"Surgeon General: Here’s how to make your own mask. No sewing required....A rule of thumb is that if you can see a lot of light through the material, ..

"Surgeon General: Here's how to make your own mask. No sewing required....A rule of thumb is that if you can see a lot of light through the material, viruses can easily penetrate. Presumably the technique demonstrated by Adams in the clip of folding several times to create multiple layers solves that problem to some extent even with a less tightly woven fabric..."

https://hotair.com/archives/allahpundit/2020/04/04/surgeon-general-heres-make-mask-no-sewing-required/