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The Multiplicative Power of Masks
We now know that masks have an outsized effect on slowing the spread of COVID-19. And yet, some people oppose wearing masks because they view this as a personal choice rather than a public health issue. This misses the big picture because masks protect the wearer and the people around them. This two-way protection makes widespread mask-wearing a powerful way to extinguish an epidemic. on masks, we'll see how when 60% of people wear 60% effective masks, disease transmission drops by as much as 60% — roughly what's needed to stop the spread of COVID-19. –Aatish Bhatia

https://aatishb.com/maskmath/

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Mask Study
this from Guardian LIVE today 23 Dec 2020

Face masks lower the risk of spreading large Covid-linked droplets when speaking or coughing by up to 99.9%, a lab experiment with mechanical mannequins and human subjects has found.A woman standing two metres from a coughing man without a mask will be exposed to 10,000 times more large droplets than if he were wearing one, even if he is only 50 centimetres away, researchers reported in the journal Royal Society Open Science.The study focused on particles larger than 170 microns in diameter, which are understood to be the main driver of SARS-CoV-2 transmission.“There is no more doubt whatsoever that face masks can dramatically reduce the dispersion of potentially virus-laden droplets,” senior author Ignazio Maria Viola, an expert in applied fluid dynamics at the University of Edinburgh’s School of Engineering, told AFP.According to the Institute for Health Metrics and Evaluation (IHME) in Seattle, Washington, 55,000 lives could be saved in the United States over the next four months if a policy of universal mask use were adopted.The WHO updated its Covid-19 guidance to recommend masks be worn indoors in the presence of other people if ventilation is inadequate earlier this month.

https://www.ed.ac.uk/covid-19-response/latest-news/masks-block-spread-of-covid-linked-droplets

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New mask study shows FFP3 masks give best protection (better than surgical blue loose fitting)

https://www.bbc.co.uk/news/health-57636360

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update Dec 2021

here are a couple more studies (among many)

but remember also this simple quote

"N95 mask  is extremely wonderful. The pores in the mask are 0.3 microns wide. The virus is 0.12 microns wide. So people ask how does it work? But you try having 3 big rugby players rush 4 lunch thro' a door -they're not going to get through."


1/ Lancet
Physical distancing, face masks, and eye protection to prevent person-to-person transmission of SARS-CoV-2 and COVID-19: a systematic review and meta-analysis
https://www.thelancet.com/journals/lancet/article/PIIS0140-6736(20)31142-9/fulltext

2/ScienceDirect
A rapid systematic review of the efficacy of face masks and respirators against coronaviruses and other respiratory transmissible viruses for the community, healthcare workers and sick patients

https://www.sciencedirect.com/science/article/pii/S0020748920301139?via%3Dihub