We started studying cloth masks in March. Preprint now at medrxiv.org/content/10.110… Take-home: Cloth masks are not an N95, but they work reasonably well for aerosols 1-2 microns and larger, which is the size that we think mostly mediates transmission. See thread. /1
.@jinpan @charbeleharb & Leng tested vacuum bag, microfiber, coffee filter, MERV 12 filter, cotton, acrylic, bandana, CDC sewn & non-sewn designs, surgical mask, face shield for material filtration efficiency in a filter holder and inward and outward protection on a manikin /2
For submicron particles, vacuum bag, microfiber, and surgical mask filtered out >50% of aerosols. Other materials were way below 50%. /3
At 1-2 microns and larger, many materials filtered out >50% of aerosols. Inward (protect wearer) & outward (source control) protection were decent, too. Stiffer materials (microfiber, coffee filter, MERV 12) were worse because they didn’t fit well to manikin & left gaps. /4
For better performance, we recommend 3-layer mask w/ outer layers of tightly woven fabric + inner layer of something designed to filter aerosols. Should produce overall efficiency of >70% at the most penetrating particle size and >90% for >1 micron if the mask fits well. /5
Here's a news release about our findings: vtnews.vt.edu/articles/2020/… /6

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More from @linseymarr

5 Oct
CDC's new guidance is out: "COVID-19 can sometimes be spread by airborne transmission" cdc.gov/coronavirus/20… /1
"Some infections can be spread by exposure to virus in small droplets and particles that can linger in the air for minutes to hours. These viruses may be able to infect people who are further than 6 feet away from the person who is infected or after that person has left...." /2
"This kind of spread is referred to as airborne transmission and is an important way that infections like tuberculosis, measles, and chicken pox are spread." /3
Read 7 tweets
25 Jul
Nearly tears of joy seeing new paper in medical journal with correct analysis and interpretation of aerosol transmission: thelancet.com/journals/lanre… See thread for summary of main points. /1
Where we have been: "Current infection control policies are based on the premise that most respiratory infections are transmitted by large respiratory droplets—ie, larger than 5 μm— produced by coughing and sneezing, then deposited
onto exposed fomite or mucosal surfaces." /2
Pathogens (e.g., M. tuberculosis, influenza, RSV, rhinovirus, many more) are consistently found in small aerosols <5 microns and if size-resolved data available, in much higher amounts than you would expect based on aerosol or droplet volume. /2
Read 11 tweets
9 Jul
My thoughts on WHO's updates:
who.int/publications-d… is an improvement
who.int/news-room/q-a-… is still in denial. See thread below /1
WHO's updated scientific brief on transmission of COVID-19 is a step in the right direction because they now acknowledge that airborne transmission may be occurring outside of specialized medical procedures, although they remain fixated on medical procedures as... /1
the main source of aerosols when we know they are generated by normal respiratory activities such as breathing, talking, singing, laughing, etc. /2
Read 11 tweets
9 Jul
Exhaled breath is a significant source of SARS-CoV-2 emission medrxiv.org/content/10.110… Emission rate in exhaled breath of 1000-100,000 RNA copies/minute in 5/24 patients (<14 days). HT @TundraDesert /1
The 5 patients' hands all negative, only 2/22 phones positive, 0/26 swabs of handles positive, discounting direct contact. Other surfaces positive, prob by fecal or aerosol/droplet settling. /2
Previously, I estimated an emission rate of 10,000 copies/min from air measurements in hospitals in Nebraska and Singapore. We may be heading toward a robust number for those who emit virus into air. /3
Read 4 tweets
16 Jun
Flushing the Toilet May Fling Coronavirus Aerosols All Over nytimes.com/2020/06/16/hea… Part 2: everything old is new again. See thread for prior work on this topic. /1
Beautiful video of droplet generation from toilet flushing by Lydia Bourouiba from 2013 lbourouiba.mit.edu/research/proje… /2
Emission Strength of Airborne Pathogens During Toilet Flushing ncbi.nlm.nih.gov/pmc/articles/P… showed that bacteria can be emitted during flushing /3
Read 9 tweets
12 Jun
I got a little too excited at first about this paper about airborne transmission and masks. pnas.org/content/early/… See the thread for more /1
Fig. 2 shows cases leveling off exactly when masks are required, but there should be a delay. Is this actually when effects of stay-at-home and distancing orders from weeks ago kick in? /2 Image
In part A of this figure, I feel like I could just as easily draw a straight line from the stay-at-home date to May 10 and get an equally good fit. Again, effect of masks should not kick in immediately. /3 Image
Read 7 tweets

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