We have some hand-sewn masks made of two layers of cotton. I modified two with the addition of wire in the top hem, allowing the wearer to bend the wire to fit the bridge of the wearer's nose. This significantly increases the mask's ability to seal to the wearer's face when inhaling, and may improve filtering when exhaling. (The positive pressure of exhaling tends to lift any mask that isn't held by adhesive or external force.)
Steel wire between 22 and 30 gauge, or copper wire between about 18 and 26 gauge should work for this. I used enameled copper magnet wire, 24 gauge, because that's what I have on hand. If you cut the wire at a sharp angle, you can preform holes in the fabric by separating it with a sewing pin, then gently press the wire through the preformed hole. I used the space between my fingernail and fingertip to back up the fabric, so the wire would protrude through, and then I could move along to the next one. I ran the wire back and forth, a running stitch, with about 6mm of space between each hole. It seems to work pretty well.
There is currently a lot of dispute about the effectiveness of DIY masks. I've found sources that say two layers of the sort of cotton used for t-shirts is about 50% as effective as an N95 mask, while one layer of tight weave quilter's cotton over one layer of flannel is somewhere near 80% as effective as an N95. https://www.npr.org/sections/goatsandsoda/2020/04/03/825639323/scientists-probe-how-coronavirus-might-travel-through-the-air has an extended discussion about current views on aerosol and droplet spread of viruses, which is poorly understood.
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