Dentists have been using face masks as part of their routine personal protective equipment (PPE) for many years. However, their close proximity to patients during treatment and the potential for the production of aerosols during dental treatment has focused much attention on the face mask and other items of PPE. During the Covid-19 pandemic global shortages of PPE have forced consideration of protective equipment sharing measures such as extended use, re-use or reprocessing of single-use masks and respirators.
The aim of this rapid review was to summarise the guidance and systematic review evidence on extended use, re-use or reprocessing of any type of surgical mask or filtering facemask respirators
Methods
The Medline, Pubmed, Epistemonikos, Cochrane Database of Systematic Reviews and three preprint repositories (Litcovid, MedRxiv and Open Science Framework) were searched for systematic reviews exploring the extended use, re-use or reprocessing of any type of surgical mask or filtering facemask respirators (FFPR). The World Health Organization, European Centre for Disease Prevention and Control, the US Centers for Disease Control and Prevention, and Public Health England websites were searched for guidance documents. Two reviewers searched for guidance documents which were screened for inclusions systematic review were also double screened for inclusion. Data extraction was carried out by two reviewers with the quality of the systematic reviews being assessed using the AMSTAR-2 checklist. The findings were integrated and narratively synthesised.
Results
Guidance documents
- 6 documents were identified, 3 from the US Centers for Disease Control and Prevention, and one each from the European Centre for Disease Prevention and Control, Public Health England and the World Health Organisation.
- The guidance documents were published or updated between March 17th 2020 and May 21st 2020 and written for the COVID-19 pandemic.
- None of the guidance documents included a systematic review of the literature.
- All the guidance documents depicted extended use, re-use or reprocessing as extraordinary, last resort measures favouring extended use over re-use because of reduced risk of contact transmission.
Systematic reviews
- 4 relevant systematic reviews (3 from the same team) were included.
- The reviews were considered to be predominantly of high-quality.
- 3 were on different interventions for reprocessing of FFPR.
- 1 related to surgical mask.
- No reviews in our sample examined the impact of extended use or re-use of filtering facepiece respirators or surgical masks on the ability to meet technical standards or on healthcare worker acceptability outcomes such as comfort.
Main findings
- While extended use or re-use of single-use surgical masks or respirators (with or without reprocessing) is generally not recommended, guidance from various organisations supports such measures (preferably extended use rather than re-use) as a last-resort measure during critical shortage.
- Comparisons across guidance documents and systematic reviews highlight limited evidence, varying levels of detail, and areas of inconsistency, especially in relation to re-use of respirators (with or without reprocessing) during and after aerosol generating procedures.
- The reprocessing of surgical masks is not recommended.
- Reprocessing of respirators under controlled and standardised conditions is recommended, but there is inconsistency regarding how or when this should take place and further research is needed in this area.
- Where extended use or re-use is being practised, healthcare facilities and institutions should ensure that policies and systems are in place to enable these practices to be carried out in the safest way possible in line with available guidance.
Conclusions
The authors concluded: –
Extended use and re-use of single-use surgical masks and respirators (with or without reprocessing) should only be considered in situations of critical shortage. Where extended use or re-use is being practiced, healthcare organisations should ensure that policies and systems are in place to ensure these practices are carried out as safely as possible and in line with available guidance. Areas of guidance lacking clarity and consistency warrant further attention and investigation.
Comments
This rapid evidence review was conducted in line with Cochrane interim guidance for rapid reviews and a summary was published as part of the Oxford Covid-19 Evidence Review Service on the 5th June. The reviewers examined six guidance documents none of which included systematic reviews of the evidence. The reviewers search for reviews identified 4 relevant systematic review all of which are new having been conducted between March and April 2020. These reviews are all from Canada with 3 being from the same team, and all were either in peer review or pre-prints. There are considered to be mainly of high quality although only 2 studies in one of the included reviews looked at the effect of reprocessing on SARS-CoV-2.
Prior to the Covid-19 pandemic dental teams were used to changing masks after each patient. Guidance from a number of sources (C0DER Working Group 2020) has suggested longer periods of use up to 4 hours or on a sessional basis where there is no visual damage or obvious soiling. UK guidance documents suggests the risk assessment of sessional use while a small number of other international guidance documents have advised of the possible re-use of FFPR masks following re-processing. This review would suggest that extended use or re-use should be a last resort measure.
Links
Primary Paper
Extended use or re-use of single-use surgical masks and filtering facepiece respirators: A rapid evidence review. Elaine Toomey, Yvonne Conway, Christopher Burton, Simon Smith, Michael Smalle, Xin-Hui Chan, Anil Adisesh, Sarah Tanveer, Lawrence Ross, Iain Thomson, Declan Devane, Trish Greenhalgh.
Oxford CEMB- Covid -19 Evidence Blog
Review protocol and associated materials
Other references
Recommendations for the re-opening of dental services: a rapid review of international sources, Version 1.3 16th May. The (CoDER) Working Group. (accessed 11th June 2020)
Dental Elf – 30th Apr 2020
How much extra protection does an FFP3 mask offer in the dental surgery?
Dental Elf – 25th Mar 2020
Photo Credits
Photo by Mika Baumeister on Unsplash
Recently a german, technical university tested autoclaving FFP2-masks. They tested 10 times autoclaving them (121 °C, 20 min), and no relevant changes could be measured for filtration, particle sizes or on inspection with electron microscopy. Deepl.com should translate:
https://www.tu-darmstadt.de/universitaet/aktuelles_meldungen/einzelansicht_259072.de.jsp
That`s why I autoclave FFP2 and chirurgical masks in my dental office.
Hi
Thank you for your comment. The example you higlight is a single small study while the review included infromation from 4 systematic reviews. Overall though the reviews authors do note that:-
‘Reprocessing of respirators under controlled and standardised conditions is recommended, but there is inconsistency regarding how or when this should take place and further research is needed in this area.
Where extended use or re-use is being practised, healthcare facilities and institutions should ensure that policies and systems are in place to enable these practices to be carried out in the safest way possible in line with available guidance.’
Derek, you wrote:
“No reviews in our sample examined the impact of extended use or re-use of filtering facepiece respirators or surgical masks on the ability to meet technical standards or on healthcare worker acceptability outcomes such as comfort.”
The “Technische Universität Darmstadt” did these technical tests. That`s why their study is more relevant for me than the four reviews. (“Comfort” I can evaluate myself, I don`t need a study/review for that). It`s simply the best available evidence, IMHO. I would like to look into the primary studies of the 4 reviews, though.
Hi Michael.
The quote about the reviews in the sample is actually a quote from the review authors. All three of the reviews included within the rapid review that relate to the N95 mask as available in preprint so you should be able to access them.
One of them Gertsman et al. Microwave-and Heat-Based Decontamination of N95 Filtering Facepiece Respirators (FFR): A Systematic Review. https://osf.io/4whsx/ is probably the one most relevant to the use of autoclaves. In their abstract they state
” All interventions successfully destroyed viral/bacterial contaminants. Other than autoclaving, which significantly increased aerosol penetration, moist and dry microwave and heat conditions did not significantly impact functional parameters or fit. However, several conditions caused physical damage to at least one N95 model.”
Given the pressues on the supply of PPE it would be good to see the results of more high quality studies on this.
Derek, thanks for your hint to this review, https://osf.io/4whsx/, which kept me going, found another, new study, see below!
But I think the sentence in its abstract you cited is misleading:
“Results: All interventions successfully destroyed viral/bacterial contaminants. Other than autoclaving, which significantly increased aerosol penetration, moist and dry microwave and heat conditions did not significantly impact functional parameters or fit. However, several conditions caused physical damage to at least one N95 model.”
In the full text I find: “and three (studies, M. L.) 17,20,44 used an autoclave.” About “aerosol penetration after decontamination”: “Results in autoclave conditions varied: in one study 20 no increase was noted, but the other autoclave intervention 44 noted an increase of over 18% in both arms.” “In both studies examining high-temperature, high-pressure autoclave decontamination, 20,44 masks were found to be deformed, shrunken and stiff post-decontamination.” “The precision of aerosol penetration for autoclave interventions was poor due to the vastly different results described by Viscusi et al.’s (2007) 44 and Lin et al. (2017). 20” “Mask deformation after autoclaving was consistent in both studies 20,44 that measured this.”
“More notably, Viscusi et al. (2007) 44 demonstrated that autoclaving the masks for 15 and 30 minutes significantly increased penetration to three and seven times the maximum acceptable value respectively, which is unsurprising as the authors reported significant observable mask deformation. Interestingly, Lin et al. (2017) 20 reported no significant change to aerosol penetration after 15 minutes of autoclaving at the same temperature and pressure conditions. The origin of this discrepancy is unclear, but it is possible that it stems from the fact that Lin et al. utilized mask fragments rather than full masks and had to use a modified airflow rate during filter testing to account for this.”
“Autoclaving does not appear to be a suitable decontamination option for FFRs as it consistently caused significant physical deformations to the masks, although these results come from only two studies that observed one mask model each.”
I think it`s fair to say, that the evidence this review presents is very weak, based only on two, somewhat contradictory studies for autoclaving and based on only two mask models. And both studies contradict my experiences with autoclaving about 5 types o) N95/FFP2-masks: No deformation, no stiffness can be seen after several cycles of 121 °C for 20 min. But I have one of the most expensive (and best) autoclaves one can buy, a Melag Vacuklav 40 B+ Evolution. Older autoclaves cannot control their temperature so well. The Melag has a double wanded chamber wall, filled with water, which should protect from overheating in the drying phase.
My results are identical with the report from the TU Darmstadt, which tested several different FFP2-masks. German health care providers had to invest heavily in new, expensive autoclaves in the last 10 years, so I assume that TU Darmstadt could use such an autoclave. They confirm microscopally what I see macroscopally (translation by deepl.com):
https://www.rsm.tu-darmstadt.de/home_rsm/news_rsm/news_details_387200.de.jsp
(…)
The evaluation of these mask properties for all samples examined showed that, within the framework of the test conditions, no influence on the filter effect of the breathing masks could be detected even after the application of up to ten sterilisation treatments.
(…)
On the Melag pages I found a hint to the following study from the netherlands, confirming that autoclaving at 121 °C is safe for FFP2-masks (free, full PDF available):.
https://repository.tudelft.nl/islandora/object/uuid%3Af048c853-7e1d-4715-b73d-3b506b274a30
Sterilization of disposable face masks by means of standardized dry and steam sterilization processes
(…)
We openly shared our positive experiences with the steam sterilization process with other hospitals in the Netherlands that were are also preparing for the outbreak. We were informed that their attempts to steam sterilize mouth masks at 134 °C gave poor results as masks started to deform and became sticky while the elastics lost its resilience.
(…)
They used a Getinge, which is also a high quality autoclave. The 121 °C program is a program recommended for some plastics which cannot stand 134°C. So good temperature control in a modern autoclave may be critical here.
So it`s 3:1 (4:1, me included ;-) ) for autoclaving now. And the study against it is the oldest one (2007), which means old masks, old autoclave.
BTW some german dentists reuse their private FFP2-masks every week, keeping the mask drying for a week. After this time the corona virus will be dead. (But many other germs will survive, that`s why I prefer autoclaving.)
PS: Streaming steam at 100 °C (for 5 minutes) is a method missing in the review which would be cheap and suitable for disinfecting FFP2-masks, but they would have to dry afterwards outside the cheapest device with this method, a baby bottle steam sterilizer. The method was invented by Robert Koch himself (https://en.wikipedia.org/wiki/Robert_Koch) for his microbio lab, because autoclaves exploded too often in his time. I did a german presentation on it (http://www.logies.de/dampftopf.zip), because I disinfect my dental handpieces this way for about 15 years now (now after putting them through a Assistina 3*3 for validated internal cleaning). Some literature:
Towle D, Callan DA, Lamprea C, Murray TS. Baby bottle steam sterilizers for disinfecting home nebulizers inoculated with non-tuberculous mycobacteria. J. Hosp. Infect. 2016;92(3):222–5.
Towle D, Callan DA, Farrel PA, Egan ME, Murray TS. Baby bottle steam sterilizers disinfect home nebulizers inoculated with bacterial respiratory pathogens. J. Cyst. Fibros. 2013;12(5):512–6.
Gräf W, Kunz B, Loisl B. Zur hygienischen Aufbereitung dentaler Übertragungsinstrumente (Hand- und Winkelstücke, Turbinen) in der zahnärztlichen Praxis. Zentralbl Hyg Umweltmed 1995;198(1):72–83. Available at: http://www.ncbi.nlm.nih.gov/pubmed/9409896. Accessed September 10, 2010.
(This german publication started this topic for me.)
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