Getting the right PPE: five minutes with . . . Sarah HallettBMJ 2020; 368 doi: https://doi.org/10.1136/bmj.m1287 (Published 30 March 2020) Cite this as: BMJ 2020;368:m1287
All rapid responses
The 10 pm news tonight showed the hugely impressive new hospital in London. They also then showed the new staff, many well within 2 metres of each other and none wearing masks. Why?
Competing interests: No competing interests
I am writing to highlight possible methods for the decontamination of respirator masks, such as FFP2 or FFP3 masks. While researching the topic, I came across an online report from the Stanford Medicine Anesthesia Informatics and Media Lab. The report I found is currently linked to from this web address, https://aim.stanford.edu/covid-19-evidence-service/, and the most current version of the report can be found at this web address, https://stanfordmedicine.app.box.com/v/covid19-PPE-1-2, but the report looks to be getting regular updates, so the current address for the report may change as the report is updated.
The report itself includes what looks like an embedded scientific paper by Liao, L et al, "Can N95 facial masks be used after disinfection? And for how many times?", which reports on experiments involving dry heat in an oven, UV treatment, alcohol, chlorine based bleach or steam. They quickly rejected the alcohol and bleach treatments as these resulted in significant damage to the physical integrity of the masks, and also found that while a steam treatment for 10 minutes could be used on 3 occasion to decontaminate a mask, after 5 treatments the filtration efficiency of the mask had dropped from >95% to ~85%. For all of their results, any one treatment was found to be able to remove >99% of viable E coli under test conditions, with the repeated testing a means to find out how many times the mask could be put through a decontamination method in an attempt to mimic repeated cycles of use, contamination and then decontamination before further use.
They found that 10 UV treatments using 254nm in a small 8W UV steriliser could be performed without significant damage to the mask, but found that dry heat at 75º C for 30 minutes could be performed on the same mask 30 times without significant damage, and without any statistically significant reduction in filtration efficiency. This temperature and time combination was obtained from a previous study on the original SARS-CoV from 2003, where it was found liquid culture medium contaminated with the original SARS virus could be sterilised with treatment at 75º C for 30 minutes (Duan et al, 2003, Stability of SARS Coronavirus in Human Specimens and Environment and Its Sensitivity to Heating and UV Irradiation, Biomed Env Sci 16;246-255, accessible via ResearchGate.net at this address https://www.researchgate.net/publication/8995908_Stability_of_SARS_Coron....
The principle of reuse of respirator masks seems to be well established, since this is discussed in a CDC guide on Pandemic Planning, accessible at https://www.cdc.gov/niosh/topics/hcwcontrols/recommendedguidanceextuse.html, which refers to two possibilities, one being 'Extended Use' which is when a single mask is worn when seeing multiple patients, and the possibility of 'Reuse' which is considered to be the use of mask followed by its removal, and then putting on again some time later. This page advises that "limited reuse has been recommended and widely used as an option for conserving respirators during previous respiratory pathogen outbreaks and pandemics". They further recommend that any respirator masks which are used for aerosol generating procedures (AGPs), any that are contaminated with blood, respiratory or nasal secretions or other bodily fluids, are obviously damaged or become hard to breath through are disposed of and not reused. They also suggest that the use of a cleanable face shield over the mask would reduce those risks, and another possible option would be to wear a disposable surgical face mask over the respirator in an attempt to minimise larger droplet or contact contamination of the respirator mask itself.
The CDC guidance advises any decision to reuse respirator masks should follow attempts to seek guidance from the manufacturer, should ensure all possible steps to limit respirator mask contamination have been taken, eg use of barriers to prevent droplet spray contamination, and consider additional staff training or frequent reminders of the need to minimise any unnecessary contact with the surface of the respirator mask, and maximise use of hand hygiene since any used mask must be considered to have surface contamination of the external surface, including the straps or ties.
The European CDC have published a technical report titled "Cloth masks and mask sterilisation as options in case of shortage of surgical masks and respirators" which can be accessed from this web address https://www.ecdc.europa.eu/sites/default/files/documents/Cloth-face-mask.... This lists other options including hydrogen peroxide vapour and gamma irradiation, with links to a Dutch report from their National Institute for Public Health and the Environment which found that hydrogen peroxide could be used at least twice to decontaminate respirator masks before there was appreciable loss of function. Their report can be found here https://www.rivm.nl/en/documenten/reuse-of-ffp2-masks.
A more detailed description of the use of hydrogen peroxide for the decontamination of respirator masks can also be found in the "Final Report for the Bioquell Hydrogen Peroxide Vapor (HPV) Decontamination for Reuse of N95 Respirators" from 2016, accessed from the FDA website at https://www.fda.gov/media/136386/download. This report gives a detailed description of the process of HPV treatment, beginning with tests using spores of Geobacillus stearothermophilus to demonstrate effective antibacterial sterilisation. They move on from there to test different numbers of cycles on respirator masks, with results suggesting that respirator masks could be treated with at least 20 cycles of HPV decontamination before failure, due to the loss of elasticity of the straps.
Going back further, there is a 2006 report from the American Institute of Medicine (IOM), accessible from https://doi.org/10.17226/11637, which provides an introduction to the design of respirator masks and how the filtration function works, before going on to discuss the issues involved in reuse of masks.
I am aware of other possibilities, including a Nebraska Medicine protocol for UV sterilisation, "N95 Filtering Facepiece Respirator Ultraviolet Germicidal Irradiation (UVGI) Process for Decontamination and Reuse", accessible from https://www.nebraskamed.com/sites/default/files/documents/covid-19/n-95-..., but I wanted to try and put what I had learned so far into the public domain as soon as possible. I am aware of worldwide shortages of PPE, and if any of these suggestions themselves lead to local protocols for safe reuse or lead to further links being made by other researchers which help to further define the parameters for safe reuse of respiratory masks then this will have served its purpose.
Competing interests: No competing interests