Post #1394: The U.S. CDC: Argh.

Posted on January 11, 2022

I saw this headline in today’s Washington Post.  It appears that the U.S. CDC is almost ready to maybe sort of recommend that you wear a good mask, not just any mask.

I guess, as pictured above, they’re looking back on the entire history of the pandemic, assessing where we now sit, and asking whether or not they might, possibly, at this stage, as a last resort, recommend an easy, cheap, and effective method for radically reducing the population’s exposure to COVID-19.


If you read this blog, you know I’ve been strongly in favor of use of high-filtration masks for a long time.  Since before the CDC even recommended wearing masks.  Just search the “mask” category and you’ll see what I mean

With this latest near-pronouncement from the U.S. CDC, I hardly even know where to start.  In the interest of saving time, I’ll skip the rant, and remind you of a few useful things.

1:  An N95 isn’t just better than a standard blue procedure mask, it’s vastly better.

2:  If you insist on wearing a cheap blue procedure mask, at least learn the “tucked and tied” technique.

3:  Leave the KN95s on the shelf.

1:  An N95 isn’t just better than a standard blue procedure mask, it’s vastly better.

Here, I’m just repeating a part of Post #938, from almost exactly one year ago.

Here’s a simple question.  Even if you think you really, truly understand masks, take 15 seconds to see if you can get the correct answer.

Question:  An N95 respirator (mask) filters out 95% of airborne particles.  A procedure mask with ear loops filters out about 30% of airborne particles.  (That’s based on an actual test of those masks as published more than a year ago in JAMA).   Let me loosely call that an “N30” mask.  Roughly speaking, how much better is an N95 mask, compared to an N30 ear-loop procedure mask?

  1. Obviously, it’s about three times better, because 30 x 3 = 90, which is close to 95.
  2. Obviously, it’s about 14 times better, because (100 -30)/(100 – 95) = 70 / 5 = 14.
  3. Obviously, this must be a trick question.

The answer is B, it’s 14 times better.  Why?  The mask rating (N30, N95) shows you what the mask keeps out.  But the viral load you inhale isn’t about what the mask keeps out.  It’s about what the mask lets through.  It’s about 1-minus-the-mask-rating.  And in any given situation, the ear-loop surgical mask will let through and expose you to 70% of what’s floating around.  While the N95 exposes you to 5%.  And 70/5 = 14.

In case you still don’t quite get it, let me do the math the other way.  How much better is that N30 ear-loop surgical mask, compared to wearing no mask at all?

Question 2:  Assume that you need to inhale 100 copies of COVID-19, at a sitting, in order to get infected.  Assume that you are going to inhale one cubic meter of air, at a sitting.  How dense can the COVID-19 particles in the air be, before you inhale enough to get infected, based on wearing:

  • No mask.
  • N30 mask (ear-loop surgical mask, worn loosely)
  • N95 respirator.


Question 2, same math, but rephrased.  Suppose there’s a room filled with COVID-19 aerosol.  Suppose that, without a mask, you can sit in that room for no more than 10 minutes before you get infected.  How much more time does your cheap, blue ear-loop surgical mask buy you?  That is, how long could you sit in that room and remain uninfected, wearing an ear-loop procedure mask? And then, how long wearing an N95 respirator?


  • No mask — 10 minutes.
  • N30 mask (ear-loop surgical mask, worn loosely) – 14 minutes (10/.70)
  • N95 respirator — 200 minutes (10/.05).

That cheap blue mask buys you a whopping four additional minutes of time, before you get infected.  Which not only makes my point, but which shows you why you want to stay away from close, crowded situations, mask or no mask.

Sure, a loosely-fitting ear-loop surgical mask is better than no mask at all.  But not by a whole lot, in the overall scheme of things.

I hope you now get why I’m so persnickety about masks.  The difference between a good mask and a poor mask isn’t a little bit.  It’s a lot. It’s an order-of-magnitude difference in performance.


Still wearing those 20-cent blue procedure masks that you bought a year ago?  Can’t bring yourself to pay a whopping 89 cents each for genuine 3M N95 respirators, even though the 3Ms are good for hundreds of hours of normal use before the filter material clogs? Or maybe just just plain don’t like N95s of any sort, despite the wide variety available?

Then you should at least learn the tucked-and-tied technique.  By itself, this improves the filtration ability of the typical surgical style mask from roughly an N30 to roughly an N60.

Takes a few seconds to do.  Costs you nothing.  Doubles the effectiveness of the mask.  What’s not to like?

Or watch that directly in YouTube.

In the U.S., KN95 is a style of mask, not a legally-enforceable filtration standard.

The CDC will be doing nobody any favors if they recommend using an N95 or KN95 mask.  I’ll go so far as to say that adding KN95 to the recommendation is simply an incompetent mistake.

In the U.S., N95 is a U.S. standard maintained by the U.S. National Institute for Occupational Safety and Health (NIOSH).  A NIOSH-certified N95 respirator must fit tightly to the face, using straps that pass behind the head (never ear loops), and, when properly fitted, filter out at least 95% of of the hardest-to-filter particles (0.3 micron).

Masks may then be further certified for medical use by the FDA.  Masks certified for medical use must meet additional standards, including resistance to splashes.  It is completely possible to have a NIOSH-certified N95 that is not suited for medical use.  Most or all NIOSH-certified N95s sold for industrial use — such as the ones you can easily purchase at your local Home Depot or other hardware store — filter to the N95 standard, but are not certified for medical use.

In the U.S., KN95 means nothing.  It’s a Chinese standard, and has no legal meaning in the U.S.  Anybody can make a mask and sell it as a “KN95” mask.

Practically speaking, in the U.S., KN95 refers to a style of mask, not to a guaranteed level of filtration.  A mask that will fold flat, unfold into some sort of cone shape, and use ear loops rather than behind-the-head straps.

I have tried several KN95 masks over the course of the pandemic, and none of them worked well enough to use.  They all fit too loosely, allowed too much air to leak around the face seal, allowed my glasses to fog, and were generally insecure due to loose-fitting ear loops.

My point is, the things you can buy in the drug store labeled “KN95” are in no way a substitute for a NIOSH-certified N95 respirator. Not even close.  I sincerely hope that some CDC bureaucrats will get out from behind their desks, walk into a few hardware and drug stores, buy a few packs of what are routinely sold as “KN95” masks in the U.S., and assess them for air-tightness and likely filtration ability.  And come to the realization that, as I just said, the typical KN95 in America is not even in the same league as a NIOSH-certified N95.

In theory, the FDA had, at one time, a list of certified Chinese manufacturers whose masks could be used in U.S. hospitals under an emergency use authorization.  The FDA has long-since cancelled that EUA, and so, technically speaking, there are no KN95 masks certified for medical use in the U.S.

The bottom line is that, for the average consumer, you have no idea what you are buying when you purchase a KN95 mask. For myself, at least, every one I tried failed due to obvious air leaks.  And that doesn’t even begin to address the actual filtration ability of the cloth itself, which you have no way of testing, and which was never tested or certified by an U.S. agency.

Maybe if you’ve never worn a properly-fitted N95, you wouldn’t know the difference.  But once you’ve worn an N95, and realize that absolutely no air is supposed to leak around the mask, you will instantly reject any hardware-store KN95s on the basis of lack of air-tight fit.

If you must use an ear-loop mask, I’d recommend a made-in-Korea KF94, such as the LG Airwasher.  (KF94 is a filtration standard more-or-less equivalent to N95 in terms of particulate filtration.)  If it’s genuinely made in Korea, that provides a known filtration ability, and the ear loops are adjustable for tight fit.  Of all the masks that I asked my daughter to try, that was by far the most preferred (Post #1246, What mask should I wear?  We have a winner).

And at the end of the day, it’s all about wearing the best mask that you are willing to wear.