Reuse of Masks After the Sterilization by Dry Heat Method
In the current COVID-19 pandemic, wearing a mask is recognized as one of the most effective personal hygiene actions to prevent infection. Especially for the medical staff who have to contact the infected patients, the face mask is the most basic resource. Unfortunately the shortage of mask supply is prevalent globally due to the explosive increase demand. Lately a Korean research team announced that they have developed a nanofiber mask that can be reused after washing. While this new product is considered as a potential solution for the mask shortage problem, it needs further testing to prove its stability and requires regulatory certification before becoming available in the market. Then, are there any other solutions that can solve the shortage problem immediately? Refer to the following methods recommended by the German government.
The Federal Ministry of Labour and Social Affairs (BMAS) and the Federal Ministry of Health (BMG) of Germany suggest a dry heat sterilization of used masks as a solution. They note that the Coronavirus (SARS-CoV-2) is deactivated when it is exposed to a dry heat of 65-70∩ for 30 minutes. Similarly, a mask can be decontaminated, using a similar heat procedure, and it can be safely used up to two more times. This dry heat purification method could be useful to address the shortage of masks, particularly for the medical staff who treat patients infected by COVID-19.
Can be applied the dry heat sterilization for all kinds of masks?
It is important to note that masks should be able to maintain its tissue at 65∩. According to BMAS and BMG guidelines, a heat-resisting property of respiratory protection mask (FFP mask) should be checked at the 65∩ temperature. For instance, the EN 149 masks, a CE certification standard, require a test for heat-resisting property under the condition of 70∩ for 24 hours. Accordingly, the proposed dry-heat sterilization method can be applied to the EN 149 masks. But most masks licensed for distribution in the U.S., Canada, Australia or Japan, have been pre-treated at 38∩ only, so they require further testing of a heat-resisting property before dry-heat sterilization. We note that KF masks, popular in Korea, also are pre-treated only at 38∩ .
In Korea, two different types of masks are used broadly: one for personal health and the other for industrial use. They are separately managed by the Food and Drug Administration (KFDA) and Ministry of Employment and Labor (MOEL). The KF standard does not require a test of the heat-resisting property, so further testing is necessary to assure its filtering function is maintained after the suggested dry-heat sterilization. Nevertheless, because KF94 applies the same standard with FFP2/3 of Europe and N95 of the U.S, and because it is based on an electrostatic charging technology using melt-blown as the basic material, those KF masks can be sterilized by dry heat. In comparison, although the industrial masks are tested more strictly at 70∩ for 24 hours , they are not suitable for public health purposes because their respiration valve has no proven function of preventing virus emission .
Is the filtering function of mask maintained after the dry-heat sterilization?
A 3M researcher team reported that the filtering function of their masks (3M 1860 and 1870, certified as N95, the U.S.) is not affected when they tested repeatedly 5 to 10 times under the condition of 60∩ and 80%RH for 30 minutes . Similarly, a Stanford University research team has found out that N95 certified masks were able to maintain their filtering function after testing repeatedly for 20 times under the condition of 75∩ for 30 minutes .
Peter Tsai, a professor emeritus at the University of Tennessee in the U.S., who invented the electrostatic charging technology-based mask (N95), claimed that the filtering efficiency would not decrease even if a mask is treated by the dry-heat sterilization several times at 70∩ for 30 minutes . He cautioned that the tissue of the filter should not directly contact the metal surface of high thermal conductivity during the sterilization, and users should clean their hands with soap after handling the non-sterilized masks.
Which equipment can be used for the dry-heat sterilization procedure?
Both BMAS and BMG state that an oven commonly used in hospitals or laboratories can be properly used for the dry-heat sterilization procedure . General-purpose cooking ovens are not recommended for this procedure, because their accuracy of temperature control between 65 to 70∑C is not yet assured, and they introduce an additional concern about infection through food . Furthermore, since this sterilization procedure has not been tested against all general pathogens, the current recommendation only applies only for the recycled use by personnel operating in the COVID-19 environment. Masks used by patients of tuberculosis or other strong infectious diseases should be discarded .
The entire world has been suffering unprecedentedly from the COVID-19. We hope this short report on the dry-heat sterilization procedure can make a small but effective contribution to addressing the mask shortage problem faced by medical staff and public health teams around the world. All of us at the Jeio Tech, the manufacturer of world-leading Lab Companion oven, look forward to welcoming back the normal days. We will do everything we can to support people through the coronavirus pandemic.
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 Use of protective masks in healthcare facilities, BMAS (The Federal Ministry of Labour and Social Affairs, German)
 Guidelines for health mask standards, Food and Drug Administration (KFDA) (2018. 7.)
 Disinfection of Filtering Facepiece Respirators, 3M, Technical Bulletin Board (2020. 3)
 Can N95 facial masks be used after disinfection? And for how many times?, Dr. Lei Liao et. Al., Stanford University and 4C Air, Inc. (2020. 3)
 Information and FAQs on the Performance, Protection, and Sterilization of Face Mask Materials, UTRF News (2020. 3)
 Safety certification system for respirators and cases of certification, Korea Occupational Safety & Health Agency (KOSHA) (2017. 7)
 Correct use of dust masks for industrial use, Ministry of Employment and Labor (MOEL), KOSHA
 Can face masks be safely disinfected and reused?, Stanford AIM Lab COVID-19 Evidence Service Report, (2020. 3)