Post #602: Radical change in CDC guidance, and implications for your behavior.

Posted on April 4, 2020

Source: COMMENTARY: COVID-19 transmission messages should hinge on science. March 16, 2020, Lisa Brosseau, ScD, author, on line at the University of Minnesota Center for Infectious Disease Research and Policy.

In the illustration above, Person A has COVID-19 but doesn’t know it.  He’s talking loudly and at close range to person B, who soon will have COVID-19.  Person C is standing to the side, wondering why none of them are wearing masks.  Presumably, this snapshot was taken in an android nudist colony.

In all seriousness, go read that article if you want to know what’s probably going on with the continued spread of COVID-19.

Last Friday’s change in CDC guidance looks modest, but it is in fact radical, and has important implications for how you need to behave.

Before last Friday, the CDC said that COVID-19 was largely being spread by droplet transmission of symptomatic individuals.  I.e., sneezes and coughs of individuals who were clearly sick.  And drops that wouldn’t (typically) travel more than 6 feet.

Hence, under the old and apparently incorrect model, social distancing with no masks was an adequate response.

If you read this blog, you already know that this didn’t seem quite plausible.  It really didn’t fit the facts.  And didn’t fit the continued spread well past the point of the start of significant social distancing in Fairfax County (March 13).

That has changed.  You need to be aware of how significant the change is.

As of Friday, CDC says that transmission by asymptomatic individuals is an issue, and in fact, droplets can be created merely by talking (not just coughing or sneezing).  Those two changes are radical.  They still haven’t admitted to a third point, that the very tiniest droplets (aerosols, less than 5 microns in diameter) are implicated in spread among the public.  But, as I read it, it’s a pretty good guess that’s true.  Post #601 goes through some of that, including the Mount Vernon, Washington choir practice superspreader event.

It’s going to take a while for the implications of the change in CDC guidance to sink into the general consciousness.  But when it does, we’re all going to realize that this radically alters the game.  Under prior CDC guidance, you could feel reasonably safe if you avoided people who were obviously sick.  That was dead wrong, because the CDC was wrong about how this is spreading.  Now, you need to treat yourself, and everyone outside of your household, as if they a) are carrying COVID-19, and b) can spread it merely by talking to others in person.

The situation may actually be a little bit worse than that, but that’s a good place to start.

And so, everybody needs to wear a mask, now, when in public among people not in your household.  Period.

The CDC has said that everyone should wear a (cloth) mask in public.  That directive is really aimed at preventing you from spreading disease, and only to a lesser degree at preventing you from catching a disease. And they specify cloth because they don’t want you to go out and buy a real surgical mask or respirator.  But, as I discuss below, I’m not sure that’s a valid fear any more.


Above, if A is wearing a cloth mask, that’ll greatly reduce the number and spread of droplets.  It helps protect B.  But if A is not masked, and B is wearing a (typical home-made mask), that (typically) provide some protection, but not the best possible protection.  It typically won’t filter out the very smallest of those droplets.  And those are the ones that can travel the greatest distance.  Penetrate most easily into the lower lungs.  And so on.

The reason that putting a mask on A does protect B others against exposure to all types of droplets is that the smallest droplets are simply carried by air currents.  They don’t have enough mass to behave like projectiles.  So even if the mask doesn’t filter those droplets out of exhaled breath very well, the mask slows the velocity of your breath.  And that means those tiniest particles will not travel as far.  (Look at prior posts for my “wet finger whistle” test to prove that to yourself.).

To recap:

Just talking, in person, can transmit COVID-19 from an asymptomatic (seemingly healthy) individual. That’s what the CDC now says.  They tell you to wear a mask, and they tell that the purpose of the mask is to protect others.  Unsaid by the CDC, a well-made home-made mask will also provide significant, though not complete, protection to the wearer.

So wearing a mask is helpful, for protecting others, even if that mask does not filter out the smallest aerosol particles.  And, a well-made home-made cloth mask is very helpful for reducing (not eliminating) most (not all) of the droplet and aerosol loads that the wearer faces.  Just not the smallest particles, in particular.

I went over that back in Post #593.   Just a quick refresher:

Some cloth can achieve filtration of an aerosolized virus that is almost as effective as a real surgical mask (rated for hospital use).  But that tends to produce a lot of back-pressure, which will make that less comfortable to use, and likely lead to some leakage of air around the mask.

And, in terms of protecting others, a home-made mask is just vastly better than nothing at all.  Note that the middle column of results (home-made mask) looks a lot like the one at the far right (surgical mask).

The source for both of those is this incredibly helpful academic article:

“Testing the Efficacy of Homemade Masks: Would They Protect in an Influenza Pandemic? ,Anna Davies (a1), Katy-Anne Thompson (a1), Karthika Giri (a1), George Kafatos (a2) ,DOI:,Published online by Cambridge University Press: 22 May 2013

I hope you are up to speed now.

Vienna Face Covering Task Force

I’m on a mission to get everybody to mask up.  In fact, I’m on a mission right now to mass-produce masks that will plausibly protect the wearer well, for use by residents of Vienna who are smart enough to know they now need a good mask, if they don’t already have one.

I’ve already figured out that the best material, not in the hospital supply chain, readily available, is the cloth inside high-end furnace filters.  (And, as shown in prior posts, many people, including doctors and nurses, have come to the same conclusion).  That material was manufactured to filter fine particulates from the air, and the filtration ability is know (MERV, or, for Filtrete (R) filters, MPR).

There are a lot of issues with that.  But for sure,that’s a fool’s errand if:

  • A)  People won’t wear masks, or
  • B)  Almost everyone already has a proper medical mask, or
  • C)  You can just go out and buy a proper medical mask, or
  • D)  I can’t make masks that provide good protection, or can’t make enough, or can’t make them fast enough.

Should you buy a KN95 mask?

But let me start with C.  I’m just mean enough, or fair enough, to say the anti-social thing that needs to be said:  If you go on Ebay, you can still find people selling what they claim to be N95 (US standard) or KN95 (Chinese standard) masks.  Used correctly, those are the best commercial protection against droplets of all sizes, including aerosol (under 5 micron) droplets.  But at this point, “society” deems it unethical for you to purchase one, because, in theory, you are diverting masks from the hospital supply chain.

I’m not sure that “society” is correct any more.  That was an important consideration six weeks ago, when large volumes of such masks were available through mass-marketing channels.  See Post #555, dated 2/28/2020.  The folks who bought those masks, six weeks or more ago, likely did divert some out of the hospital supply chain, because at that point, Home Depot would have been re-ordering them to re-stock the shelves.  Now read Home Depot’s current policy. .  Look on Amazon, ditto.

And what you see Home Depot saying goes for every mass-market outlet of N95s in the US.  At present, the supply of those N95 masks appears tightly controlled in the US at the supplier/wholesale level.  Nobody who is not in the health care business or first response can get a fresh shipment.

Couple that with the FDA’s decision (finally) to allow use of cheap-and-plentiful Chinese KN95 masks in hospitals, and the realization (finally) by hospital administrators that N95 masks can be reasonably safely re-used if proper protocols are adopted and … I think the mess that we saw in Washington state, where they literally had to use home-made masks in the hospital, is entirely in the past.

I believe that tight control of US manufacturing and wholesale channels is why the only people I see selling them are the sleazeballs rip-off artists opportunists capitalists on Ebay.  Most of what I see appears to be Chinese-based or Chinese-connected, one way or the other.  There is no shortage of such masks in China (see Post #601).  The volume of those that could plausibly be sold retail via Ebay could not possibly affect overall supply of Chinese-made KN95 masks available to the US.  (Recall, I just got a business-to-business offer to order up to 10 million masks.)  Further, much of what you see is (claimed to be) Chinese-made KN95s that are still not exactly legal for hospitals to use.  (The FDA says they may use them, but caveat emptor.  Hospitals still incur legal risk if they do, because they are not officially certified for hospital use — that’s how I understand it.)  And for sure, whatever remaining US stock that got siphoned off months ago, to be sold now on Ebay, almost certainly isn’t going to get into the hospital supply chain, at this point, no matter what.

But I’m an economist.  Where others see this as price-gouging opportunism, I see the market attempting to clear.  It seems perfectly rational to me.

Here’s how I view this, all things considered. If you are:

  • In a high medical risk risk category — elderly (over 60), or with significant chronic conditions.
  • Sole source of care for dependents (children or elderly)
  • In a high-risk public-facing position such as grocery store clerk
  • In some sort position critical to the US or state government.
  • And you could probably make up a few more categories here.

I would say, if you fall into one of those categories, don’t screw around with a home-made mask.  Buy a KN95 from some source, if possible.  If you want, I can do the arithmetic for the high-medical-risk categories to show you that investing in prevention, for those folks, probably actually saves masks.  But the fact is, this is just what I now call Rule #1:  Don’t be the dumbass who fills the next empty hospital bed.

If you are outside of those categories, and you feel guilty about buying one, then consider buying two and trying to donate the extra to a local hospital or first-response provider.  I bet you can’t, at this point.  As I understand it, from talking to an official of the VVFD, at present, our local first responders aren’t looking for donations of that type.  But you can try.  And that way, if you succeed, you’ll have pulled one back into the hospital supply chain, that presumably escaped some months ago, or that was lurking in China, ready to be bought and used.

But I’m not your momma.  You have to decide for yourselves.  And besides, at this point, a strategy of buy it on Ebay doesn’t work for the US as a whole.  I’m not even sure it would work for Vienna as a whole.  Not a lot of through-put available from that source, that’s my best guess.

But, please, if you do, you only need one.  They can be reused indefinitely (until the elastic sags), by the simple expedient of putting them aside in an open paper bag after you wear it.  (Again, I explained that in the recent series of posts on masks.)  It takes months and months of use before they begin to clog, and you’ll know it because if that happens, they will become hard to breath through.  The longer it sits, in an open paper bag, the more of the potentially viable virus that may have contaminated it will decompose.  One day of sitting is almost certainly adequate, buy two and rotate if you are worried.  Any more than that is almost certainly excessive.

And DO NOT WASH THEM.  That reduces the filtration effectiveness.  Just put them aside to dry after you use them.  AND IF YOU GET ONE, WEAR IT CORRECTLY.  Ticks me off to see a store clerk with an N95 put on willy-nilly.  I don’t begrudge them the N95, I bewail their lack of comprehension.  These masks are nearly useless if you put them on in such a way that they leak.  There’s a proper procedure for fitting an N95 respirator.  Take five minutes to look it up.

What if you won’t or can’t do that?

Now on to point D above.  Let’s assume that many people in Vienna won’t or can’t buy a  KN95.  I can respect that. But that all the rest of the fool’s errand issues can be put aside.

The next question is, what about home-made masks?  While a cloth mask is helpful, what I really want is a mask that plausibly has a known and high filtration level, particularly against aerosol (under 5 micron) particles.  That is, I want to get as close as I can to the N95 standard of filtration (see Post #593 for more than you ever wanted to know about filtration standards).

So I want to make that mask.  I want to make a lot of those masks.  I want somebody in Vienna to start making sure everybody around here wears them (or something better) in situations that require it.

This is not to discourage using commercially-made masks or home-made fabric masks.  Any mask is better than no mask, and the right fabric can do quite well at filtering the bulk of aerosol particles from the air.  I just think that, once the full import of the new CDC guidance sinks in, there ought to be a lot of people here in Vienna who need a decent mask.

You can see my first attempt at an expedient “respirator-style” mask in Post #595 While I know that works (at least with MERV-8 filter material), that’s labor-intensive.  And, at least at this point, it requires that you have a common dust mask to use as the form to which the MERV-rated material is attached.

Expedient surgical mask using furnace filter material.

Now I believe I have settled on my first “surgical-mask-style” expedient mask design.  In a nutshell, I’m going to take a cheap, non-medical-grade “surgical” mask, and upgrade it.

The joy of this is that it can be rapidly produced.  In fact, I should be able to stuff the materials in an envelope, mail it out, and anyone could do the final assembly.  Like Ikea furniture, minus the Allen wrenches.

So you start with the cheap “single-use” masks that are still sold on Amazon.  Currently, these are all back-ordered.  But they will eventually be available again, and can be tracked down in other places.

I’m not entirely sure what these are.  These are not used by hospitals.  They are not certified as to filtering ability.  So it’s OK to buy them.  That’s my interpretation, as Amazon continues to offer them for sale.  Amazon has removed anything that hospitals might use, reserving those items for health care and first response providers.

I hope that these are, in fact, genuine Chinese single-use masks (not to be confused with surgical masks).  If so, they will actually provide a modest amount of filtration on their own.

Then I’m going to upgrade that with a piece of high-MERV or high-MPR material extracted from a furnace filter.  See Post #593 for a quick background on MERV, HEPA, MPR (Filtrete (R)) and other household filtration standards, and how these compare to standards for  medical masks.

This will not be quite as good as a respirator-style mask, because you will have some air leakage around the mask.  If you’re really paranoid, do what the pros do:  Get some medical tape and tape the mask to your face.

But these should be easy to make.  And with a sufficiently high MERV rating, they should be fairly effective at filtering aerosol-sized particles.  Let’s put it this way:  This will almost certainly provide you with more protection than any cloth mask you are likely to be able to make.

I will need:

A high-end furnace filter.

A cheap non-medical-grade “surgical” mask.

Duct tape or equivalent.


Dissect the filter (as discussed in Post #595).  Ideally,  locate a filter that does not have the wire backing.  Otherwise, gently (and tediously) separate the filter cloth from the wire backing.

Cut out a piece of the filter media a little larger than the mask itself, while the mask is being worn on your face.  Use thin strips of duct tape to attach it to the cheap mask.  Wear the mask, paying careful attention to fitting it as closely as possible to your face.  If the mask gets loose, has straps break, and so on, peel off the MERV-rated filter media and stick it onto a new mask.

You probably want to consider duct taping a piece of thin cloth over the MERV filter material, just to keep it from getting damaged, and to act as a pre-filter for large particles.

That’s it.  Assuming I can breathe through it, that ought to work just fine.  No sewing.  And assuming that every American has a roll of duct tape somewhere, I can mail them out for final assembly in the home.

You will not be able to wash this filter.  Or, at least, based on tests of washing real N95s, that will reduce filtration efficiency.  So I want to go with an all-synthetic base mask in any case.  Masks made with organic material are eventually going grow bacteria if kept damp.

Care and maintenance will be the same as is now suggested for re-using an N95.  Take it off carefully, put it in an open paper bag, and leave it for a while.  Arguably, one day is enough.  Again, arguably, three days should be far more than adequate time to allow any trapped virus particles to decompose.

At present, I’ve ordered a bunch of those cheap surgical masks (from, yeah, Ebay, because they are months-back-ordered everywhere else), and I’m looking over various furnace filter types.  Arguably, the people with the most experience (Washington State) argue for high-end Filtrete (R) filters, but at the volume I need, I’m not sure I can afford that.  Likely I’m going to end up with MERV-13 filters.  Two layers of that will arguably provide something like an N75 level of protection.  Not an N95, but not too shabby, either.

I would also like to make furnace filter cloth available to Vienna residents who want to sew their own cloth masks, the kind with a filter pocket in the mask, for those who cannot lay their hands on/feel uncomfortable trying to extract furnace filter materials on their own. You only need a modest-sized piece, so it would be inefficient for every person who wants a mask to buy a furnace filter.

Now if the materials will just show up.  And fi we can just convince everybody to mask up in public, as the CDC suggests.  And find an effective distribution channel.  And so on.

We need a Vienna Face Covering Task Force.