Post #595: Expedient masks, part 3: Expedient mask prototype

Posted on April 2, 2020

Rapid conversion of dust masks to MERV-8, proof of concept.


I just heard that the Mayor of Los Angeles has asked everyone to wear masks in public.  So, now more than ever, we need effective masks for everyone.  I reviewed what’s for sale, and a) it ain’t much, and b) likely, most of it is gone by now.

Just breaking:  Looks like the CDC is finally going to issue guidance to wear masks in public.  Thank goodness.  I’ve just been told that Laredo, Texas will now fine people for being in public without a face covering.

Maybe the Army Corps of Engineers can produce and distribute a few optimized mask designs, with the idea that its within their mandate as sort of reverse Roemer’s Law.  A bed not filled is a bed not built.

In Post # 593, I showed the key tables from a scholarly article on home-made respiratory masks.  That analysis looked at the air-filtering properties of various types of cloth.  And, in fact, you could achieve particle filtration equivalent to a surgical mask, merely by using two layers of tightly-woven a tea towel.  But that two-tea-towel mask would have generated much higher back-pressure than a surgical mask (i.e., hard to move air through it).  That would have resulted in moving a lot of air around the mask (at the edges), rather than through the mask.  So, likely, you’d end up inhaling significant amounts of unfiltered air.

Return to Post #593 for the discussion MERV standards for air filters.  There was a reason I included that.  Some people (and other people) (and even some doctors) (and some nurses) have already come to the conclusion that the obvious material for home-made masks is the fabric inside of furnace air filters.

At the end of this, I’m going to make something that I think is obvious.  Take a dust mask — one with no guarantee of filtration at all — and tape a piece of MERV-8 cloth over it.  That should provide excellent protection against droplet transmission.  The shape of the face seal is formed by the factory-made dust mask, and is far better than I could achieve by hand.  The filtration is performed by the MERV-8 material, which should be adequate for droplet protection.  It uses a minimal amount of the MERV material.  I believe I could get this down to maybe — well certainly under 5 minutes a mask.  And if I can swap out MERV-16 for MERV-8 and still breathe through it, the same techniques would allow me to mass-produce N95-equivalent masks.

Seriously, I think this is a way to help solve America’s current shortage of masks for the public.  So this is my last plea:  If you know someone in a position of responsibility, please pass this along.  Because this way, not only do you have a mask that protects others.  This way, if I up the MERV level, you have a mask that protects you.  Using filtering materials that are outside the hospital supply chain. 

And if you can suggest ways to do this better, or have an idea for a better design using MERV-rated furnace filters, email me (

Let me now list the advantages of the MERV-based mask.

  1. It has a known level of filtration, the MERV rating.  With cloth, you have to guess.
  2. You can achieve a N95 level of filtration with a (rare) MERV-16 filter.  This would provide protection against aerosol (airborne) transmission.
  3. You can filter out some small droplets with a MERV-8 or higher.  This would provide protection against droplet transmission.  You can get almost all of them with a MERV-13 or so.
  4. Most American homes have some supply of these in the basement, so there is a large supply of this material readily available and already distributed to homes.
  5. The only tool required to extract the cloth from filters and work it into shape is a set of common household shears.
  6. The MERV-8 cloth is easy to work with (well, sort of), if somewhat fragile.
  7. FWIW, except for Filtrete (R) filters, I believe that the material in MERV-rated furnace filters is in fact melt-blown cloth, the same stuff that is in masks.

And so, while I could talk about how to sew a cloth mask, for that, I think I’ll just just put that off, for the time being.  At some point, that will come back into play, because I would need sewn bodies for my MERV-based masks.  But for now, let me just convert a dust mask that I have, and see how that works out.

I’m not even going to review MERV.  Refer back to Post #593 if you want to brush up on it.  For now, you’ll just have to take me at my word that MERV-16 requires 95% removal of particles down to 0.3 microns, in one pass.  Just line N95.  And we can work backwards from there.

For the record, I know MERV-8 ain’t great.  You can see the specs here.  But its what I happen to have.  MERV-8 only guarantees to trap 70% of particles between 3 and 10 microns in size.  And only 20% of those those 1 to 3 microns.  (Aerosols are those 5 microns and below).  But a MIRV-12 — readily available for home use, traps a minimum 90% and 80%, respectively.  And, you can double up the fabric, get two passes, and presumably raise those to something like 99% and 96%, respectively.

(And MERV-16 is basically the same spec as N95:  It gets 95% of particles down to 0.3 microns.  But I just got my hands on a MERV-16 filter, and I don’t think I could breathe through the material..)

So this is a proof of concept.  I.e., I’m wrecking a relatively low-valued MERV-8 filter.  Once I’ve fine-tuned this, I’ll move up the MERV scale.

Section 1:  Converting a MERV-8 pleated filter, and common dust mask, to a MERV-8 mask.

You’re going to take a standard home air filter, MERV-8, cut it out of its casing, leave the wire backing in place, cut out a rectangle, mold it around a common dust mask, trim and tape in place overtop the dust mask.  Then replace the elastic with something more substantial.

Tools and materials:

  1. MERV-8 or better home air filter
  2. Kitchen shears, large scissors or similar
  3. Sharp razor blade (optional)
  4. Packing tape, duct tape, Gorilla (R) tape, or similar.
  5. Piece of cord (shoelace, parachute cord, kitchen twine, garden twine).

1  Remove the filter from the cardboard casing, taking care not to damage the fabric.

You can’t peel them out without damaging them.  I learned that the hard way.  Take a pair of scissors and cut them out, cutting right through the metal mesh on the back.

On mine, the carboard on the face was randomly glued to the fabric.  Don’t pull that off.  For now, just cut around it, leaving chunks of cardboard on the fabric surface.  You can cut them down later, and either leave them or carefully get them off the fabric with a sharp razor blade.

Step 2:  Roughly flatten the fabric and wire assembly.

Leave the wire mesh attached.  I tried removing the fabric from the wire, but a) it was tedious and b) being as careful as I could be, I was still damaging the fabric.  I would prefer to work with the fabric alone, but that’s not gonna happen.

Step 3:  Cut roughly to size.

Carefully roll the mask in one direction, mark, then roll it in the other dirction, and mark.  Give yourself maybe a half-inch seam allowance all the way around.

Step 4:  Roughly shape the filter material by wrapping it round one fist.

Check for fit against the mask shape.  Fine-tune as needed.   Avoid creating “folds” in the material, to the extent possible.  I have a large fold at the bridge of the nose that may, to small degree, compromise the filtering, but … when I look at it, it’s not obvious that it does.

Once you are comfortable with the fit, trim the excess.  It’s OK to be a bit sloppy because you’re going to cover the joint between the two materials with plastic tape in the next step.

Step 5:  Attach the filter material to the dust mask.

Lay down a ring of tape on the mask seal.  This is just small pieces of high-quality tape.  It’s important that these overlap a bit, because these are going to become, in effect, the new mask seal.

Lay the dust mask into the shaped filter/mesh assembly, start in one place, and tape it all around.  When you are done, take a couple of strips of tape about 1″ wide, and just tape your way around the circumference of the mask.  Just to stabilize it, and make sure it sticks.

Step 5:  Replace the elastic with a cord of some sort.

Knot the cord, tape it to the mask.

6:  Put it on on, bend to adjust fit as necessary, and tie tightly.

Time and materials summary:

This entire process took me an hour, and left me with enough MIRV-8 material to make, looks like, maybe another 7 or 8 masks, with a lot of waste (because the mask min dimension is just over half the MERV-8 sheet that I got from my 14 x 30 x 1 filters.

Performance:  Yeah, it works.

I’ve been wearing this for an hour now, and it’s hot, uncomfortable, and it stinks a bit of plastic..  Which is to say, behaves like just about every other face mask.

I was worried that I would compromise the mask seal, but I don’t think I did.  For one thing, my glasses aren’t fogging up.  I don’t notice any leaks with a wet finger held near the edge (and sharp exhalations).  And, when I exhale, I can feel my hot breath diffuse out the lower part of the mask.  I want it going through the mask, not around the mask.

Back pressure is fine.  Not at all hard to breath, despite narrowing the breathable area somewhat with the plastic tape  .As it should.

The plastic tape is a little itchy, and I can feel a couple of the seams.  But I actually think the plastic tape, the overall metal mesh, and heavy-duty tie, in total, results in a pretty good seal against my face.

Tomorrow I’m going to review what other MERV masks are out there, and either improve this one, or build one of a different design.

And keep in mind, there is no obvious barrier to do this with MIRV-16 material.  The only real hitch there is that few domestic furnaces or air cleaners use MIRV-16.  And MIRV-16 — at least in terms of the specification — matches N95 for capture of 0.3 micro particles, and such.

I, Christopher Hogan, PhD., place this posting entirely in the public domain.