Breathing Resistance Rating and Fabric Density

We offer a large number of different fabric choices, each with four different lining options. This creates a wide variation in the total thickness and density of fabric you’ll be breathing through, each with a different degree of resistance to air flow. In order to help you compare masks, we have developed a testing procedure to measure the total “breathing resistance” of each mask and liner combination. Every combination is given a “Breathing Resistance Rating” shown in its individual product listing.

Values range from around 10 for our lightest-weight fabric with all-silk lining, to almost 100 for the densest fabric with two extra layers of cotton lining. It is very important to note that this rating is purely a measurement of breathing comfort. It is not in any way a measurement of filtering effectiveness. Different materials may offer better or worse filtering of one kind of particle or another, independently of how hard they are to breathe through. A mask with a very high breathing resistance may in practice be less effective, because more air escapes unfiltered around the edges (or because you stop wearing it).

Personal Preferences

These are my personal opinions about each of our lining choices, not to be taken as medical or scientific advice.

1. Cotton-Silk-Silk. This is our easiest-to-breathe-through, lightest-weight option, and it’s what I wear. But it’s more expensive because silk is expensive and hard to work with.

2. Cotton-Cotton. This was for a long time our standard 2-layer design. It’s comfortable, easy to breathe through, and it’s what I wore until we introduced the silk option.

3. Cotton-Cotton-Silk. This option provides two layers of cotton, and then a layer of silk against your face, which feels just lovely. If you’d like a high-filtering, three-layer design, but also want silk to protect you from cotton shedding and just because it feels good, this is a good choice.

4. Cotton-Cotton-Cotton. This was for a long time our standard 3-layer design. This is the style my daughter and her friends prefer. I find it too difficult to breathe through in hot weather, but wore this style a lot in March and April when it was still cool. I would consider it an indoor/winter option.

Bottom line: In the summer, I would choose Cotton-Cotton, or Cotton-Silk-Silk if you don’t mind the extra cost. In the winter or if you’ll be wearing it indoors, consider one of the 3-layer designs if you’re OK with a thicker mask. Above all, choose a mask that you think you’ll actually wear. There’s a saying in photojournalism that the best camera for the job is the one you have with you (also “f/8 and be there”). The same goes for masks: if you’re not wearing it, it’s not doing anyone any good. Three-layer masks provide more filtering, but on a hot day, the two-layer or silk versions are significantly more comfortable. For prolonged interactions indoors where filtering is more important, maybe go with a three-layer option (or, to be perfectly honest, a real N95 mask if you have access to one). But for brief visits to the store, or outdoor events, personally I choose the thinner options.

You can always add a filter insert to the any of these—a piece cut from a furnace filter, or another piece of cloth, for example. (Note that the straw hole option blocks the filter pocket, so those have to be used as-is.)

Technical Details of Breathing Resistance Rating Measurements

The Breathing Resistance Rating is a measurement of the pressure required to maintain a standard rate of air flow through a test sample of each mask fabric. The idea is that, regardless of how hard a mask is to breathe through, you are going to have to maintain the same total amount of air flow through it to get enough oxygen. For thicker fabrics you’ll just have to work harder to get that same air flow, and pressure is a measurement of that breathing effort.

We use a 5cm x 5cm sample of each fabric and measure the pressure (in units of 0.01 inches of water column) required to maintain a flow rate of 2 liters per minute through the sample. This corresponds to an air velocity of 2 meters per minute, and is at the high end of the typical breathing exchange rate. (Actual breathing exchange rate is higher than 2 liters per minute, but the surface area of the mask is larger than 5cm x 5cm, so the rate per square cm works out to be in the correct range. In any case the exact flow rate is not crucial as long as it’s consistent, since the measurements are only meaningful in comparison to measurements of other identical designs of mask under the same conditions.)

The test setup consist of a series of flow rate meters (covering the range of 0-20 liters/minute in three ranges), a low-range manometer that measures differential pressure, and a magnetic clamp to hold the fabric sample. Air from an air compressor goes through the flow rate meters, into the measurement chamber, and then out through the fabric sample.

The measurement for each individual outer decorative fabric is added to the values for the silk and cotton liners (4 and 30 respectively). It is assumed that the total resistance is linearly cumulative. In the example shown, note the ball in the middle flow rate meter centered on the 10 liter/minute line, and the reading of 0.10 inches of water column. This mask fabric thus has a rating of 10. Combined with a double-layer silk liner the total rating would be 18. Combined with a single layer of cotton lining, the total would be 40.

Microscopic Views

To provide further insight into the nature of our fabrics, we provide a magnified view of each fabric choice, with a superimposed scale (0.1mm minor divisions). It is possible to determine the thread count (the sum of the number of thread per inch in the horizontal and vertical directions) from this image, but thread count is not really the most meaningful parameter, because two fabric with the same thread count may have very different filtering and breathing resistance properties, depending on the nature and diameter of the threads. (For example, mosquito netting and burlap both have about the same number of threads per inch, yet are completely different.)

Here are two of our Batiks, both 230 thread count (72 threads per inch in the warp, 158 threads per inch in the weft). Yet one has a Breathing Resistance Rating of 11, while the other is 30. As you can see in the photos, that’s because the second one is made with thicker thread, packed more tightly to the same spacing. (Note that they have opposite orientations in these two pictures: in the first one the warp is aligned with the scale, while in the second one the weft is aligned with the scale. The thread count of a fabric is the sum of the number of threads per inch in both directions.) Click any of the images below for a full-screen view.

Our cotton liner fabric is 200 thread count (72 in the warp, 128 in the weft) plain weave percale (meaning every thread crosses over and under at every intersection). The breathing resistance rating is 30, indicating a fairly dense, tightly-woven material.

Our silk liners are 19 momme 3/1 satin-weave charmeuse. The thread count is about 485 (125 in the warp and 360 in the weft), but you can’t really use thread count to describe satin weave fabrics, because the threads are not crossing at every intersection. (Bed sheet manufacturers commonly lie about the thread count of their fabrics in this and other ways. There is no such thing as a 600 thread-count cotton sheet. Anything over 220 is probably a lie.)

Despite the very high density of threads, the breathing resistance rating of the silk liners is only 4.

Silk is measured in momme weight instead of thread count: 19 momme means that a 100-yard roll of 45 inch wide fabric weights 19 pounds. (This unit is one way you can tell how old and tradition-bound the silk trade is.) The filtering ability of silk fabrics is not as well tested as others, but there is no reason to believe it would be inferior. Individual silk fibers are about 10 microns in diameter (1/100 the size of one of the minor divisions in this scale), and respiratory droplets are typically in the same 5-15 micron range. Silk also absorbs water similarly to the way cotton does, which is a factor in filtering efficiency for water droplets.