Table 2 Table of different decontamination methods according to the defined variables (see Fig. 1 for scoring with emojis.)

Dry heat

Efficacy

to

The decontamination method at a temperature of 65 to 70 °C shows a > 3 log reduction of most microorganisms tested, though not all of them (see Table 3). [7, 38,39,40,41,42]. Initial sorting of the masks must be performed very carefully, as dry heat treatments tend to fix proteins and organic matter to the mask material [43]. In other words, the presence of even an almost invisible stain means that the mask must be disposed of. Double exposure to heat over a long period of time can achieve decontamination values similar to those obtained with water vapor at 121 °C [7]

Risk

The exposure to dry heat does not cause the deposition of harmful molecules. It is therefore safe for HCWs. No other precautionary measure should be taken in addition to those related to the handling of potentially infected masks for personnel. The machines used must have an efficient and appropriate ventilation systems with an external exhaust from the room in which they are being treated (which is normally the case for rooms with this type of equipment) [7]. Using this method, and because the masks are only disinfected, it is essential that decontaminated masks are distributed to their original owner for the reasons mentioned in the "Efficacy" section

Cost

to

No cost estimates have been provided for this process, but the basic equipment is not expensive. For a two-step process, costs may be higher due to the need for two different types of devices as well as the added manpower to transport masks between the first and the second sterilization stages

Time

to

The duration of a disinfection cycle is less than an hour. The masks are treated in the disinfection device for 30 min and are ready for use shortly after. No desorption, evaporation or drying period is necessary. Personnel simply have to wait until the temperature of the masks is cool enough to handle safely before distributing them. For a two-step process, time is significantly longer

Complexity

Drying cabinets, laboratory drying rooms, ovens or washer-disinfectors are the devices offering the possibility of disinfecting the masks with dry heat. Their presence is probably common in high-resource hospitals. One cycle is enough to effectively eliminate SARS-CoV-2 and most other microorganisms

Reusability

Two disinfections per mask are recommended, resulting in three uses in [7, 44]. This method maintains the filtering functions and the physical integrity of the masks [38,39,40,41]

Ethylene oxide

Efficacy

This process eliminates SARS-CoV-2 as well as other microorganisms that may be present on the mask [15, 21]. Experimental study has shown a > 6 log reduction of microorganisms tested [20]

Risk

Several articles in the literature exclude the use of ethylene oxide because of its toxicity. This gas is known to be carcinogenic and teratogenic if the wearer is subjected to chronic inhalation [2]. Following the proper procedures should, however, avoid this problem. In Geneva, the experiments carried out concluded that this method needed to include a 48 h desorption time to guarantee the absence of harmful particles in the masks [4, 11]. It is extremely important to respect this desorption time before using the masks. The personnel in charge of sterilization must avoid any exposure to gas and handle the used masks with gloves and mask. As ethylene oxide is a flammable and explosive gas, safety measures associated with this risk must be considered. An additional risk factor could be that if the masks need to be reprocessed at a site external to the hospital, there could be inherent issues with oversight and quality control

Cost

to

The price of autoclaves, the necessary equipment and the cost of the workforce has to be considered. The costs have not been estimated in the included studies. However, there is a need for a specific external structure (see section "Complexity") and trained workforce, which can make the process less accessible. According to the Swiss pilot plan, a cycle of 30 masks requires 10 min of direct labor, meaning 20 s per mask [14]. As this method of sterilization is often used in industry it may, be feasible to perform at a large scale, which could reduce the cost per mask quite significantly. The cost of 30 masks sterilized in the HUG is 86 CHF (€80) which means 2.60 CHF per mask (€2.40). In an industrial process in Switzerland, 8000 masks can be sterilized for 2500 CHF (€2325 which means 0.30 CHF per mask (€0.28)

Time

The total time needed for sterilization is around 51 h. The sterilization process itself lasts for 3 h. It is necessary to wait an additional 48 h before handling or reusing the masks. According to the HUG project, no residue of ethylene oxide was present on the tested masks after the 48 h desorption time. It is very important for the health of personnel that the desorption of ethylene oxide is complete. Additional time may be added in the case of the reprocessing needing to take place in a structure external to the healthcare facility

Complexity

The infrastructure and equipment necessary for ethylene oxide sterilization is lacking in most hospitals. Thus, a dedicated worker external to the care center is necessary for the implementation of this method. This complicates the recycling process, as the masks must leave the medical center, be sterilized and then brought back. This particular sterilization technique is most often implemented in industrial processes, which may also imply a greater sterilization capacity. The cycles carried out in Switzerland contained 30 masks at a time, which is advantageous. One cycle is sufficient to guarantee complete sterilization of the masks

Reusability

Up to three sterilizations can be performed, resulting in a maximum of four uses for each mask [45]. This method does not affect the filtering properties or the physical integrity of the mask [2, 46]

Hydrogen peroxide (gaseous)

Efficacy

This process eliminates SARS-CoV-2 as well as any other microorganisms that might be present on the mask [21, 40, 42, 47,48,49]. Experimental study has shown a > 6 log reduction of microorganisms tested [20, 50, 51]

Risk

The personnel in charge of sterilization is no more at risk than for another disinfection process using an autoclave. Wearing a mask and using gloves are recommended. After placing in the sterilizers, the masks must remain at least one hour in order to guarantee the full evaporation of the hydrogen peroxide [14] and personnel must be kept from breathing it in when opening the autoclave. For HCWs as well, it is important that the evaporation time of the hydrogen peroxide vapors is respected before handling or reusing the mask. Following this, the masks can be used again. The residual quantity of the decontaminant is below the permissible exposure limit (PEL) [52, 53]

Cost

The price of autoclaves, the necessary equipment and the cost of the workforce has to be considered. The Swiss pilot plan calculated a price of 15 CHF (€14) per sterilized mask [12]. It is also estimated that a cycle requires 10 min of labor, which amounts to one minute per mask (as most of the tested autoclaves had a capacity of 10 masks per cycle)

Time

The duration of a sterilization cycle varies between 1 h 24 min and 2 h depending on the program used. The hydrogen peroxide vaporization programs themselves last 24–55 min [22, 31]. The masks must then rest for one hour to allow the hydrogen peroxide to evaporate for the reasons mentioned previously in the "Risks" section

Complexity

A single passage in the autoclave guarantees sterilization of the masks. However, autoclaves compatible with hydrogen peroxide sterilization are rarely present in healthcare facilities. The adoption of this method would probably imply the acquisition of this equipment. The presence of a zone dedicated to the safe evaporation of hydrogen peroxide is also necessary. On the other hand, these programs are effective for 10 to 20 masks at a time [4, 11]

Reusability

to

The number of sterilization cycles that a mask can withstand without damage varies from between 2 to 10 cycles depending on the model of autoclave used. As long as the masks have not reached their maximum number of possible decontaminations, their filtering capacities and integrity are maintained [4, 9, 46]. When they reach the maximum number, they must be discarded

The hydrogen peroxide vapor sterilization systems proposed by the Battelle Memorial Institute and the article by Bergam et al. served as references for the FDA and the CDC [2, 54, 55]. The method proposed by Battelle is interesting as it allows up to 20 sterilizations per mask [3, 8]

Moist heat

Efficacy

This process eliminates SARS-CoV-2 as well as any other microorganisms that might be present on the mask [23, 56,57,58]. Moist heat can be generated by different ways. Experimental study of microwave-generated steam has shown a > 6 log reduction of microorganisms tested [24]. Moist heat disinfection in an oven has shown a > 3 log reduction of microorganisms tested [59, 60]. Moist heat disinfection in a rice cooker has shown a > 3 log reduction of microorganism tested [61]

Risk

The exposure to steam does not cause the deposition of harmful molecules. It is therefore safe for HCWs. No other precautionary measure should be taken in addition to those related to the handling of potentially infected masks for personnel

Cost

The devices used (microwave and bags, rice cooker, containers and oven) are not expensive compared to other techniques. This accessibility is a significant advantage for low resource environments

Time

The masks undergo a cycle close to a minute in the microwave and then dry for a maximum of an hour. In one experiment, the masks were dried for 30 min or 60 min under conditions of approximately 20 °C and 60% relative humidity [23]. The total time for a disinfection cycle should be approximately one hour. For moist heat generated by a rice cooker, time of treatment is also less than an hour [61]. The disinfection process lasts 15 min and masks must be dried after. For moist heat in an oven, masks are exposed during 20 to 30 min. However, the masks are located in a container during the process of disinfection that must have been heated before. This process can request 3 h of preheating the containers without having a mask in them [59, 60]

Complexity

A single pass in the microwave, rice cooker or oven guarantees elimination of the microorganisms tested [23, 24]. The complexity of implementing this method is low as the necessary equipment is accessible. Furthermore, the MSB X/Y bags for microwave-generated steam can be used as storage bags before the disinfection [23]. The capacity of the microwaves, ovens or rice cookers can be a limiting factor

Reusability

For microwave-generated steam, masks can be disinfected more than 3 times. In one study masks’ fit and function were preserved after 20 cycles [24]. No information about reusability is available for oven moist heat and rice cooker moist heat disinfections. However, one disinfection by these two methods does not affect the filtering properties or the physical integrity of the masks [59,60,61]

Ozone

Efficacy

All of the studies showed that the ozone disinfected well, and has the potential for sterilization [26, 62, 63]

Risk

Ozone is toxic and a lung irritant but unstable; it degrades into oxygen. With adapted equipment and ventilation, this method is low risk, but system needs to be functioning well without leaks, etc [64]

Cost

Dedicated equipment and oxygen gas is needed to produce the ozone on site, but otherwise this method is not excessively expensive [65]

Time

Relatively short time is needed, tests on masks were conducted with exposure times of between 5 min and two hours [25, 74, 75]

Complexity

Some specialized equipment and a sealed chamber for the masks is needed

Reusability

The only area of the facemasks that seemed to have an issue is the straps, which also puts into question the usability of ozone as a successful decontamination in the first place [62, 63]. In order to recommend this treatment, masks must be verified for compatibility with this method

Steam sterilization

Efficacy

This process eliminates SARS-CoV-2 as well as other microorganisms that may be present on the mask [35, 66]. Experimental study has shown a > 6 log reduction of microorganisms tested [20]

Risk

The exposure to steam does not cause the deposition of harmful molecules. It is therefore safe for HCWs. No other precautionary measure should be taken in addition to those related to the handling of potentially infected masks for personnel

Cost

The costs depend mainly on the presence or absence of the water vapor sterilizers and their capacity

Time

The duration of the process until the masks can be is usually less than 1 h. The sterilization programs themselves last 15–20 min. The masks go out dry so they can be used directly after their sterilization

Complexity

One sterilization cycle is sufficient for the mask to be used again. Steam sterilizers are found in the majority of sterilization units in healthcare facilities, though there is a need to accommodate for a drying area for masks. One cycle is sufficient to guarantee sterilization of the masks

Reusability

The number of sterilization cycles varies by country. The Austrian Ministry of Health recommends a single sterilization and therefore two possible uses per mask. In the Netherlands, health authorities state that two sterilization cycles are feasible (resulting in 3 uses per mask) [7]. Concerning the filtration properties of masks after sterilization, a Dutch study indicated that these do not vary significantly between masks sterilized once and 5 times. However, there is nonetheless a decrease in filtration capacities after sterilization [67]

Ultraviolet Germicidal Irradiation (UV-C)

Efficacy

This process eliminates SARS-CoV-2 as well as any other microorganisms that might be present on the mask. Most of studies have shown a > 3 log reduction of microorganisms tested. [6, 29, 35, 58, 68,69,70,71,72,73,74]. Two studies have shown a > 6 log reduction of microorganisms tested [6, 29]. It is generally accepted that the efficacy of disinfection by UV strongly depends on the dose administered and thus on the intensity of the lights available [2, 75, 76]. This method destroys SARS-CoV-2 if a sufficiently high-energy UV apparatus is used. According to laboratories in the Nebraska [6], 2–5 mJ/cm² are necessary for the inactivation of the coronavirus. In practice, they recommend not exposing the masks to any less than 300 mJ/cm². The study showed that the bacterial and viral load was reduced by 6 logarithms when subjected to 60–300 mJ/cm². A study from Switzerland concludes that a 60 mJ/cm² UV irradiation is sufficient to reduce by > 3 log of virus [68]. The efficacy also depends on the sufficient exposure of the mask material to the light. It is very important that the entire surface of the masks is reached by light, either directly or by reflectors, which can pose certain challenges to implementation. The efficacy depends also of the distance between the lamp and the mask and time of exposure [77]

Risk

The exposure to UV does not cause the deposition of harmful molecules [52, 53]. It is therefore safe for HCWs. In addition to the protective measures linked to the handling of used masks, the personnel in charge of disinfection must of course avoid any exposure to the UV rays. It is therefore recommended to dedicate a room to the process, and to turn on the lamps remotely after the room has been closed

Cost

The initial cost of buying the UV machine is quite high. UV lamps and all the material necessary for the reflection of light are expensive. However, the cost per cycle is much lower than the one for HPV sterilization if a high number of cycles are performed. It is, however, still a method mainly accessible to healthcare facilities in high resource environments. It is also worth noticing that the cost varies according to the quality of the installation

Time

A disinfection or sterilization cycle (> 3 log reduction or > 6 log reduction) lasts less than one hour. The exposure time required is between 15 and 40 min and no additional steps are necessary [6]. Calculations must be made to ensure that the distance between the lamp and the mask is correct. The time needed for this type of decontamination increases as the mask is placed further form the source [77]. Time of sufficient exposure depends also on the intensity of the lights as said before

Complexity

This method uses equipment which is not present in most hospitals. Moreover, it requires the installation of a room intended solely for decontamination. One cycle is enough to effectively eliminate SARS-CoV-2 and most other microorganisms

Reusability

The possible number of cycles for each mask remains to be defined. According to one study, masks exposed to 1 J/cm² can be reprocessed up to 10 times without the filtering properties or the integrity of the mask being affected [5, 44]. This same study also concludes that the exposure for the majority of models of N95 masks with 20 cycles causes almost no alteration, though some models of masks showed damage or dysfunction

One study tested UV disinfection followed by dry heat disinfection (65 °C). This method could be considered if the UV lamps available are of low intensity, as the dry heat further reduces the viral and bacterial load [78]