These are pre-cut, 5"x4", ready to insert into your masks!
Polypropylene fabric is a man-made fabric made from Thermoplastic polymers (plastic). It is breathable, has moisture-wicking properties and is most commonly used in Sportswear, cold weather undergarments, military wear, and tote bags.
The Government of Canada has recommended non-woven polypropylene for the filter layer in masks. Below is an excerpt from the Government of Canada webpage on Non-medical masks and face coverings:
Filters add an extra layer of protection against COVID-19 by trapping small infectious particles. Consider wearing a mask that includes a filter or filter material as one of its layers, such as:
non-woven polypropylene fabric, which can be found as:
a craft fabric
the non-woven fabric that's used to make some reusable shopping bags
a disposable filter inserted into a pocket on the mask