Post #780: Making an aerosol-filtering respirator from a dust mask and Filtrete (r) 1900

Posted on August 23, 2020

This post shows one way to upgrade a common dust mask so that it filters aerosol-sized particles.  The basic idea is not exactly rocket science:  Cover it with something that filters aerosol-sized particles.

If you can get your hands on a good dust or surgical mask — the kind that seals against the face — the approach I’m going to outline here is the best of all possible worlds.  Short of being able to buy a real (e.g., 3M) N95 mask again.  You use the factory-made seal of the dust/surgical mask to provide a tight seal against the face, just like an N95 respirator.  The addition of a layer of 3M Filtrete material then filters out a significant fraction of aerosol particles without compromising the breathabilty of the mask. If done with care, and the right inputs, you end up with the functional equivalent of a proper N95 respirator.

The end result here is about as good as it gets, for a D-I-Y mask.  You can see my other attempts at expedient masks in much earlier posts.  This latest attempt replaces any of the earlier ones.

But the catch is, you need to start with a mask that has a good face seal.  If you start with a lousy mask, you’re going to end up with a lousy mask with another layer on it.

It is easy enough to adapt the process below to off-the-shelf “KN95”-style masks.  The problem is, in my limited experience, they’re all crap.  Or, at least, the ones I got at my local hardware store have such obvious problems that I’m not going to waste Filtrete ™ trying to cover them.

So I’m still pondering how to get from an off-the-shelf generic “KN95” style mask, to something worth wearing.  Something that plausible provides an assured level of protection from aerosols.  I haven’t gotten there yet.

This mask-making exercise serves two purposes.  Mostly, it’s to support of my next post, where I test whether or not you can use the smell of cigarette smoke to estimate aerosol filtration ability of a mask.   But also, the 3M mask I’ve been wearing since the start of the pandemic is worn out.  I know that because it’s becoming hard to breathe through.  So I need a new mask anyway.

To be clear, I’ll be wearing the mask I make in this post. It’s now my every-day mask, for shopping and such.  Foolish or not, that’s my expression of the confidence that I have in the end product.

In a nutshell:

  • Take a common high-end dust mask with a good face seal.
  • Cover it with a layer of 3M Filtrete (R) 1900 (or higher), taken from an air filter.
    • OR two layers.  It remains breathable with two layers.
  • Use a hot glue gun and hot glue to fuse the Filtrete to the dust mask
    • OR use fusible interfacing and an iron to “glue” that together.  That’s neater, but takes longer.
  • Replace the “rubber bands” on the cheap mask with stronger elastic.
    • OR use string or cloth for a tie-on mask.

The result is a lightweight, breathable mask that uses a factory-made mask-to-face seal and should filter a minimum of 60% to 75% of 0.3-micron aerosol particles.  If you upgrade to Filtrete 2500, you’ll have higher filtration capability.

Don’t have a close-fitting dust mask?  The generic strategy is to buy any tight-fitting mask, with a good face seal, and cover it with breathable 3M Filtrete ™ to provide aerosol filtration.  Any mask that is breathable and has a good face seal should work.

Why Filtrete ™?  It is designed to filter aerosol-sized particles while generating very little “back pressure”.  That is, it’s very breathable.  And yet has a known and documented ability to filter aerosols.  You can achieve this level of filtration with other materials, but those will have much higher back pressure.  High back pressure makes the mask uncomfortable to use, and means that more air goes around the mask, rather than through it, compromising filtration.

Why use a “cup-type” high-end dust mask?  I just happen to notice that my 3M dust/surgical mask has the exact same face seal as my last remaining 3M N95 respirator.  That’s ideal, because that’s designed to give a good, tight seal against the face. And it does.  It gives a remarkably better seal than …

Generic “KN95” masks are more like a folded piece of paper, and won’t seal up as tightly against the face.  The ones I got leak extensively around the nose, and no amount of manipulation of the nosepiece would fix that.  The ones I got also had considerably more back-pressure than my 3M dust mask.  But use what you’ve got.  So, you can start with a cheaper mask, but the end result isn’t going to be very good, because the leaks and high back-pressure compromise mask performance form the start.

If you need to ask why aerosol-sized particles matter in the context of COVID-19, you’ve come to the wrong place.

Tools and materials

In order, top to bottom left to right:

  • One 3M Filtrete ™ air filter, 1900 or higher.  You’ll need approximately 6.5″ x 10.5″ piece of Filtrete ™.  One air filter will provide enough material for dozens to hundreds of masks.
  • One high-end dust mask, the “cup” kind that seals against the face.
    • (OR any tight-fitting mask that seals against the face).
    • (BUT not a cheap “KN95”.  Too leaky, too much back pressure).
  • Stapler (just a garden-variety office stapler).
  • Sharpie or other marker.
  • A few small pieces of clear high-quality tape (such as Gorilla tape).
  • Two 15.5″ pieces of thin elastic.
    • (OR four 20″ pieces of string, or thin cloth ties, if no elastic is available.)
  • Hot glue gun (“mini” size preferable) and glue stick.
    • (OR fusible interface and iron).
  • Scissors, heavy-duty shears (for cutting apart the air filter).
  • Scissors, light-duty (for cutting cloth).
  • Two clothespins or two paper clips.
  • Not shown:  Yardstick, some paper towels.



Task 1:  Strip down a 3M air filter and remove the Filtrete cloth.  I covered air filter disassembly, including pictures, in Post #595.  The only difference here is that the 3M filters aren’t attached to the metal mesh, so they come off the mesh much more cleanly.

Edit:  See an easier and less sloppy way to do this, for a Filtrete filter, in Post #807.  You don’t need to cut the metal mesh, as shown here.

No matter how you do it, it’s a sloppy, messy process.  Just try to damage the Filtrete ™ fabric as little as possible.  Yours will look something like this.

  • I used heavy-duty shears cut all four sides off the filter (first photo).
  • I cut through the whole assembly, including the metal mesh.
  • I then peeled any cardboard backing off (second photo).
  • I then “unzipped” the fabric off the metal mesh (third photo).

I tried unsuccessfully to peel the cardboard “boxing” apart, rather than cut it off.  In the end, it seemed easier just to cut off the sides of the filter, as shown in the first picture above.

There will be an air flow arrow on the filter.  You should ideally mark which side is the “air intake” side of the air filter, and in a later step orient the Filtrete material so that this side faces the out (away from you) on the final mask.  I’m not sure that it matters much, and I’m not going to repeat that in the steps below.

Just cut away and discard any part of the Fitrete ™ cloth with flaws.  That includes blobs of glue, rips, holes, or anything along those lines.  One filter is going to give you more than a lifetime supply of Filtrete.

Task 2:  If your mask has cheap “rubber band” straps, cut them off now.

The cheap rubber bands on a single-use dust mask aren’t good enough to hold up the heavier 3M-covered mask, for multiple uses.  You’ll replace them with elastic (or string or fabric) at the last step.

If your mask has good-quality elastic, leave it on.  If it has cheap “rubber band” straps, cut them off.  Leave the staples that were holding the rubber bands in.  You will need those for proper alignment later.

Task 3:  Cut a generous rectangle out of your Filtrete ™.

For ease of assembly, the pleats in the 3M fabric will run vertically when you are wearing the mask.  For most masks — 3M dust mask included — that means that the long dimension of your piece of 3M Filtrete runs perpendicular to the pleats.

To figure out how much to cut for your mask, either eyeball it, or do this. 

    • Go to one corner of the fabric.
    • Start at one side and “roll” your mask from one side to the other, across the fabric, perpendicular to the pleats.
    • Add an inch, and mark that.
    • Go to the top, and “roll” your mask from top to bottom, down the fabric, in the same direction as the pleats.
    • Add an inch, and mark that.
    • Use those marks to cut out a rectangle of fabric.

The process is briefly illustrated below:

Or just eyeball it.  As long as you get it large enough, you’re fine.

Task 4:  Tack the Filtrete to the mask at the elastic attachment points.

Use the least amount of glue that you can.  The glue stiffens the mask-to-face seal, and you want as little of that as possible.

  • Center your mask on the Filtrete from top to bottom.
  • Square it up so that the top-to-bottom mask dimension aligns with the pleats.
  • Roll it to to the left, glue it to the Filtrete at the left-hand elastic attachment point.
  • Roll it to the right, glue it to the Filtrete at the right-hand elastic attachment point.

An important note on gluing:  The nose of the hot glue gun can melt the 3M Filtrete.  Throughout this mask-making process, lay a thin bead of glue on the outside of the mask, then press the 3M Filtrete into that.  Use a paper towel to squeeze out any excess, and wipe it onto the Filtrete ™.  You’re going to trim off that excess Filtrete ™ at the final step.  This approach leaves the least hot glue on the mask, and results in the most flexible mask seal.

Illustrated below:

Task 5:  Roughly form the mask cover using two folds to take up the slack.

Starting from the elastic attachment points, shape the Filtrete material around the mask as neatly as you can, ending up with one fold-over at the bridge of the nose, and one at the point of the chin.

Temporarily paper-clip or clothes-pin those folds in place.

When you are done, this should look kind-of like a generic “KN95”-style mask, no matter which mask base you started with (dust mask or KN95-style mask).  Beauty is not the goal here.



Task 6:  Roughly trim the Filtrete.

Trim off most (but not all) of the excess Filtrete ™.  This makes it a lot easier to do the next hot-glue step.

Task 7:  Hot-glue the Filtrete all around the mask edge.

Use as little glue as possible.  Don’t get glue on the actual mask seal — the part that will touch your face.  Try to make this as complete a seal as possible — one continuous glue line.  And do this by laying a bead of glue on the outside of the mask, then pressing the Filtrete into that, as mentioned above.

  • Start at one elastic attachment point.
  • Work up toward the bridge of the nose.
  • Go to the other side.
  • Work up toward the bridge of the nose
  • Work down to the point of the chin
  • Go to the other side.
  • Work down to the point of the chin.

The point of that is:  Work toward the folds at top and bottom.  Tighten up the material as you go.  Taken out the slack, and add it to the fold as you glue.  That way you don’t trap any excess material around the edge.

Task 8:  Final trim of excess Filtrete material.

Now go around the rim of the mask, cutting the Filtrete flush with the mask edge.

At the folds, cut the excess off at an angle that will allow you to paste the folded material onto the mask itself.

Task 9:  Tape the folds down.

You could use glue, but high-quality clear tape ensures a good seal and looks nicer.  Use hot glue if you prefer.

Edit:  I’ve now made about a dozen of these, and hot glue is simpler and quicker.  Just put a dab of glue between the cloth layers, a dab of glue on the bask, and press down.

Task 10:  Staple on new elastic, if needed.

  • Knot the ends of the elastic.
  • Align them at the old elastic attachment points.
  • Staple on.
    • Elastic is on the outside of the mask.
    • Staple so the bar of the staple traps the knots on the elastic.
    • The “points” of the staples will be inside, but they won’t bother you.
    • This is a weak point, so feel free to staple this twice.

Step 11:  Done.  Fit the mask to your face and try it out.

That’s it.  If this all worked, you now have a mask that a) has a good face seal, b) filters out a significant fraction of aerosol-sized particles, and c) will last a fairly long time, if the elastic and staples hold out.

So what if it looks ugly and sloppy.  (The mask, I mean.)  It seems to work exceptionally well.  Breathable, good face seal, and should filter a high fraction of aerosol particles.  That’s what I’ll be testing tomorrow.