course, they would thereunto have to be decontaminated after each use. This procedure is not at all ubiquitous in the medical system, as FFRs are intended for single use by manufacturers. However, in recent years the scientific literature has laid out various possibilities to decontaminate FFRs and reuse them several times [2,3,4,5,6,7]. All these methods must meet the following requirements: the masks must be decontaminated, the effectiveness of the mask must not be damaged, the user must not be endangered in any other way (e.g. poisoning) and the masks should be ready for reuse quickly. In addition, the methods should avoid the usage of disinfectants, as these are in short supply in the current situation.
Some of the methods examined in the above-cited articles meet these criteria. In the following we classify these methods according to their mode of operation and the availability of the corresponding equipment as follows:
1. UVc-radiation (disinfection on an industrial scale using professional UV-sterilization systems)
2. Heat treatment with moist heat (heat treatment by means of ovens or dryers) 3. Microwave-generated steam (microwave ovens, on-site application with minimum effort)
The SARS-CoV-2 virus does not differ dramatically from other viruses in its chemical and physical resistance. First investigations show that SARS-CoV-2 can be eliminated in a few minutes with the usual disinfectants, UVc irradiation, or temperatures above 56°C [8, 9, 10, 11] . The results for the SARS-CoV-2 virus are comparable with those of the more recent pathogens of the last years (SARS-CoV-1, H1N1, H5N1) [4, 6, 7, 12, 13, 14, 15] . The table summarizes some of the existing (preliminary) results and compares them directly with data of the influenza-A pathogen H1N1. The disinfectants used included, among others, ethanol (75%), hand soap solution (2%), Clorox (10%), acetone and household bleach [8, 9] . 2) Warm moist heat
The germicidal effect of heat is widely known -the addition of moisture accelerates this.
Steam condenses on colder objects until these have reached the temperature of the steam.
To decontaminate FFRs, the masks must be exposed to a warm, humid atmosphere for a sufficiently long time. Several studies have shown that influenza pathogens (H1N1 and H5N1) can be removed up to 99.9% using a rather simple procedure [3,4,6]. In one of the publications, a sealable 6l vessel was filled with 1l of tap water and heated to 65°C in an oven. The container was then removed from the oven and a respirator mask was positioned above the water surface using a rack. The container was then closed again and placed back into the oven for a 20-minute treatment. The samples were then air-dried overnight. This method can also be used in the COVID-19 pandemic to remove SARS-CoV-2. Upscaling the method is straightforward. To shorten the drying time, placing the treated masks in a drying cabinet is useful.
Comment: There is a risk of contaminating the room in which the oven is placed, especially if the oven has a ventilator when turned on. Hence it is absolutely necessary that the container within which the mask is placed is sealed!
This method is related to the one above, as it includes steam used for disinfection. Two there is a risk that the mask could be damaged due to strong local heat generation. It would be possible to remove the metal part (nasal clamp) temporarily given this can be done easily.
The methods listed have been known for rather long and have been published in scientific journals about ten years ago. They offer the possibility to decontaminate the currently most urgently needed FFRs, and thus to use them repeatedly. Furthermore, modifications are possible for the listed processes. The following example demonstrates a modification that is realizable in a timely manner and uses devices available on the market without major, with the help of existing infrastructure in the industry. In particular, two of the described the methods (warm moist heat and microwave-generated steam) require essentially standard kitchen appliances. All three methods were shown to be successful for disinfecting masks from pathogens. The method using UVc radiation seems to offer a very robust and very effective solution, but requires more technical skills to calculate the correct irradiation time to achieve the required dose.
The strategies reviewed in this work could help ease the current precarious situation globally -and thus save lives.
Disclaimer: The methods and procedures outlined here are intended for "self-help" in emergency situations only and are not certified in any way. Anyone choosing to execute on these methods does so at his/her own risk.
It would be highly desirable for a governing instance to validate or officially recommend any of these methods for the time of the pandemic.