I see a lot of people - on both "sides" - making simple mistakes about masks. I've studied PPE, taught PPE, and overseen PPE programs, including teams implementing fit testing in hospitals, long before covid.

A thread
1. Masks work

It's basic physics.

I made one of the earliest calls for universal masking, for that reason.

April 2, 2020
2. Not all masks are created equal

N95/KF94/KN95 > surgical > cloth

Here's my 90-second explainer from last winter
3. Cloth masks work, just not as well as high filtration masks

--> a 50% effective mask reduces inhaled dose by...50%

--> when two people where this level mask, the combined efficacy is 75%

It's wrong to say these "don't work"; correct to say they are "less effective" than N95
4. Surgical masks work

I cringe when I see 'experts' say these masks leak so they don't work. Yes, some aerosol escapes out the side, but direction of airflow matters. Reducing the plume in the speaker's cone of emissions as they directly interact with others is key. And...
...in places with good ventilation and filtration, these 'escaped' aerosols out the side of the surgical mask dilute further before reaching someone. This is the exact situation in hospitals. So...
...when someone say surgical masks don't work in hospitals, they're ignoring the benefit of reducing the aerosol jet in front of the person and thinking of PPE in isolation, ignoring airflow dynamics in the space. N95s better? Sure, and should be norm for high-risk areas.
5. The benefit of mask depends on the 2 Fs: filtration and fit

Most spend their time focused on filtration, but fit is critical. I wrote on this in April *2020*. (You've heard of the 'Rainbow Passage' and 'user seal check', right? RIGHT??)

6. There is a hierarchy of controls

PPE is what we use as last resort. Note it is the 'least effective' control. Early in pandemic, we needed all. The vaccines moved us up the hierarchy. Masks were necessary as the bridge to the vaccines.
7. Set control strategy (eg, masking) based on risk

Two key risk factors are vax status and age. We continue to ignore this when setting control strategy, and instead opted for 'one size fits all'.

This is why I didn't write about everyone needing an N95 until winter 2021. In my assessment, 3-layer or surgical masks were fine for the risk we faced. Alpha changed this for me.

(I brought back in the 'Rainbow Passage' and 'user seal check' (fit!))
Quick nod to @AbraarKaran @RanuDhillon @sri_srikrishna who have been steadfast in calls for N95s since early on. Here they are in June 2020:

For me, I updated my recs on masks again once vaccines were widely available, to account for the change in risk to those who are vaccinated (and still recommended masks for unvaccinated)

Side note: On masks (and all controls), I think there is a danger of being on 'red alert' all the time. Pandemic is not like a hurricane where event happens and you march steadily toward recovery. Rather, it ebbs and flows. Controls should, too.

8. One-way masking is fine

If you're vaccinated, and boosted, and wearing an N95, that's as low risk as anything in your life, *regardless of what anyone around you is doing*.

9. For the newly "N95 or bust" crowd, I have something that will blow your mind. You know there are even better masks than N95s, right? RIGHT??

Google "N100"



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More from @j_g_allen

20 Dec
1. Vax/boost are key (I said about 40x...)
2. Not reckless for NFL to stop routine testing of vaxxed. Were doing 1x a week which is a surveillance strategy, not a control strategy. (you've seen viral kinetics of Omicron, right?)


3. Overestimating transmission risk on the field (they have good ventilation/filtration in locker room, trainer, weight room...right? RIGHT??)
4. Overestimating superspreader risk in large volume spaces, like outdoor stadiums (and even indoor arenas)

5. Not just a $ issue. (So easy to attack in this way.) Many tens of thousands of incomes and livelihoods on the line (which, is, you know... public health), and what the sports leagues do influences what schools and others do (see: NBA closing, March 2020)

Read 4 tweets
4 Dec
It was never a question of if, only a question of when. And when that when happened, we knew buildings would play a key role in the a response.


1. ANTICIPATE (Dec 2, *2019*)
2. RECOGNIZE (Feb 9, *2020*)

"Even with this uncertainty, it is clear that we can enlist buildings to help us in this fight."

3. EVALUATE (March 4, 2020)

"there is still some debate about how the new coronavirus that causes Covid-19 is spread. This has resulted in an overly narrow approach taken by the federal CDC and WHO. That’s a mistake."

Read 6 tweets
29 Nov
This picture is creating misplaced outrage bc Biden is in a surgical mask instead of an N95.

1. Yes, N95s or KF94s are better if you need to wear a mask


2. Everyone is fully vaxxed and boosted, so masks provide limited benefit (especially considering 5 and 6)
3. Even though surgical mask has lower efficiency, the combined efficiency of two 70% masks is 91%. Saying they’re useless is incorrect.
4. In addition, surgical masks get dinged bc ‘leaky’ out the sides, but this misses point that they still help lower the concentration in the cone of emissions directly in front and slow the speed of the emission jet. (particularly relevant when you factor in 5 and 6 next…)
Read 5 tweets
27 Nov
What can schools do right now, if feeling anxious and don't want to wait 2 weeks til Omicron gets sorted out? Implement the simple tips from our Lancet COVID19 Commission report: covid19commission.org/safe-work-trav…

(BONUS...improvements to school buildings come with many other benefits!) ImageImage
Where’d the ‘2 weeks’ come from? 👇
Read 4 tweets
20 Nov
Some people have have high ACE2 expression, and some have “almost none”

For the life of me, I can’t figure out why so few have engaged with this research that’s been out as preprint from @celldeathlab for over a year…

Read 4 tweets
4 Nov
Jan 2020: rapid tests (@RanuDhillon; @sri_srikrishna)
Feb 2020: ventilation; filtration; portable air cleaners (me)
Mar 2020: airborne (@linseymarr)
Mar 2020: masks (@zeynep)

We knew what it was, how it spread, how to test, and how to stay safe before vaccines *very* early on
Feb 2020: ventilation; filtration; portable air cleaners (me)

Read 5 tweets

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