. Whether reuse of N95 masks after decontamination would prevent the infection in healthcare workers is not known currently. Future studies should incorporate this aspect as an outcome while planning.