We went through a procedure of bending at the waist , reading aloud , looking left and right , and looking up and down to see if those affected the deposition into the mask and the filtration . Our conclusion is that a face mask does protect the wearer by removing potentially virus-laden particles , but to varying degrees . We found variability of filtration within masks and between manufacturers . We also found a filtration efficiency in the 40 percent to 50 percent range , with the results of this initial study published in JAMA Internal Medicine in 2020 . An important finding especially when considering there was a shortage of N95s early in the pandemic , is that expired N95s ( up to 11 years ) had more than 95 percent of their filtration capability , as did sterilized and reused N95s .”
Following this research , Bennett said he and his colleagues decided to test masks worn by members of the public , to determine if improvements could be made .
“ We tested cotton bandanas that had a filtration efficiency of about 50 percent ; neck gaiters had only 37 percent filtration efficiency ,” he said . “ We realized that the material efficiencies were not so bad in a lot of these masks , but that the fit to the face was very important . We compared medical procedure masks with ear loops to different modifications of homemade masks and bandanas that we could do , such as tie the loops in a knot at the at the base and tuck the corners in make the mask fit better ; this improved filtration efficiency . We got to 80 percent filtration efficiency when putting a nylon sleeve over the face mask , so it really was an issue of making the mask fit to your face . Finally , the CDC kicked around the idea of double-masking to protect people even more , so we thought it ’ s worth testing because there wasn ’ t much consensus that it would do all that much . We found that double-masking did improve over single masks , but what was most dramatic was that if you put a cloth mask or cotton bandana on top of the ear-loop type of mask , it increased the efficiency quite dramatically , going from 40 percent to 80 percent filtration efficiency . But once again , an improvement in the fit of the face coverings was key , reducing potential leaks .”
Bennett added , “ Masks have some degree of effectiveness , with some being better than others , and they ’ re worth wearing , obviously , especially in a poorly ventilated space where particles can stay in the air for a long time . But how well does a mask contain aerosols ? By that we mean source control , so that it doesn ’ t get emitted to other individuals . There have been a number of scientists who have tried to address this issue . One study using a single subject who coughed , spoke , and sneezed , used a light-scatter background so you could see the particles being emitted . What ’ s important to note is that when the subject was masked , the number of particles were reduced dramatically , and a two-layered face covering pretty much eliminated large particles .”
Bennett continued , “ What we seem to know about transmission of viruses by aerosol is that particle size can determine distance traveled , and that masks reduce exposure , some better than others . The non-woven , multi-layer ones or those with a good fit are the best , but there are more questions to answer — one of them is the efficiency of mask containment , and is it the same as protection ?”
Masks have some degree of effectiveness , with some being better than others , and they ’ re worth wearing , obviously , especially in a poorly ventilated space where particles can stay in the air for a long time .”
Wrapping up the IDWeek presentation and assembling all the perspectives was Hilary Babcock , MD , MPH , professor of medicine at Washington University School of Medicine .
“ Casting our minds back to February 2020 , the initial CDC guidance for the new coronavirus was to use airborne , eye protection and contact precautions , which was , of course , N95 eye protection , gowns and gloves ,” Babcock said . “ It was important to start with full protection while getting more data and information about this new virus , and that ’ s often referred to as a precautionary principle . We should take all precautions until we know that it ’ s safe to scale back , but it does disregard some of our prior experience with prior coronaviruses . And it can be difficult to scale down from these high-level precautions , even as we do get more information , and we saw a similar thing with H1N1 where we started with N95s and a higher level of protection . While we did scale back some afterwards , we kept some of those higher level of protection recommendations for aerosol-generating procedures and for some other clinical situations . In implementation , of course , we were limited at the beginning of the pandemic by supply , and so there was a necessary prioritization of respirators to our highest risk interactions , and as we tried to define what those high-risk situations were , particularly around aerosol-generating procedures , is when we first entered this wave of controversy around how to define what an aerosol-generating procedure was and what the right PPE in that situation was .”
Babcock explained that the standard definitions for aerosol-generating procedure had always included intubation , extubation , bronchoscopy , and positive-pressure noninvasive ventilation . “ But we soon saw a wide expansion of those definitions to include any surgical procedure , whether the patient was intubated or not . Walking a patient was suggested to be an aerosol-generating procedure as well , but in addition , we also gained real-world experience of using isolation masks in many settings with COVID patients when we didn ’ t have N95s to use for all patient care .”
She continued , “ Soon there were swarms of opinions on the best risk-mitigation strategies for this new situation in which we found ourselves , and there was a lot of yelling back and forth between aerosol scientists and engineers , biologists , epidemiologists , and infectious disease physicians . There were many arguments on social media that confounded locations , patient factors and virus factors , all getting reduced to # covid is airborne . Early on , WHO said COVID is not airborne in the way that we epidemiologists understand ‘ airborne ’ in a healthcare setting , and a lot of the aerosol scientists were very unhappy with this description ; they made a lot of graphics to show all of the particles in the air and how they could spread through enclosed spaces , to support their point . Some virologists pushed back , saying that we ’ re devolving into people screaming that something was airborne all the time , as opposed to understanding the more complex issues around how we define transmission and what that definition meant .”
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