N95 masks are the most protective masks.  They seal tightly around the nose and mouth, so that very few viral particles seep in or out.  N95 masks are made with tangled fibers to filter airborne pathogens, and they are at a minimum of 95 percent efficient at filtering aerosols.  In a Duke study, less than 0.1 percent of droplets were transmitted through an N95 mask while the wearer spoke.  Disposable surgical masks, like the N95 mask, are made from unwoven, tangled fibers.  These masks are 98.5% efficient at filtering large droplets and 89.5% efficient at filtering aerosols.  Both N95 and disposable surgical masks are ranked highest and are best suited for healthcare settings.

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“Hybrid” masks have been ranked as one of the safest homemade options and offer protection close to that of an N95 or surgical mask.  Preferably, masks should have more than one layer and be made from fabrics that are woven as tightly as possible. The World Health Organization (“WHO”) recommends that “fabric masks have three layers: an inner layer that absorbs, a middle layer that filters and an outer layer made from a nonabsorbent material like polyester."  In a study by the University of Chicago, “hybrid” masks, combining two layers of 600-thread-count cotton paired with another material like silk, chiffon, or flannel, filtered at least 94 percent of small particles (less than 300 nanometers) and 96 percent of larger particles (greater than 300 nanometers).  Because of the efficiency of “hybrid” masks at filtering large droplets and aerosols, they are appropriate face coverings to be worn in public, indoors, and in crowded settings.

In the same study by the University of Chicago, common fabrics used in cloth masks were evaluated on their filtration efficiencies. Fabrics were evaluated in single layers as well as in multiple layers.  2-layer 600-thread-count cotton masks were found to provide similar levels of protection as hybrid masks.  The two layers of 600-thread-count cotton were 99.5 percent efficient at filtering large droplets (greater than 300 nm) but are not as effective at filtering aerosols. These masks might also provide less protection against coughs and sneezes.  2 layer cotton masks are ranked after the hybrid masks with 99.5% efficiency at filtering large droplets and 82% efficiency at filtering aerosols. The appropriate coverings to wear in public, indoors, and in crowded settings.  Tea towels, dishcloths, and 100% t-shirt fabrics were evaluated in a study by the University of Illinois.  The fabrics were exposed to high velocity and low-velocity droplets to mimic coughs and sneezes.   It was found that even in a single layer, these fabrics were considerably efficient at blocking high-velocity droplets.  With 2 to 3 layers of these fabrics, the efficiency increases and becomes similar to a medical mask. Tea towel or dishcloth masks have 98% efficiency at filtering large droplets and 72.5% efficiency at filtering aerosols.   100% cotton t-shirt masks have 97% efficiency at filtering large droplets and 51% efficiency at filtering aerosols. These two mask types are efficient enough at filtering out large droplets, but their ability to filter aerosols ranks them behind the two-layer cotton mask.  These mask types should only be worn in outdoor areas and not in crowded settings or indoor.

Natural silk face coverings were evaluated and ranked.  In the study conducted by the University of Chicago, a single layer of natural silk was only 56% efficient at filtering large droplets and 54% efficient at filtering aerosols. Masks made with only a single layer of natural silk should only be worn in outdoor areas.  Ranked after single layer silk masks, bandanas and scarves offer little protection.  A study from the Journal of Hospital Infection found that scarves only reduced the risk of infection by 44 percent after a person shared a room with an infected individual for 30 seconds. At 20 minutes of exposure, scares only reduced infection risk by 24 percent. Scarves and bandanas are only 44% efficient at filtering large droplets and 49 percent efficient at filtering aerosols. Scarves and bandanas should only be used as a last resort, but in the end, they are better than no face covering at all. 

Ranked last on the list of the best and worst face coverings are masks with built-in valves or vents.  Though these masks are 90 percent efficient at filtering large droplets and aerosols, they should never be worn because they do not protect others.  Masks with one-way valves can expel infectious particles into the atmosphere, which only helps fuel transmission. The CDC advises against the use of these types of masks as well.

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