Post #807: A vastly simpler mask liner using Filtrete ™

Source for base data: 3M, ASHRAE.  See Post #593 for writeup of these and other filtration standards.

I’ve done several posts about making masks out of Filtrete ™ home air-filter material.  Each time I return to that task, I find simpler ways to work with Filtrete ™.

In this post, I’m going to show just how easy it is to take a Filtrete ™ home air filter and make some simple, flat, easy-to-handle, fiber-free pieces for use inside cloth masks.  In a nutshell, extract the Filtrete ™ fabric from the air filter and hot-glue it between two layers of the thinnest synthetic fabric you can find.

Then end-user can then cut it to size, for use as a liner for a cloth mask.  That seems to work just fine, and nothing more labor-intensive is needed.  The materials run well under $0.25 per mask liner, depending on what size filter you buy, and how large you cut your mask liners.

A few tips and tricks for doing that are given below.  Of which, the only one that might not occur to you is to use kitchen “parchment paper” as a non-stick surface as you are gluing.


Background:  Why make a Filtrete mask or mask liner?

The current situation seems to be the following.

COVID-19 almost certain can be and is spread by aerosols, that is, particles under 5 microns that have the ability to float in the air for considerable periods of time. See Post #771 for my last posting on that topic.

Ordinary citizens can’t easily get masks that will filter aerosols well.  N95 masks stop 95% of aerosol particles, but these cannot be obtained through any mainstream retail channel now.  They are reserved for health care personnel.   The US is being flooded with cheap “KN95” masks, but a) that’s not a US standard so KN95 is no guarantee of filtering ability for masks sold at retail, and b) you really have no idea what you are getting when you buy a “KN95” mask.  (see Post #747 for a description of the chaos of the KN95 market).

In short, the only way for most of us to have a mask with a known ability to filter out aerosol particles is to make it ourselves. 

And for that, 3M Filtrete ™ has two big advantages over other materials that you can easily get your hands on.

First, it has a known and good ability to filter out small particles.  That’s what Filtrete ™ household air filters are known for.  That’s what they are designed to do.  As you can see from the table at the top of this post, two layers of Filtrete ™ 2500 fabric will filter out 95% of 0.3 micron particles.  That’s the same filtration ability as an N95 mask.

Second, it’s very breathable.  Filtrete ™ air filters produce very little back pressure relative to their filtering ability.  In terms of the tradeoff between filtering the air and being breathable, it’s about as good as it gets.

The main drawback is that the surface is fibrous and easily clings to fabric.  You can stuff a raw piece of Filtrete ™ into a tight mask pocket, but it’s a fussy process.  It’s much easier to handle if you cover the surface of the Filtrete ™ with a thin, breathable fabric.

This is the first time I had used the 2500 material.  By eye, it looks identical to the 1900.  But when I put a micrometer on it, it was about one-third thicker.  Which was reassuring, because it’s supposed to be about one-third better at filtering aerosol-sized particles.


Detailed instructions.

Materials and tools:

  • One Filtrete ™ 2500 or 1900 air filter.  This will cost you as much as $25, but will make many, many mask liners.
  • Hot glue gun and hot glue sticks.
  • Pair of scissors.
  • Sharpie or other marker.
  • Pliers (optional)
  • Work gloves (optional)
  • A foot or two length of kitchen “parchment paper”.  Long enough to span your air filter from top to bottom.
  • Large cutting board or equivalent.  Again, ideally, long enough to span your air filter from top to bottom.  (A couple of books would do just fine.)
  • A yard or so of extremely lightweight synthetic fabric.

I used lightweight floating row cover (spunbonded polyester), sold in lawn-and-garden stores.  That’s pennies a yard, but typically only sold in large pieces.  As you can see (left), you can easily read a magazine through it.  That’s what you’re shooting for.  You could also use tulle (often used for  wedding dress veils). That runs about $2 a yard.

Step 1:  Tear apart a Filtrete 2500 or 1900 filter.

Before you start:  Mark the exhaust side of the Filtrete fabric.  That’s the side at the tip of the air flow arrow on the side of the filter.  That’s also the side with the circular plastic “change filter warning” device on the mesh.  You will want to mark the final product so that this side faces IN toward the user.

Tear open the cardboard surround.  Just make a hole, stick your index finger in, and go all around the air filter.

 

 

 

Peel off the remaining cardboard that’s stuck to the metal mesh.

 

 

 

 

The two pieces of metal mesh are glued to the Filtrete only around the edges.  Take off the piece that doesn’t have the  plastic “filter change warning” device on it.  Work your finger between mesh and Filtrete ™ fabric, and just work it all the way around the edge.  You might want to wear gloves at this step.

 

Pull off the mesh.  Note that the Filtrete ™ fabric is only messed up at the very edge.  The interior portion of the fabric is undamaged.  You’ll cut off the messed-up bits in a second.

 

 

Unzip it off the metal mesh on the other side.  At some point, you’ll have to pull out the “change filter” device by brute force.  Until that point, just grab the end of the fabric and pull it off.  Once the glued edge comes off, the rest is easy.  I would only take what you need for now, and leave the rest attached to make it easier to store.

 

Discard the dinged-up fabric.  That’ll be about an inch on each side, plus maybe the first and last six inches of the length of the fabric.  Mark the torn-up spot where the “filter change warning” device was, if you took that much fabric off the filter.

 

Step 2:  Set up your gluing area and glue.

Here’s what you see above, from bottom to top.

  • The dark brown table I’m working on.
  • A piece of kitchen parchment paper, so that the fabric does not stick to the table.
  • A piece of floating row cover (lightweight synthetic fabric.
  • The Filtrete ™, folded over the cutting board, which is then keeping the Filtrete ™ flat.
  • A weight, to hold the Filtere ™ folded over.

To glue it up:

  • Run a thin line of hot glue down the lightweight fabric, maybe an inch or so from the edge of the cutting board.
  • Scoot the cutting board over and press the Filtrete ™ into the glue line.
  • Repeat.

When you are done with one side, flip the Filtrete ™ over and use the exact same method to glue another piece of lightweight fabric to the other side.

The resulting product will look something like this (below).  (This is only half the width of the piece above.  The other half went off to college with my daughter.)  Note the little “ins” marking surface that should face the user when worn.

When cut, this composite will have a little bit of loose cloth at the edges, but not enough to matter. It remains easy to handle, and any loose edges will be well away from the user’s mouth.

The bottom line is that even if you don’t have a mask pocket in a cloth mask, you could cut a piece of this and use it as a mask liner.  It adds almost nothing to the mask back-pressure but adds considerably to the filtration ability of the cloth mask.   Per the table above, if you started with Filtrete ™ 2500, the liner should catch 77% of 0.3 micron particles.  Not quite as good as an N95 mask (which captures 95% of those particles).   And, particularly with cloth masks, some air will leak around the mask rather than through it.  But, all in all, not bad for something that you can get your hands on.

Don’t wash these Filtrete ™ mask liners.  That definitely degrades the Filtrete ™.  Either take them out and let them dry when you launder your cloth mask, or throw them away and replace them.  They should cost you less than $0.25 each for the materials.