The Decontamination of Mask and Reuse : Evidence Review

Decontamination methods

Physical and chemical methods of decontamination were identified. Physical methods include decontamination by steam, heat (dry and humid), and irradiation [9-18]. For chemical methods, decontamination by ethylene oxide, bleach, vaporous hydrogen peroxide and other chemical reagents were tested [9, 12, 16, 19, 20]. In addition to this, method involving hot water heating was proposed by Chinese researchers for general public use [21].

Following requirements were set when evaluating above mentioned methods, such as biocidal efficacy, filtration performance of tested masks, safety to the users. In the upcoming sections, we will discuss each method, describing its strengths and weaknesses (table 1).

Table 1.

Decontamination methods characteristics

Steam treatment

Steam treatment is one of the promising tested methods. When N95 grade FFRs and melt blown fabrics were steamed on top of beaker for 10 min, testing with NaCl as the aerosol showed that filtration efficiency has a sharp drop after 5 steam treatments and continues at cycle 10 (80%). In terms of pressure drop, it remained at 8-9 Pa without significant changes. Decline in filtration efficiency was speculated to be due to the decay of electrostatic charge. It was suggested that if steam treatments saturate the fibers many times and condense water droplets on the fiber, it is possible that the static charge decays after multiple treatments. Biocidal efficacy was not evaluated in current study; therefore, this method is still concerning in an environment with high viral aerosol concentrations [9]. Whereas when surgical masks and N95 masks were put into intact plastic bags and steamed on boiling tap water in a kitchen pot for certain time, surgical masks and N95 remained blocking efficiency against the vaccine strain of avian infectious bronchitis virus H120, which was used as surrogate of SARS-CoV-2. It was suggested that both surgical masks and N95 respi rators can be reused for a few days with steam decontamination between uses [10]. However, in this case, conclusion was made based on single treatment procedure. Therefore, multiple treatment cycles need to be performed.

Evaluation of microwave steam bags for decontamination of FFRs were performed using MS2 bacteriophage containing droplets. Steam bags had zipper lock area, steam exhaust port and internal pleat. FFRs were placed into separate bags filled with 60 ml tap water. The bags were sealed and irradiated on high power for 90 seconds in commercially available microwave oven (2450 MHz, 1100 W). The tested steam bags were found to be 99.9% effective for inactivating MS2 on FFRs. The presence of some viable virus on the FFRs was speculated to be due ton on-uniform distribution of steam over entire surface [11]. Efficiency of steam treatment with autoclave was evaluated using N95, Gauze, Spunlace masks. Aerosol penetration rate was found to be increased through Gauze and Spunlace masks, whereas N95 penetration rate was not affected by treatment. Bio efficacy was not evaluated, but method was considered to be promising for decontamination of N95 respirators [12].

Immediate-use steam sterilization (IUSS) procedure was evaluated using steam sterilizer autoclave. N95 masks were packed in paper-plastic sterilization peel pouches for IUSS and treated. The IUSS cycle was performed with a chemical and biological indicator to confirm that no biological or chemical contamination is present. Five subjects were tested for individual differences and fit test were performed before and after three IUSS cycles. Masks retained their structural integrity and filtration efficacy, indicating that IUSS is a promising procedure [13].

Treatment in a commonly used kitchen rice cooker-steamer for 8-10 minutes of heating and 5 minutes of steam, resulted in a greater than 5 log₁₀ reduction in bacteriophage MS2 and methicillin-resistant S aureus (MRSA) on N95, cotton and quilting fabric cloth face masks. No visible changes were observed in any of the masks or respirators after 5 cycles of decontamination. Even though, mask performance was not evaluated, it is suggested that kitchen rice cooker-steamer can be effective for decontamination of face masks and N95 respirators as well as cloth masks [14].

Overall, the results of steam mediated masks decontamination studies suggest that this method is promising. It is found to be non toxic to users, cost effective, since equipment is widely available at home and hospital, rapid and easy to use. However, multiple treatments may increase penetration of masks and decrease filtration efficiency. This drop in filtration performance is caused by direct water molecule contact resulting in decay of electrostatic charge. Therefore, it may possible to alleviate the electrostatic decay if the fibers do not come into contact with vapor directly. In addition, to alleviate biocidal efficacy, uniform distribution of steam needs to be ensured.

Decontamination by dry heat

When dry heat at 60°C and 70°C was applied in electric oven to used masks, it was found that seven types of common respiratory pathogens were killed at both 60°C and 70°C for 1 hour. H1N1 strain, which was used as the indicator virus, was also inactivated. The filtering efficacies of the N95 respirators for bacterial aerosols were 98% for 1 and 2 hour and 97% for 3-hour treatment. The filtering efficacies of the surgical face masks were 97% for 1 and 2 hour and 96% for 3-hour treatment. Results indicated that the dry heat at 60°C and 70°C for 1 hour can ensure the decontamination of surgical masks and N95 respirator while maintaining their filtering efficiency and shape for up to at least 3 rounds [15]. However, in another work, where decontamination by dry heat was performed using oven at 70°C for 30 minutes, found that dry heat was not effective for decontamination of MRSA and the MS2 bacteriophages [16].

When dry heat was used against MS2 and MRSA at 100°C for 15 minutes, it did not result in a greater than 3 log₁₀ reduction of either organism at any inoculated sites on any masks or respirators [14]. Difference in results can be due to the variations in the treatment time.

Dry heat was also applied using rice cooker for 3 minutes, without adding water. Penetration of tested masks (N95, Gauze, Spunlace) remained similar, however, biocidal effect was not evaluated [12].

Dry heat can be used for mask decontamination. However, results are controversial. Advantages of dry heat include its convenience and low cost. The major concern over the effectiveness of dry heat is lower penetration rate. Therefore, heat applying time is needed to be tested with more variability to ensure its efficacy.

Decontamination by irradiation

Irradiation is also one of the promising methods for mask decontaminat ion. Ultraviolet germicidal irradiation (UVGI) sterilization cabinet, which provides UV-C light at wavelength of 254 nm with intensity of 8 W, was used for decontamination. Efficiency of melt blown fabric slightly changed after 10 cycles of UVGI treatment. Pressure drop after UVGI treatments remained similar. Biocidal effect was not evaluated in this study [9]. Other study using UVGI treatment tested against H1N1 Influenza virus as aerosols or droplets that are representative of human respiratory secretions . Treatment at wavelength of 254 nm with intensity of 1.6-2 mW/cm² for 15 minutes provided more than 4 log₁₀ reduction of viable H1N1 virus [17].

3 commercial N95 respirators were inoculated with MRSA, MS2 bacteriophages and Phi6, which is enveloped RNA v irus. UV-C treatment administered as 1-minute cycle in a UV-C box or a 30 minute cycle by a room decontamination device reduced contamination but did not meet criteria for decontamination of the viruses form all sites on the N95 respirators [16]. Indicating non-uniform distribution of UV-C.

In another study UVGI treatment was applied to circular coupons, punched from N95 respirators and tested for filter penetration and flow resistance. UVGI had a small effect on filtration performance and no effect on flow resistance at doses up to 950 J/cm². Results suggested that UVGI could be used to disinfect respirators, although the maximum number of disinfection cycles will be limited by the respirator model and the UVGI dose required inactivating the pathogen [18]. Treatment with a 40-W UV-C for 50 minutes to each side (outer and inner) of respirators did not affect the filter aerosol penetration, filter airflow resistance or physical appearance of the FFRs. Whereas 2450 MHz microwave irradiation (1 minute for each side) partially melted material adjacent to the metallic noseband [18].

Decontamination by UVGI irradiation is promising method. Its throughput capability is benefited by a relatively short irradiation time and there are no known health risks to the users. However, penetration depth may be limited, high dose may be needed to disinfect virus inside the mask. In addition, this method is limited by the available working surface area of a biosafety cabinet equipped with a UV-C source or other area being irradiated by UVGI source.

Decontamination by chemical methods

Solution based methods, such as ethanol (75%) applied by immersion and air dry, chlorine-based (2%) applied by light spray and air dry, drastically degraded the filtration efficiency to unacceptable levels. Pressure drop remained comparable. This indicates that the loftiness and structure of the melt blown were unchanged and the resultant efficiency degradation is the result of less apparent electrostatic charge on the electret. It was hypothesized that small molecules such as solvents can adsorb onto the fabric fibers on either screen or possibly lift the frozen charges, which would decrease the filtration efficiency [9]. Submersion in 70% ethanol, 100% isopropanol solution or bleach raised the penetration of particles through masks. Bleach even destroyed the structure of gauze filters [12]. Therefore, advise were given against liquid contact, such as alcohol, chlorine-based solutions or soaps.

In addition to degradation of filtration performance, chemical decontamination methods tend to leave unpleasant odor and residues on masks. FFRs treated with bleach retained a bleach odor following off-gassing period. Even though the amount was below action levels, odor might cause adverse effects in users with certain health conditions. Also, bleach corroded the metal parts on the FFRs and discolored other area [19]. Little or none of the gaseous sterilizers, such as ethylene oxide (EO), vaporous hydrogen peroxide (VHP) remained on the FFRs following decontamination and off gassing. However, EO treatment of FFRs produced detectable residues of 2-hydroxethyl acetate (HEA), toxic byproduct, possibly formed by a reaction of EO with rubber parts of the respirators. EO did not cause any visible changes to masks, where as VHP caused slight tarnishing of metallic nosebands [19, 20]. Among chemical methods, VHP seems the most promising. High level disinfection cabinet using submicron droplets of aerosolized peracetic acid and hydrogen peroxide was effective for decontamination of N95 respirators. 3 consecutive cycles or a single cycle achieved more than 6 log₁₀ reductions in MRSA, MS2 and Phi6 [16].

As a result, alcohol, chlorine-based solutions, soaps and bleach are not advised to use because they can significantly degrade the filter, either because they alter the electrostatic properties of the filter fibers, affect particle penetration levels, or deform the FFRs leading to its degradation. Moreover, chemical residues left on mask can cause risk to wearers. VHP is most promising chemical method. However, it is labor intensive, costly and it has limited widespread application.

Hot water treatment

Sterilization by hot water treatment was proposed for individuals at low risk for infection and without access to new masks, and not for healthcare professionals. Method includes following steps, such as soaking disposable surgical non woven mask in hot water above 56°C for 30 minutes with no rubbing and twisting during soaking process. This step is followed by drying by electric hair drier for about 10 minutes to recover electrostatic charge and testing the electrostatic effect with the paper scraps. With this method filterability remained similar to new masks. However biocidal effect was not evaluated; therefore, this method is suggested only for general public with low risk for infection [21].

National policies on extended use and reuse

Due to the current COVID-19 outbreak, some countries are allowing extended use and reuse of masks. The Korea CDC does not have a clear position on mask reuse. However, at present, it is recommended to use only once in health care workers (high risk group), and there is a consensus that ordinary citizens can use it multiple times.

Countries, such as Canada allows use of masks after the expired deadline, France, Mexico, New Zealand and S weden allow prolonged use of masks up to 4 hours if they are not removed, damaged or contaminated. Germany allows reuse of masks after treatment at 65 70°C in a drying cabinet for 30 minutes with maximum of 2 decontaminations. In Netherlands a short process with hydrogen peroxide gave an acceptable result, however, hydrogen peroxide sterilizers are not available in all institutions. VHP and steam treatment are also considered by Europe and the United States [22].

Even if the mask is decontaminated for reuse, no change in structure, maintenance of the firmness of each component of the mask, and maintenance of filtration function are required for reuse. In particular, in the filtration function, it is important to maintain the electrostatic capacity along with no change in the physical filtration function such as diffusion, interception, and inertia impaction of the filter fabric.