Thread by @jeremyphoward

I led the team that studied mask efficacy in early 2020 and published our results in the Proceedings of the National Academy of Science.

I spent three months earlier this year revisiting this topic, and today I'm publishing my notes and links here:
fast.ai/2022/07/04/upd…

An admission: these notes were meant to be the basis of another academic paper, and I gave up on it. In Jan 2022 when I finished this research, I looked around, and it seemed like no-one much cared about avoiding COVID any more.

So I figured it wasn't worth spending time on.

It seems like in the last couple of weeks there's signs that folks might be more open to protecting themselves and others by wearing a mask.

But the vast majority of public health advice I see on mask use is scientifically inaccurate. So I'm digging out this research for you.

Masks work.

An observational study of Beijing households analyzed the impact of mask use in the community on COVID-19 transmission, finding that masks were 79% effective in preventing transmission, if used by all household members prior to symptoms.
gh.bmj.com/content/5/5/e0…

However, with omicron “surgical masks are no longer sufficient in most public settings, while correctly fitted FFP2 respirators still provide sufficient protection, except in high aerosol producing situations such as singing or shouting”
smw.ch/article/doi/sm…

Surgical masks are much less effective than N95's, because they are made to stop liquid splashes during surgery, rather than made to stop airborne transmission.

But you can improve them with a 30 sec trick.

But there's really no need to where anything but an N95 nowadays. They're widely available, inexpensive, and the best ones are very comfortable and breathable.

This one fits nearly all faces:
health.com/condition/infe…

You can re-use an N95 until the straps wear out. I find that's about 30 times with my usage.

Mask maker 3M says "There is no time limit to wearing an FFR. Respirators can be worn until they are dirty, damaged or difficult to breathe through." (I find the straps wear out 1st.)

If you use a good mask like the Aura and you're not a healthcare worker you don't need a fit-test.

Non-experts get an average fit factor of 88, well over the recommended goal of 10. (In healthcare the goal is 100, to provide a 10x safety margin.)
tandfonline.com/doi/abs/10.108…

There isn't a shortage of N95s so you don't need to reserve them for healthcare workers. In fact, not enough people are buying them, so factories are closing down
bloomberg.com/news/articles/…

Masks needn't be a substantial burden. A study found that “in healthy healthcare workers, [N95s] did not impose any important physiological burden during 1 hour of use, at realistic clinical work rates”
rc.rcjournal.com/content/55/5/5…

For dozens more links to academic studies on masks, see the links in my full research notes:
fast.ai/2022/07/04/upd…

Many thanks to the super team of @arimoin, @larrychu, @zeynep, @lexfridman, @austinvhuang, @Hernandez_Danny, @ArneDelft, @HeleneMarivdW, @AmyPricePhD, @reshamas, Z. Li, C. Ramirez, L. Tang, V. Zdimal, C. Bax, G. Watson & V. Tang, who taught me so much when we wrote our 2020 paper

BTW, here's our original 2020 paper, written in April 2020, but not published in PNAS until Jan 2021 (although it was available on preprints.org throughout that time):
pnas.org/doi/10.1073/pn…