The filter is based on the mask described in this National Institutes of Health article,
The main difference between the advanced filter and the quick filter is that the layers of the advanced filter alternate grain direction every two layers as recommeded by the WHO/NIH, rather than every other layer as the quick filter. Additionally the advanced filter has 9 layers rather than 8 if you use a standard Hanes T-shirt. It is not known how significant these differences are.
Our design uses the same filter construction and material as shown in the NIH article, but with a mask that seals better to the face. This should force more of the air through the filter rather than allowing it to go around, or through the thinner parts of the mask described by the NIH. The filter is constructed out of squares cut out of boiled T-shirt material like so:
I used these common Hanes T-shirts:
First, boil your T-shirts for 10 minutes and allow to air-dry. This will cause the shirts to shrink and make all the air pathways through the cloth smaller.
There are 9 total layers, the first 8 layers have 2 layers with the knit grain lined up one way, the next 2 lined up rotated 90 degrees, etc. The 9th layer is added and knit grain alignment is unimportant. Particularly thick T-shirts may need only the first 8 layers.
Using a sharpie or other marker, draw up a grid of squares on the T-Shirt, each square should be 10cm x 10cm. Make sure the T-shirt is NOT stretched out at all when you mark these:
Stack up the layers of cotton as described above, and sew them together around the edge. You do not need to be an expert. I have no idea how to sew, and it worked out just fine. I used the backstitch described in this article. This filter can easily be washed and reused.