Introduction: No-Sew Filtrete™ Face Mask Liner for the COVID-19 Pandemic

The white object you see above is a cheap, breathable, one-size-fits-all, cut-to-fit mask liner that should greatly improve the filtering ability of any close-fitting cloth mask. It's a simple way to upgrade a bunch of existing cloth masks so that they have some ability to filter tiny, aerosol-sized particles. A cloth mask will never be a substitute for an N95 respirator. But adding this mask liner should help.

Background: On 4/3/2020, the US CDC changed its guidance to citizens regarding COVID-19. People are now encouraged to wear a cloth mask when in public. They specify "cloth" because they don't want citizens buying up proper medical masks or respirators that may be needed by front-line medical personnel.

The problem is, cloth masks are, at best, so-so in providing protection to the user. And, in particular, most provide little protection against the finest "aerosol" droplets (those under 5 microns). In effect, if a piece of cloth was woven so tightly that it could capture those well, you'd hardly be able to breathe through it. You'd end up breathing around it, rather than through it, which defeats the whole purpose.

High-end furnace filters are designed to filter out those fine particulates without creating a lot of back pressure. Which seems impossible, until you realize that the don't work at all the way you might think. Suffice it to say, they don't work by forcing the air through really tiny spaces.

But what matters is that they work. Any furnace filter that says "virus" on it is, in fact, designed to capture things as small as a virus-carrying particles. And they can do that AND remain breathable.

In a nutshell, you're going to build something that looks like the body of a simple surgical mask. Really, just a flat piece of cloth, with a few pleats in it. But very light-weight, very breathable, and, in theory, very good at capturing fine particulates. With a "rim" of lightweight cloth that can be easily cut, so that you can get a good fit to an existing cloth mask.

The user will take that, open it up the way you open up a surgical mask, trim the edges to get a good fit with their existing cloth mask, and then wear it under their existing cloth mask.

The end user needs to have a tight-fitting cloth mask. That mask should have a metal nosepiece that ensures a seal at the bridge of the nose. This liner will do no good whatsoever for somebody who's (e.g.) just using a bandana. But if your end user has something like a homemade cloth surgical mask, this should work well.

All the rest is details.


A Filtrete™ air filter, MPR 1900 or higher. Higher is better. One medium-sized filter will provide enough filter medium for many, many mask liners.

A piece of "floating row cover" or other very thin non-woven cloth. I used a thin spun-bonded polyester fabric sold in hardware and garden stores as "floating row cover". This will be used to enclose the Filtrete™, making it easier to work with and ensuring that no stray fibers escape as you use it.

Some fusible interfacing or "Dritz Stitch Witchery" or similar. If you are buying the stuff that's already pre-cut in a narrow roll, 1" wide is about right, but 2" wide would be too wide (and/or, you'd have to cut that down). You could probably use 3/4". It's not critical. This will be used to glue the fabrics together, using an iron.

A few staples.

Two pieces of thin cardboard (for patterns and guides).

A couple of feet of aluminum foil (to make a heat shield for ironing).


Iron (and ironing board, if you have one), thin cotton cloth (such as a bandana), office stapler, tape measure, scissors, and a sharpie. And something to use as a thin, flat weight, such as a metal ruler or a large wooden ruler.

Step 1: Strip the Filter Material Out of the 3M Filtrete™ Air Filter.

There is no good way to do this. Here's what I did.

Put a mark on the filter material itself, on the fresh are (air intake) side. I'm not sure if this matters, but if you think it does, you're going to want to put that side UP when it comes time to pleat the mask liner.

Peel off as much cardboard as you can. Get the metal mesh entirely free from the cardboard. You might have to take a knife to the cardboard, cutting along the edges, to do that.

Once the metal mesh is no longer held together by any cardboard, gently peel the the metal mesh from the white filter material and/or unzip the white filter material off the metal mesh. You will mess up some spots on the filter cloth due to (e.g.) heat-welding at the edges, and a few spots of glue elsewhere. Just avoid those as you cut this up for mask liners.

As far as I can tell, there is no obvious way to do this. Just try to do as little damage as possible to the filter medium.

One high-end filter will yield a huge amount of material. I used good-sized MPR 2500 filter, and I ended up with a piece of filter cloth 2' x 12'.

Step 2: Make Patterns and Cut Materials

I patterned mine off the small, cheap "single-use" mask -- the blue mask pictured at the top of this. I think you want it to be a bit on the small side, because it's going to fit under somebody else's cloth mask. The cheap "single-use" mask I use for my patter was 6.75" wide, and 6.5" tall when completely un-pleated.

(Just FYI, the blue "single-use" mask is not rated for use by hospital workers and could not legally be used in a hospital. And, calling it cheap is an understatement. It really can't filter anything. I'm just using it for size.)

Cut one piece of cardboard, 6.75" x 6.5", as the pattern for the Filtrete™. The longer dimension is the side-to-side dimension. Mark a line, along that dimension, and write "pleats" to remind yourself how this should be oriented. The long dimension is the top and bottom edge of the mask. The short dimension is the sides of the mask.

Cut a second piece of cardboard 10.75" x 10.5" (or thereabouts) for the thin cover material. Exact dimension and exact cutting of this material does not matter.

Put a line across the Filtrete™ pattern, parallel to the top and bottom and write "pleats" on it. That's to keep you oriented as you go through this.

Cut out one piece of Filtrete, and two pieces of floating row cover.

ON THE FILTRETE, you want the pleats that you "inherit" from the air filter to go in the same direction as the pleats in your finished mask liner. The long dimension of the template lines up with the pleats in the air filter fabric.

If you are using fusible interfacing, cut some 1" (or so) strips off that. Longer is better, I guess. You just need enough to go around the perimeter of the Filtrete™. Don't make them too wide, or they'll stick out beyond your fabric, and you'll glue your iron to your ironing board. Otherwise, if you have the precut stuff on rolls, 1" wide is about right.

Step 3: Assemble and Iron

First, make a thin "heat shield" by wrapping the Filtrete template in aluminum foil. Couple of layers is adequate. Keep it neat.

Second, get a thin cotton cloth and get it damp, to use in ironing. Something about the weight of a bandana will work well. It has to be thin because you have to be able to see the outline of the heat shield through it. Wet it, and just squeeze it dry in your hand.

Third, turn your iron on to "Wool", or whatever setting your fusible interface instructs you to use.

Now assemble and iron:

Put the first layer of floating row cover on your ironing board.

Center the Filtrete on that.

Add strips of fusible interfacing around the edge of the Filtrete, like framing a picture. You are aiming for a continuous line of interfacing, all around the Filtrete. It's OK to overlap the interfacing, because it's going to melt at this step.

Weight down the Filtrete by placing something flat perpendicular to the pleats. I used a metal ruler.

Adjust the interfacing so that it's all lined up again.

Add the top piece of floating row cover.

Put your hand on it and gently remove the weight.

Put the heat shield directly over the Filtrete.

Put your damp cloth over the whole assembly. (This is a standard technique for ironing, not unique to this project).

Hold the heat shield in place, and iron all around the edges of that. Use firm pressure, and shoot for two or three seconds of hissing steam at every spot. Go over it a couple of times if you want, but with cloth this thin, fusible interfacing is pretty much bullet-proof.

Pull off the damp cloth, let it cool a few seconds, pick it up, let it cool a few seconds more, and inspect to make sure it bonded. You need to give the fusible interfacing (glue) enough time to cool down before you test it.

Step 4: Pleat and Staple

I can belabor this step, but by far the best thing you can do is get a picture of a surgical mask or single-use mask, and study it a bit. You will find that, for the great majority of them:

They have three pleats (some have more)

The pleats lie flat when the mask is flat

The mask expands toward the side that you pleat from (front vs. back)

The pleats are all oriented in the same direction

The openings of the pleats face downward when worn (bottom vs. top).

That's what you are trying to replicate. As noted, this step determines what the front of your mask is, and what the bottom (chin) is. So you need to pay attention to a couple of details.

First, make a template for the width you want: 3 and 5/8ths inches. Mark two parallel lines that far apart on the back of one of your cardboard templates.

Now, if you are picky about direction of air flow, turn your "sandwich" from the last step so that the fresh air side is up. I wasn't that picky.

Here's the one and only trick: The outside of this mask is the side that you are looking at, as you pleat it. Because of this, you must flip your stapler upside-down when you staple the pleats in place. Otherwise, you end up with the raw ends of the staple against your face. (Alternatively, flip the pleated assembly before you staple.)

Got it: Flip the stapler upside down.

Pleat it with your hands: Now, when I made this, I just put in two large pleats, following the pleating from the filter. This is quick, easy, gave me about the right size, and it's certainly what the filter medium "wants to do". It seems to work.

If you have the dexterity to put in three smaller pleats, that's probably better. I didn't.

Staple it. When you have your pleats in place, so that the pleated cloth assembly is the width you want, keep the assembly flat with your hand, push one edge over the edge of a table, turn the stapler upside-down, and staple. Then spin it around, put the other side over the edge, and, with the stapler upside-down, staple that.

Mark the outside and the top. Do your end user a favor at this point. Take a sharpie, on the edge material, mark what is the outside, and what is the top. As discussed above. Outside is the side you pleated from. Top is the side that the pleats fall away from.

Step 5: Final Pressing

Take your iron and damp cloth and your heat shield and press the cloth flat, outside of the Filtrete area. Do not iron on the Filtrete.

That's it, you're done making it.

Step 6: Use It

Unfold the mask liner, so that it looks like a surgical face mask. Flex it a bit so that it is good and pliable. Place it into your tight-fitting face mask, and trim the excess cloth to fit. Ideally, set this up so that the metal nosepiece of your close-fitting cloth mask sits on top of the Filtrete. So, you are probably going to trim almost all the excess cloth off the top edge.

It is probably best if you don't clip, pin, or otherwise attach this to your cloth mask. The reason is that you don't want to compromise any seal you have, between your cloth mask and your face. Instead, just work this liner around until it fits nice and snugly.

The liner is so breathable that I double you will notice any difference in breathability.

You probably should now wash this liner. Instead, I suggest that you do what healthcare personnel now do to re-use N95 respirators. Take your mask, with liner installed, drop it in an open paper bag, and let it sit for a few days. Any virus captured on the mask surfaces will self-destruct over time.

My best guess is that this should be good for hundreds of hours of use, if not more. But I haven't yet quite figured out the arithmetic to tell me how to get from a furnace filter, with 24 square feet of cloth and a specified air flow, to this, with a person breathing through it. In any case, as with your home air filter, if this gets clogged, it should become harder to pass air through. Conservatively, if you are worried about it, toss these out after 40 hours or so. They only use about $0.25 worth of Filtrete. Really, if you end up mailing one these to a friend, the postage will cost more than the mask liner will.

If you want more filtration, you can either start with two piece of Filtrete, or if you have the room in your mask, try adding two of these finished one-piece mask filters.

If you want to see the filtration specifications for Filtrete, you can get a .pdf file from the 3M website at this URL:

The key fact you have to know is that, for aerosol filtration, everybody concentrates on the very hardest-to-capture 0.3 micron particle. That's an E1 particle in the 3M literature. And that's what makes an N95 an N95 -- they capture 95% of those 0.3 micron particles.

If you use two pieces of Filtrete, you can use a little math to estimate what fraction of particulates they shouldl capture, based on the 3M specification. I used MPR 2500 material, which captures 77% of the "E1" particles. So one layer would let 23% pass. And two layers would let .23 x .23 = about 5% pass.

This mask liner will never be a substitute for an N95 respirator, because your cloth mask does not seal up against the face the way an N95 respirator does. That said, to the extent that you breath through your mask, and not around it, this liner should greatly improve filtration of fine particles.

Finally, I keep calling this a mask liner, and not a mask. Don't try to use it as a stand-alone mask. For one, there's no nosepiece. But in addition, while Filtrete is rated for capturing particles up to 10 microns (about a tenth the width of a hair), there is no information from 3M on what it does for particles above that size. For that reason alone, if you use this, you want to use it behind a tight-fitting cloth mask. Only.

I make no guarantees or warranties about this. Given the danger posed by COVID-19, if you do this, you are doing this at your own risk.

I hope this has been helpful.