We have added Interfacings to our Face Mask Section as some patterns suggest they sit inside the mask as a lining. There are differing opinions on whether this is necessary or even desirable. Additional layers of fabric will inhibit microbes from passing through the mask, but there comes the point where each additional layer provides only minimal improvement. The other consideration is breathability. Some people have been using interfacing interlining, which can be sew-in or ironed onto the inside of the mask. We want to note that although we sell interfacing, we must point out it was not designed for this use.