A: Any multi-layered cloth mask (at least 2 layers is recommended and 3 is even better) made of a tightly woven fabric is a good option.
If you do use a two-layer mask with a built-in pocket for a filter as a third layer, here are some things to keep in mind.
The type of material recommended by the World Health Organization for the filter layer is polypropylene which, as noted in the articles linked below, is a plastic-derived material that has the added bonus of carrying an electrostatic charge that helps trap particles. This material can be removed and washed, however, it loses charge when washed and should be “recharged” by ironing it (be careful not to melt it) or rubbing it with a plastic glove.
Not recommended is cutting coffee filters or vacuum bags into small pieces and inserting into the mask pocket. These materials are more difficult to breathe through which will just cause air to flow around the filter, through the thinner layers of the mask.
If you do insert a polypropylene filter layer, it should span the entire area of the mask. Otherwise, again, air will just flow around the filter, defeating the purpose of adding it in the first place.
Many commercially made masks have started to include the option of filters that contain carbon or even HEPA-certified filters. Such filters are normally designed to absorb and remove particulates in the air through air purifiers and the effectiveness of such filters when worn on the face is still unclear. As we learn more about these types of filters, the Nerdy Girls will share any updates!
When we say ‘filter’ we are NOT talking about masks with an exhalation valve. These types of masks allow unfiltered air to be expelled and do not protect others around you from your droplets, so should not be worn.
The filter layer, just like masks, should be washed or replaced after each use. If at any point your mask becomes misshapen, loose or the fabric thins after repeated washing, you should replace it.
See links below for more information about filters in masks (as well as other useful info):
World Health Organization video about mask layers: https://youtu.be/iYE0A-5wd14