Gaps--around the perimeter of a face mask--can reduce filtration efficiency (FE) of particles by 50 percent. Research, published by molecular engineers at University of Chicago and Argonne National Laboratory, Illinois, was based on poking holes in fabrics before testing to simulate face mask gaps. After testing 15 different fabric-layer combinations, the scientists found that each of the three top performers showed significant filtration reduction when gaps were introduced. They also tested N95s for comparison, and found that N95s' filtration efficiency was diminished by minus 51% to 87%.
The best filtration-performing cloth combination, that can commonly be used in making face masks, was: one layer of 600 thread per inch (TPI) cotton (as found in some bed sheets) along with two layers of chiffon (defined as 90% polyester and 10% spandex). That combination provided 97% to 99.2% filtration efficiency as long as no gaps were present. See the video on how to close the gaps here. Read More details in the Science News Release.