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What does a mask do? Blocks respiratory droplets coming from your mouth and throat.

Two simple demos:

First, I sneezed, sang, talked & coughed toward an agar culture plate with or without a mask. Bacteria colonies show where droplets landed. A mask blocks virtually all of them.
What about keeping your distance?

Second demo: I set open bacteria culture plates 2, 4 and 6 feet away and coughed (hard) for ~15s. I repeated this without a mask.

As seen by number of bacteria colonies, droplets mostly landed <6 ft, but a mask blocked nearly all of them.
I'm aware that this simple (n=1) demo isn't how you culture viruses or model spread of SARS-CoV-2.

But colonies of normal bacteria from my mouth/throat show the spread of large respiratory droplets, like the kind we think mostly spread #COVID19, and how a mask can block them!
My mask vs. no mask demonstrations got perfectly summarized in this brief 34 second video. Masks keep your respiratory secretions to yourself! @providence_phc
Something extra cool, local reporter Kevin Kim came to our teaching lab and tried this same demonstration himself (his reaction in this clip was absolutely genuine). Story airs tonight @KHQLocalNews

The take home: masks matter!
Masks as a political/social litmus test or used to shame those who won't (or disabled folks who truly can't!) wear them is a travesty.

We wash hands after using the bathroom & wipe noses on tissues. Masks/face shields need to be just another normalized act of hygiene. #MasksWork
Well I just got back from reading some slides in the lab and this has completely overwhelmed my ability to keep up with comments and questions. Wow.

I'll circle back later tonight to answer questions. Thanks everyone!
I saw a number of people ask this: how did I sneeze on command?

I actually got a packet of pepper from the cafeteria to try to make myself sneeze - didn't work, just got watery eyes. So I just did a fake stage-sneeze at the plate.
When reporter @NewsWithKevin did this experiment, he sang twice. Once he sang Smashmouth's "All Star" and on a different plate rapped Tupac's "Things Will Never Change." The Tupac plate had more bacteria.

Literally spitting verses ...
The clip from tonight's @KHQLocalNews with reporter Kevin Kim repeating this experiment (and reacting to the results) is here. I think this turned out really well! Awesome job Kevin! @NewsWithKevin @providence_phc…
A lot of people asked this: "could you do this with ___ type of mask?" (homemade, single layer, cotton, N95 etc.)

So it's not a bad idea but I would honestly expect the outcome of THIS demonstration to be essentially the same and here's why...
I try to be really clear what this does & does not show.

DOES SHOW: the act of talking (or coughing, sneezing, singing) causes particles of liquid to come out of your mouth.

DOES SHOW: those droplets can carry microbes like bacteria

DOES SHOW: a mask blocks most of those
DOES NOT SHOW: what is the number, size, and distribution of respiratory droplets produced by coughing/talking etc.

DOES NOT SHOW: could these droplets carry viruses (like SARS-CoV-2), does a mask block THOSE

(Based on what we know you can intuit/infer this, but it's not shown)
Something I repeated enough while talking that reporter Kevin finally asked, "Why do you keep calling it a 'demonstration' and not an 'experiment'?"

To me this is purely a demonstration bc 1) it was just one single data point and 2) I essentially knew the outcome going in
I *knew* that coughing on a agar plate would get my mouth bacteria on there and I knew that a mask would decrease that amount (maybe even completely).

I used a surgical mask from a hospital. Would a homemade mask do the same and keep mouth bacteria from the plates? Certainly.
If I was trying to compare the ability of
- disposable paper mask
- hospital surgical mask
- homemade fabric mask
- N95 mask
to reduce virus transmission, I wouldn't do what I did here, which is see how well a mask blocks minute bits of spit using # of bacteria as a proxy
So COULD you test different masks with this same set up? Yes. Would it be different? Eh. Probably not.

A mask preventing your spit & breath from flying out of your mouth, even if doesn't catch it all, will stop some spread of bacteria (see here) AND LIKELY VIRUS (not seen here)
OK here's what was probably the most common question I saw: what did I sing at (to??) my culture plates.

It was "Dear Theodosia" from Hamilton, top of my lungs.

If I could have anyone's singing talent it would be Leslie Odom Jr.'s. (Spoiler: I don't.)
100% true: bacteria are incredibly different from viruses!

But since we expect respiratory droplets to be what primarily spreads #COVID19, I exploit the presence of (easily to grow and visualize) bacteria in respiratory droplets, to show where they go.
It’s Friday night and a bunch of nice people are recommending I pull out my nose hair. Things are ver normal and good ... 😅
You certainly COULD do this. But I was not be trying to quantitate # of droplets/bacteria colonies spread by diff. behaviors or exactly how far. More sophisticated techniques than this show droplet spread by just breathing...
It's a fun idea testing distance of droplets spread by talking, breathing, singing etc. (what about diff. types of songs? Trained or professional singer?)

Would it "prove" that singing can really spread droplets? I think we already know that's the case:…
This could be a fun thing you could do. However, I don't think it would model protection to the mask weaver very well (at least with this simple experiment design....)
Inhaling is an active, high-velocity process, just like exhaling, coughing, and sneezing are.

A mask sitting on top of a plate will certainly block any droplets that come flying at it. But a person breathing in droplets coughed out by another person? Different story.
Unless it's a fit-tested respirator, your surgical or cloth mask won't have an air tight seal. When you inhale, if air finds the path of least resistance, a lot will go in on the sides... (video from LaVision showing air currents w/ and w/o a man's N95)
Someone (sorry I can't find who) asked, could I try to catch droplets coming out the SIDE of a surgical mask.

I don't think a bacteria plate-based readout would answer the real question which is "where do THOSE exhalations/droplets go?" I don't know but it won't be out in front.
Well this has been like nothing I could've expected.

There are replies to a few often-repeated questions in the above thread but a lot of these keep getting re-asked. If you've asked something new, chances are good I'll miss it because, yikes, this got pretty big.
It's been really great interacting with people, seeing other language translations and so many good questions and comments.

Overwhelmingly I've only seen positive responses so thanks for that. Going to get some rest. Thanks! #Masks4All #FaceShieldsForAll
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Keep Current with Rich Davis, PhD, D(ABMM), MLS 🦠🔬🧫

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