Does wearing a mask protect you from coronavirus?

There was some confusion early on in this crisis about masks.  The WHO originally said that "there is currently no evidence that wearing a mask (whether medical or other types) by healthy persons in the wider community setting, including universal community masking, can prevent them from infection with respiratory viruses, including COVID-19.", but has since shifted to a position that a mask alone is not enough.


The US Surgeon General tweeted on Feb 29 "Seriously people- STOP BUYING MASKS! They are NOT effective in preventing general public from catching #Coronavirus, but if healthcare providers can't get them to care for sick patients, it puts them and our communities at risk! https://t.co/UxZRwxxKL9." 

I don't know what that link (which goes to the CDC) originally said, but now it recommends wearing a mask ("Everyone should wear a cloth face cover when they have to go out in public, for example to the grocery store or to pick up other necessities.").  The CDC webpage now even has a video of that same Surgeon General making a mask from an old T-shirt and two rubber bands.

So right now the CDC is encouraging mask use, and the WHO is at least no longer discouraging it.

I've posted a blog post in which I describe how I feel safe going out in public wearing my N95 mask.  I've been posting links on FB along the way, as research came out, FB is not great for finding old posts, so here they are, for those who want them all in one place.


From March 20th
https://www.facebook.com/jennifer.gruenke.5/posts/612546845366

The main study I was looking at was this 2008 study of the filtration capacity of homemade masks.  The best was one made out of a tea towel. In this image, the different colors are different people wearing the mask, the X-axis along the bottom is time, and the Y-axis on the left is a protection factor for the person wearing the mask.  A protection factor of 3, for example, only one-third of the particles make it into the mask.  That's not nearly as good as an N95 mask or a surgical mask, but it's much better than nothing.

I also mentioned this study, which showed that masks protect health care workers from bacterial infections. Not the same as viruses, but still the same general idea.

From March 31https://www.facebook.com/jennifer.gruenke.5/posts/613095540776

This is a study modeling the effect of masks in Wuhan, China.  I'm not crazy about models, but they do cite a meta-analysis, that show a strong reduction in infections in multiple studies of health care workers exposed to various pathogens.

From April 5https://www.facebook.com/jennifer.gruenke.5/posts/613347775296

This is not an official academic source, but it is reporting on filtration by different kinds of fabrics, for those making their own.  The best homemade mask filtered 79% of virus-sized particles.  They said "The best masks were constructed of two layers of heavyweight "quilters cotton" with a thread count of at least 180, and had thicker and tighter weave."

From April 22https://www.facebook.com/jennifer.gruenke.5/posts/614724386556

This was a news report as well, but there is an academic version here.  Researchers put a nylon stocking over a homemade cloth face mask, in order to improve fit to the face, and eliminate air coming in from the edges.  They found that "Use of a nylon stocking overlayer brought the particle filtration efficiency for five of the ten fabric masks above the 3M surgical mask baseline."

From April 23https://www.facebook.com/jennifer.gruenke.5/posts/614884450786

This is a study of outgoing virus, that is, how much you spread to other people with and without a mask. They were using surgical-type masks and found that they blocked coronaviruses (the cold-type; this was before covid came along)

The image below is new, but this math person has made a model for how safe it is to go outside with and without a mask. I'm not generally a big model person, but she at least gives you the spreadsheet so you can download it and change her assumptions as you wish.  And even if it's off by a factor of ten or a hundred, the risk of catching the virus outside your home while wearing a mask and being mindful of hand hygiene is really small.https://medium.com/swlh/so-youre-going-outside-a-physics-based-coronavirus-infection-risk-estimator-for-leaving-the-house-d7dcae2746c0

I definitely remember more studies, for example one in China where the health care workers in the Covid-19 ward were given the N95 masks, and those in other wards had to go without.  Zero health care workers with the masks caught Covid, whereas there were multiple cases among the workers without the masks.  But I can't find the link.