Protecting Those Who Protect Us
Disinfecting Face Masks in Healthcare Safely
As the COVID-19 pandemic spread around the world, the number of people becoming infected and needing hospital care are overwhelming our medical systems. Healthcare workers, on the front line, are facing large numbers of extremely ill and highly infectious patients. However, recently they also had to deal with a lack of basic personal protective equipment (PPE) needed to protect themselves. Basic items such as NIOSH certified N95 respirator masks (1) are now in such limited supply that healthcare providers are unable to change masks when going from one patient to the next, and the masks are also being reused from one day to the next. Extreme measures are being taken by health care systems to address PPE shortages due to the unique complications of this pandemic. Decisions are being made without the rigorous reviews that would normally occur in changing a medical practice so that healthcare workers can continue, as a united force, to focus on patient care in the short term. Many of these solutions are less than ideal and may impart additional risk to healthcare workers. The CDC provides some advice for reusing masks in times of crisis. (2)
Limited re-use of facemasks is the practice of using the same facemask by one HCP for multiple encounters with different patients but removing it after each encounter. As it is unknown what the potential contribution of contact transmission is for SARS-CoV-2, care should be taken to ensure that HCP do not touch outer surfaces of the mask during care, and that mask removal and replacement be done in a careful and deliberate manner. (3)
With Covid-19 rapidly over-stretching healthcare facilities, medical staff are being told to wear PPE such as N95 masks for multiple patients. For example, one healthcare system issued new guidance saying: (4)
Healthcare workers can continuously wear [PPE] without doffing and redonning between patient contacts, as long as they are not soiled, contaminated or damaged.
Even with these measures, a recent survey found that 45% of U.S. healthcare facilities surveyed are already out of, or almost out of respirators to use in caring for patients with COVID-19. (5) Some facilities are relying on home made cloth masks, (6) however, many of these ‘home made’ masks provide false assurance, since simple cloth masks have been found have a wide variability between mask designs (7) and some retain only about a third of the particles of surgical mask, (8) especially for small particles. (9) However, many hospitals are so low on PPE that they are sending their front-line staff in to care for patients without anything at all while others are collecting masks in mass amounts to decide what to do to conserve them.
Another approach that is being considered is whether a mask can be disinfected or sterilized between uses, or even users. In a 2007 study, it was also found that disinfection using an autoclave, 160°C dry heat, 70% isopropyl alcohol, and soap and water (20-min soak) caused significant degradation to filtration efficiency. (10) A 2009 study found that microwave and steam sterilization destroyed some N95 masks, whereas UV light, hydrogen peroxide, ethylene oxide (EtO) and bleach appeared to work. However, it was found that disinfection with bleach resulted in chlorine off-gassing even after the filter had been rinsed with water and dried. EtO did not affect the physical properties of the mask, but the long cycle times were a detriment to this method, and EtO is a known carcinogen. Hydrogen peroxide vapor was also found to not affect the physical properties of the mask, though some masks were found to absorb enough hydrogen peroxide vapor that the ASP Sterrad sterilizer would abort. (11) UV light did not affect the physical properties of the masks. (12) Nebraska Medicine, for example, has been testing using UV light to disinfect N95 masks between users, (13) and some companies are working on similar products. (14)
Hospitals frequently reprocess medical devices for re-use which involves a two-step process, cleaning and decontamination, followed by high-level disinfection or sterilization. (15) There is no practical way for N95 masks to be cleaned by a healthcare facility, instead they are inspected and those with visually gross contamination are discarded, and only the ‘clean’ ones are reprocessed. Since it is impractical to clean N95 masks during reprocessing, this article uses the term disinfection rather than decontamination to describe the reprocessing of N95 masks. Also, while some studies have shown that the various disinfection methods used do destroy viruses, so far very few of them have demonstrated a sufficient sterilization assurance level (SAL is typically 10-6 meaning that there is a less than or equal to one chance in a million that a single, viable microorganism is present on a sterilized item) (16) for the process to described as sterilization.
Hydrogen peroxide is the most common method of low-temperature sterilization used in hospitals today and hydrogen peroxide vapor is widely used for whole room disinfection. Preliminary results indicate that hydrogen peroxide vapor is an effective viricidal agent when used to disinfect N95 masks, using a room disinfection unit. (17) Similarly, Duke University School of Medicine recently announced that they are disinfecting N95 masks with hydrogen peroxide vapors before redistributing them to health care workers, and this process is being extended to other hospitals in the Duke system. (18) Battelle also just received emergency clearance from the Food and Drug Administration for its Critical Care Decontamination System which can decontaminate up to 80,000 masks per day using hydrogen peroxide vapor. (19,20) Sterilizer manufacturers have also developed methods to reprocess N95 masks, for example Advanced Sterilization Products (ASP) which is part of Fortive Corp., has qualified a protocol for reprocessing select N95 masks so that hospitals can use their sterilizers, and process up to 480 masks per machine daily. (21) Richard Peltier of the University of Massachusetts (Amherst) and Dr. Brian Hollenbeck, chief of infectious disease at New England Baptist Hospital in Boston found that N95 face masks were still effective at blocking infectious particles after sterilization. (22)
Facilities considering reprocessing N95 masks need to provide a space dedicated for inspecting and packaging used masks for disinfection. Used N95 masks cannot be take to the Prep. & Pack side of a Sterile Processing Department since they are contaminated and risk contaminating the environment where surgical instruments are exposed to the environment. Conversely, since the masks cannot be cleaned, they cannot be taken to the decontamination room since they would absorb or be exposed to additional contamination.
As mentioned above, hydrogen peroxide is the most commonly used method for chemical low temperature sterilization with the market leader, ASP having over 20,000 hydrogen peroxide sterilizers worldwide. (23) Hydrogen peroxide is a very effective sterilant, killing bacteria, viruses and other pathogens quickly with the shortest cycle times (~ 28 minutes) for any chemical sterilant used in healthcare and so this vapor must be managed to ensure that healthcare workers are not exposed to hydrogen peroxide vapor. Hydrogen peroxide vapor is very toxic (otherwise hydrogen peroxide would not be such an effective sterilant chemical) and it has an OSHA permissible exposure limit (PEL) of only 1 ppm calculated as an 8-hour time weighted average. (24,25)
Using a room disinfection system to disinfect N95 masks raises two issues that need to be managed. The first issue is ensuring that the people who are performing the mask disinfection are not exposed to hydrogen peroxide vapor. The second issue is to ensure that any residual hydrogen peroxide off-gassing is sufficiently low that the next wearer of the mask does not breathe in a significant concentration of hydrogen peroxide vapor.
In a typical hydrogen peroxide disinfection setup, a device evaporates or mists a known amount of hydrogen peroxide into the room, the amount calculated based on the volume of the room or chamber and the desired concentration. The hydrogen peroxide is then left for a dwell period during which the disinfection process occurs, and then the hydrogen peroxide is removed by drawing it through a hydrogen peroxide scrubber or to an outside exhaust. The question that operators face is how to determine when the concentration is low enough for them to re-enter the room or chamber. The vapor concentration for room disinfection is usually several hundred ppm, but the safe limit, as defined by OSHA is only 1 ppm. Hydrogen peroxide vapor is colorless and has essentially no odor, so smell cannot be used as an indicator of concentration. If the operators do not wait long enough before entering, then they risk being exposed to dangerous concentrations of hydrogen peroxide vapor. If they wait too long, then they reduce the throughput of mask disinfection at a time when the availability of masks is critical. The time to renter will depend on the equipment used, but it will also depend on the efficiency of the ventilation system and materials in the space that has been disinfected. The following procedure is intended to allow them to optimize the time before re-entering. This is the same procedure that is used in many pharmaceutical companies and other organizations that use hydrogen peroxide vapor to disinfect rooms.
Procedure to Determine When It is Safe to Re-enter a Room or Container After Hydrogen Peroxide Disinfection.
1) Run the disinfection cycle as advised by the manufacturer of the hydrogen peroxide vapor equipment.
2) Estimate the approximate time that it should be safe reenter, based on a. Manufacturer’s guidance b. Calculation if system parameters are sufficiently well known, or c. Estimate based on prior runs
3) Momentarily crack the door and place a hydrogen peroxide sensor inside the room, close the door and monitor the reading from the sensor outside.
4) When the monitor shows that the hydrogen peroxide concentration has fallen below 1 ppm, then it is safe to open the doors and enter.
5) If the monitor shows that the reading was already below 1 ppm, then on the next run, the time before placing the sensor in the room can be shortened.
Gas sensors for hydrogen peroxide monitoring for occupational safety typically have a resolution of around 0.1 ppm, i.e. these are precision instruments. The hydrogen peroxide vapor concentration in the disinfection room or chamber (typically 200 to 800 ppm) is several orders of magnitude higher than the OSHA PEL. If the sensors were left in the room during the disinfection cycle, the sensors would be saturated after exposure to such high concentrations that they would no longer be able to read below 1 ppm. Therefore, it is necessary to put the sensor into the area once the hydrogen peroxide has fallen to a lower concentration. Momentarily cracking open the door at a time when the concentration is expected to be low prevents the user from being exposed to too much hydrogen peroxide vapor. It is very helpful if the sensor can be read from outside the room. In some cases, there is a window which allows the user to look inside and see the hydrogen peroxide monitor, but a better way is to have a separate sensor in the room with a wireless (e.g. Bluetooth) connection to a monitor outside the chamber or room, such as ChemDAQ’s SafeCide hydrogen peroxide monitor.
If the room or chamber door seals are not effective then there is a risk that hydrogen peroxide vapor can escape from the disinfection room or chamber. Placing an area monitor for hydrogen peroxide outside the room or using a portable monitor is a simple way to ensure that people present are not overly exposed to otherwise imperceptible fugitive hydrogen peroxide vapor.
Off-Gassing of Hydrogen Peroxide
People who are familiar with gas sterilization may know that ethylene oxide (EtO) sterilization cycles are long, typically 10 to 12 hours, and most of this time is aeration time, i.e. waiting for the rate of EtO off-gassing to fall to a level sufficiently low enough that the medical devices can be used. While EtO is notorious for its long off-gassing, other chemicals such as hydrogen peroxide will also absorb and later off-gas. The time needed to off-gas depends on the material that has been exposed to the hydrogen peroxide vapor, the concentration of hydrogen peroxide and environmental conditions. For example, a battery or steel clamp will require essentially no off-gassing time, but a fabric, especially if it contains any cotton, paper or other cellulose materials will absorb much more hydrogen peroxide and will take much longer to off-gas after disinfection.
It is difficult to estimate how long an N95 mask will require to off-gas, but the 2007 paper described above in which N95 masks absorbed enough hydrogen peroxide to cause the sterilizer to abort (26) suggests absorption is significant. Most N95 masks are made of non-woven polymer such as polyurethane and polyester, polypropylene and polyisoprene, (27) and it is well known that hydrogen peroxide can absorb onto and diffuse into various polymers upon exposure and off-gas again when the hydrogen peroxide atmosphere is removed.28,29 In 2016 the FDA commissioned a study with Battelle on reprocessing N95 masks using a Bioquell hydrogen peroxide vapor generator, which showed that N95 masks could be reprocessed multiple times. The final procedure in this report found that the aeration time needed before the hydrogen peroxide dropped to below 1 ppm was 210 minutes and it took 300 minutes before the hydrogen peroxide vapor was below detectable limits. The off-gassing was measured by placing the 9N5 mask onto a “chuck” and drawing air through it, and sampling the air from the mask using a hydrogen peroxide monitor. (30)
ASP recently issued instructions for reprocessing N95 masks that do not contain cellulose in their sterilizers, saying that masks should be allowed to aerate for one hour prior to returning the masks for use. (31) Additionally, ASP recommends that N95 masks should be individually packaged in an appropriately sized Tyvek® Self Seal pouch or equivalent product. However, if it takes five hours for an N95 mask to aerate with air being blown through it, it is optimistic to assume that a mask left on a tray, wrapped in a Tyvek or other gas porous bag will fully aerate in an hour. Incomplete aeration is of particular concern for the next person who will be wearing the mask. Since the off-gassing is from a mask on the wearer’s face and they are breathing through the mask, then there is the potential for a much greater exposure. After the masks have been sterilized, it is recommended that they be checked for off-gassing. This step can be done relatively simply:
Procedure for Assessing Off-Gassing from Hydrogen Peroxide Disinfected N95 Masks
1) Place the masks that have aerated after disinfection into an approximately air-tight container.
2) Place a portable gas monitor with a sensor for hydrogen peroxide inside the container and seal the container.
3) Wait for 30 minutes and take a reading.
4) If the reading is less than 0.2 ppm (one fifth OSHA permissible exposure limit), then the masks can be released for use. 5) If the reading is higher than 0.2 ppm, then the masks should be removed from the box and laid out in well ventilated area to off-gas for at least two hours. The process can then be repeated.
Off-gassing poses another risk. Most of ASP’s Sterrad sterilizers and Steris Corporation’s V-Pro sterilizers have a cycle time as short as 28 minutes. To archive sterility in that short of a time requires exposing the load to very high concentrations of hydrogen peroxide vapor, and consequently the initial rate of hydrogen peroxide off-gassing is large. With the current crisis in N95 mask supplies, hospitals are likely to take full advantage of these short cycle times and run back to back loads to get as many masks reprocessed as possible. The operators therefore face not only the off-gassing from one load, but from multiple loads, and from multiple sterilizers if the facility has more than one. While the high air turn overs in a sterile processing area will dilute a small amount of off-gassing to safe levels, the off-gassing from multiple loads means that there is a risk of exposure to hydrogen peroxide vapor. It is advisable therefore, that the operator wear or carry a portable hydrogen peroxide gas monitor in order to check that he or she is not exposed to the cumulative off-gassed hydrogen peroxide from a large number of masks. The use of hydrogen peroxide aerators discussed below would also solve this problem.
In response to the current situation, ChemDAQ has developed a prototype hydrogen peroxide aeration box, containing a fan and a hydrogen peroxide scrubber, that the masks can be put inside to aerate immediately after they are taken out of the sterilizer. This aerator simplifies the above procedure in that the masks are placed in the aerator directly from the sterilizer or disinfection room/chamber. This procedure should be done in a way so as limit the potential exposure of the operator (e.g. use gas monitor to show that it is safe to unload the sterilizer, or place a gas sensor directly over the sterilizer door, open the door and step back and only unload the sterilizer when the hydrogen peroxide concentration has fallen to safe level).
Procedure for Off-Gassing from Hydrogen Peroxide Disinfected N95 Masks in Aerator
1) Place the masks in the aerator directly from the sterilizer.
2) Place a portable gas monitor with a sensor for hydrogen peroxide inside the container and seal the container.
3) Turn the fan on and wait until the hydrogen peroxide monitor reads below 0.2 ppm
4) Turn the fan off, remove the hydrogen peroxide scrubber, close the lid and wait for 30 minutes.
5) If the reading stays less than 0.2 ppm, then the masks can be released for use.
6) If the reading is higher than 0.2 ppm, then place the hydrogen peroxide scrubber back into the aerator, and the fan turned on again for at least two hours. Repeat the process as needed.
The time needed for aeration depends on the materials the masks are fabricated from, the number of masks, how they packaged and packed etc. The aeration times describe above are just guidelines and the optimum time will vary with specific conditions. Operators can use the hydrogen peroxide vapor monitor to optimize the aeration times needed to bring the off-gassed hydrogen peroxide concentration down to safe levels. It is recommended to use a monitor that has a separate wireless (e.g. Bluetooth) sensor that is connected to the exterior of the container, such as ChemDAQ’s SafeCide hydrogen peroxide monitor, so more easily see the hydrogen peroxide concentrations.
Other disinfectant chemicals can also be used, the prime candidates are those for which the vapor generators are already available for room disinfection using compounds that do not leave any harmful residues including ozone, chlorine dioxide and peracetic acid. (32) If these same chemicals are used, then the same issues concerning determining when it is safe to reenter the room, and off-gassing from the disinfected masks are the same. Similar procedures can be used, but with a sensor designed for the gas or vapor being used and appropriate gas scrubber material.
As the Covid-19 pandemic stretches healthcare systems beyond capacity across the United States, many facilities are running out of basic PPE such as N95 masks and so are being forced to reuse the masks available. Disinfection by hydrogen peroxide vapor is one of the most promising methods being tested, but there are some potential hazards from hydrogen peroxide that needs to be considered. After hydrogen peroxide disinfection has been completed, operators must re-enter the area but need to know when it is safe to do so. Since hydrogen peroxide has essentially no odor, a continuous gas monitor for hydrogen peroxide is required to verify that it is safe to enter the disinfection area. Secondly, hydrogen peroxide off-gassing may continue from the polymers comprising the masks and before these masks are distributed for reuse they need to be fully aerated. Aerating masks release hydrogen peroxide vapor in the air and cumulatively pose an exposure risk to operators. A simple procedure is described to verify that the aeration period is sufficient using a portable gas monitor for hydrogen peroxide and a simple hydrogen peroxide aerator is described
Meet the Authors
The authors would like to thank Lisa Wakeman for her valuable suggestions in writing this article.
Dr. Richard Warburton
CTO & General Counsel
President & CEO
1 For a list of manufacturers whose masks have been certified by NIOSH see https://calendar.google.com/calendar/r/month/2020/4/1?tab=mc1. N95 is one of several standards that NIOSH certifies respirator masks. https://www.cdc.gov/niosh/npptl/topics/respirators/disp_part/default.html. The requirements for an N95 mask and test procedure that are used prior to certification can be found at 42 CFR Subpart K—NonPowered Air-Purifying Particulate Respirators, subsection §84.170 to §84.182 Non-powered air-purifying particulate respirators; description.
4 Oakland hospital took in coronavirus cruise patients. Now, low on supplies, nurses asked to reuse disposable masks.Akela Lacy, March 21 2020, https://theintercept.com/2020/03/21/oakland-coronavirushospital-kaiser/
5 National survey shows dire shortages of PPE, hand sanitizer across the U.S., Healthcare Purchasing News, Mar 31st, 2020, https://www.hpnonline.com/infection-prevention/disposables-kits-drapes-ppeinstruments-textiles-etc/article/21132020/national-survey-shows-dire-shortages-of-ppe-hand-sanitizeracross-the-us, citing survey of infection prevention experts conducted March 23-25, 2020 by the Association for Professionals in Infection Control and Epidemiology (APIC).
6 Donations & Volunteer to Make Face Masks, University Hospitals, https://www.uhhospitals.org/healthcare-update/masks-and-donations, https://www.providence.org/lp/100m-masks, https://www.chop.edu/how-make-homemade-diy-facemask.
7 The efficiency of surgical masks of varying design and composition. Quesnel LB., Br J Surg. 1975 Dec;62(12):936-40.
8 Davies, Anna & Thompson, Katy-Anne & Giri, Karthika & Kafatos, George & Walker, James & Bennett, Allan. (2013). Testing the Efficacy of Homemade Masks: Would They Protect in an Influenza Pandemic?. Disaster Medicine and Public Health Preparedness. 7. 413-418. 10.1017/dmp.2013.43.
9 Evaluating the efficacy of cloth facemasks in reducing particulate matter exposure. Shakya KM, Noyes A, Kallin R, Peltier RE., J Expo Sci Environ Epidemiol. 2017 May;27(3):352-357. doi: 10.1038/jes.2016.42. Epub 2016 Aug 17.
10 Viscusi DJ, King WP, Shaffer RE. Effect of decontamination on the filtration efficiency of two filtering facepiece respirator models. J Int Soc Respir Prot. 2007;24:93–107
11 Viscusi DJ, Bergman MS, Eimer BC, Shaffer RE. Evaluation of five decontamination methods for filtering facepiece respirators. Ann Occup Hyg. 2009;53(8):815–827. doi:10.1093/annhyg/mep070
12 Viscusi DJ, Bergman MS, Eimer BC, Shaffer RE. Evaluation of five decontamination methods for filtering facepiece respirators. Ann Occup Hyg. 2009;53(8):815–827. doi:10.1093/annhyg/mep070
13 N95 Filtering Facepiece Respirator Ultraviolet Germicidal Irradiation (UVGI) Process for Decontamination and Reuse, John J Lowe, Katie D Paladino, Jerald D Farke, Kathleen Boulter, Kelly Cawcutt, Mark Emodi, Shawn Gibbs, Richard Hankins, Lauren Hinkle, Terry Micheels, Shelly Schwedhelm, Angela Vasa, Michael Wadman, Suzanne Watson, and Mark E Rupp, https://www.nebraskamed.com/sites/default/files/documents/covid-19/n-95-decon-process.pdf
14 Cambridge company building N95 mask disinfection units, Phil Doan, 3/28/2020, https://www.kitchenertoday.com/local-news/local-cambridge-company-building-n95-mask-disinfectionunits-2208119
15 For details see for example, ANSI/AAMI ST79:2017 Comprehensive Guide to Steam Sterilization and Sterility Assurance in Health Care Facilities
16 ANSI/AAMI ST58:2013 Chemical Sterilization and High-Level Disinfection in Health Care Facilitites (section 2.71).
17 Hydrogen Peroxide Vapor sterilization of N95 respirators for reuse, View ORCID ProfilePatrick Kenney, Benjamin K Chan, Kaitlyn Kortright, Margaret Cintron, Nancy Havill, Mark Russi, Jaqueline Epright, Lorraine Lee, Thomas Balcezak, Richard Martinello, doi: https://doi.org/10.1101/2020.03.24.20041087, https://www.medrxiv.org/content/10.1101/2020.03.24.20041087v1
18 Duke Starts Novel Decontamination of N95 Masks to Help Relieve Shortages, Thursday, March 26, 2020, https://medschool.duke.edu/about-us/news-and-communications/med-school-blog/duke-starts-noveldecontamination-n95-masks-help-relieve-shortages
20 FDA approves use of Battelle’s mask sterilizing technology at full capacity, Workers construct a Battelle CCDS Critical Care Decontamination System. (Battelle), 10TV, PUBLISHED: 03/29/20 10:04 PM EDT
UPDATED: 03/29/20 11:44 PM EDT https://www.10tv.com/article/fda-approves-use-battelles-masksterilizing-technology-full-capacity-2020-mar
21 Advanced Sterilization Products develops reprocessing method to reuse N95 masks, I-Chun Chen – Reporter, L.A. Biz, Mar 30, 2020, https://www.bizjournals.com/losangeles/news/2020/03/30/advancedsterilization-products-develops.html?ana=yahoo&yptr=yahoo
22 Urgent Research Shows Face Masks Can Be Safely Reused by Pandemic Medical Workers, Findings from UMass Amherst scientist may ease critical equipment shortage, March 27, 2020, Richard Peltier, University of Massachusetts, Amherst, Press Release, March 31, 2010. https://www.umass.edu/newsoffice/article/urgent-research-shows-face-masks-can-be
23 Advanced Sterilization Products advertisement, https://get.asp.com/allclear/?utm_source=Google&utm_medium=cpc&utm_campaign=campaign1&gclid =EAIaIQobChMIquPpj7zC6AIVD56fCh18eAzzEAAYASAAEgJGWvD_BwE.
24 OSHA Permissible exposure limit (PEL) for hydrogen peroxide is 1 ppm calculated as an 8 hour time weighted average. 29 CFR 1910.1000 Tbl. Z1
25 1 ppm hydrogen peroxide is one part per million by volume. For every million molecules of gas, one of them is hydrogen peroxide. At 25 oC, 1 atmosphere pressure, this concentration is equivalent to 1.4 mg/m3
26 Viscusi DJ, Bergman MS, Eimer BC, Shaffer RE. Evaluation of five decontamination methods for filtering facepiece respirators. Ann Occup Hyg. 2009;53(8):815–827. doi:10.1093/annhyg/mep070
27 3M Technical Specification Sheet for N95 mask. https://multimedia.3m.com/mws/media/1425070O/3mparticulate-respirator-8210-n95-technical-specifications.pdf. http://multimedia.3m.com/mws/media/1425060O/3m-particulate-respirator-8511-n95-technicalspecifications.pdf.
28 Radl, Stefan & Larisegger, Silvia & Suzzi, Daniele & Khinast, Johannes. (2011). Quantifying Absorption Effects during Hydrogen Peroxide Decontamination. Journal of Pharmaceutical Innovation. 6. 202-216. 10.1007/s12247-011-9114-6.
29 Problems on Hydrogen Peroxide Sterilisation － New Proposal for Safety and Effective Use, Rika Yoshida, Hiroyoshi Kobayash, Annual WFHSS and JSMI Conference 2012, 13th World Sterilization Congresshttps://wfhss.com/html/conf/wfhss-conference2012/lectures/wfhss_conf20121121_lecture_sp_s702_en.pdf
30 FINAL REPORT for Bioquell HPV Decontamination for Reuse of N95 Respirators Prepared under Contract No. HHSF223201400098C Study Number 3245 FDA Contracting Officer’s Representative Brenda Brooks, Prepared by Battelle Columbus, Ohio 43201 July 2016, p19. Available from https://www.fda.gov/media/136386/download
31 https://www.asp.com/sites/default/files/covid-19/AP-2000011- Instructions_for_Use_for_Reprocessing_N95_Masks_in_STERRAD_Sterilization_Systems.pdf
32 Karen E. Middleton, Cleaning and Disinfection: Whole room fogging, Cleanroom Technology, https://www.cleanroomtechnology.com/news/article_page/Cleaning_and_Disinfection_Whole_room_fo gging/52850.