Post #938: Yet another series of posts on masks, part one

Posted on January 4, 2021

I posted something yesterday, chiding people for wearing cheap face masks, and in particular for wearing them poorly.  Apparently I hit a nerve with more than a few people, and I’ve been challenged to offer some practical advice.

What you are going to get next on this website is a series of posts on masks.  Solely from the standpoint of protecting yourself, not from the public health standpoint of protecting others.  Starting with some straight-up “buy this” practical advice, before I go off on a deep dive on the whys and wherefores.

But because most people don’t grasp the basic math of masks, I have to do the math first.  And that’s because mask ratings and mask performance tests hide the true relative risk of various types of masks.

 If you just want to get to the quick advice, just skip to the next section.  But you really ought to try to answer the question below.

Mask ratings hide the true relative risk of poor masks  versus good masks.

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.   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?

A)  Obviously, it’s about three times better, because 30 x 3 = 90, which is close to 95.

B)  Obviously, it’s about 14 times better, because (100 -30)/(100 – 95) = 70 / 5 = 14.

C)  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 14 times as much viral load as the 95 mask.  Because 70% of what’s in the air is 14x as much as 5% of what’s in the air.

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.


Edit 1/15/2021:  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).

Yep, 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.  To the point of making my own, so I can be sure of what I’m putting on my face (Post #807, Post #780), and trying to test them (Post #790).  And why I continue to be irked about the inability of citizens to purchase true N95 respirators.  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.

Edit:  And I’ll go you one better.  In at least one hospital here in Northern Virginia, the nurses serving the COVID-19 ward wear half-face N100/P100 respirators. Like the one below.  Because if you’re really heavily exposed, allowing even 5% of viral particles past your respirator just won’t cut it.



If you just want some quick advice on a reasonably good mask to wear.

I’m not going to go even one inch into all the details.  Fact is, there is a mask, that you can buy, that is easy to wear, and that did very well in a realistic test, by real scientists, published in the Journal of the American Medical Association.  In terms of some quick advice, on what to wear, that’s about as bulletproof as it gets.

The recent test of masks published in the Journal of the American Medical Association found that two-layer nylon masks filtered out about 80% of airborne particles, once the masks had been washed (Post #924, or you can try to pull up the tables in the JAMA article itself).

Because this was actual scientific research, they specified the mask fully as: “(1) a 2-layer woven nylon mask (54% recycled nylon, 43% nylon, 3% spandex) with ear loops (Easy Masks LLC) tested with an optional aluminum nose bridge and nonwoven filter insert in place.”

Click here to buy those exact masks from the manufacturer’s website.  (To be clear, I have no financial interest in this whatsoever.  Also, the JAMA test achieved near-80-percent filtration without use of the nonwoven insert.)

I honestly don’t think there is anything unique about those masks, within that specification.  Except that they are made correctly.  If you go to the website, you will see that they are generously cut, and cover the face from throat to eyes, ear-to-ear.  And that they make them in different sizes, and they tell you how to measure your face, to choose the right mask.  And they make small ones for kids.

If I had to bet, I’d say that this particular North Carolina firm’s masks were chosen because the principal author and all of his colleagues are from North Carolina.  And because the manufacturer seems to do everything more-or-less correctly.

At some point, I’ll belabor exactly why this is a reasonable choice.  But for now:

The upshot is, based on their cheapest mask, for under $20 (including shipping), you can get two copies of the mask tested in that JAMA article.  It’s roughly an N80 after washing.  You can reduce your exposure to airborne virus 3.5-fold, compared to a standard ear-loop surgical mask. 

With no fear of counterfeits.  With an actual legit test of that exact mask, on the books.  Easy-on, easy-off.  I’m sure if you did your homework, you could find well-fitting two-layer nylon masks for less.  But at some point, it’s not a lot of money, given what’s at stake.

Is this the best mask you could possibly use? No.  You can see where this sits on the scale of risk, in the graph below (redone from above).  At least you can see the difference without using a ruler to measure the bar.  But is this a substantial upgrade, if you’re still using disposable ear-loop surgical masks?  Yes, the odds are overwhelming that it substantially out-performs a blue disposable ear-loop mask.

Be sure to wash these before you wear them.  Filtration improves greatly after washing.

And if you insist on using up that pack of blue masks that you bought, look into the tied-and-tucked method for improving the fit and filtration of those masks, in this YouTube video.