As the world opens up from quarantine, questions about the best use of face masks—many millions of which have already been sold or improvised in the past few months—will be with us for quite some time. 

Here are some key things you may still not know…

Best cloth fabrics to protect against spreading the coronavirus. A paper published in ACS Nano tested the effectiveness of different fabrics in a laboratory and determined that high-thread-count cotton does very well—600-threads-per-inch ­cotton filters out 79% of particles smaller than 0.3 microns. (Particles of SARS-CoV-2, the scientific name of the new coronavirus, are 0.125 microns in diameter). But the study also found that a mask that combines a layer of high-thread-count cotton with ­either a layer of flannel—the study looked at a polyester-cotton blend of flannel—or two layers of natural silk or chiffon does much better…even though flannel, silk and chiffon are not especially effective mask materials on their own. These “hybrid” masks filter out 94% to 97% of sub-0.3-micron particles. The fabrics supplement cotton well because they filter in a different way. While high-thread-count cotton literally blocks tiny particles, flannel, natural silk and chiffon provide “electrostatic” filtration—tiny particles stick to them as if to a magnet. Check with manufacturers or suppliers about materials.

What doesn’t work: A mask made from a single layer of low-thread-count cotton will be largely ineffective. The study found that 80-threads-per-inch cotton filters out less than 10% of sub-0.3-micron particles. Also, never use a HEPA filter, such as the filter from a vacuum cleaner bag, in a face mask. Those are extremely effective, but they contain chemicals and are made with microscopic glass or acrylic fibers that are harmful to lungs if inhaled. 

Never wash your mask in bleach. Wash it in hot water using detergent, but do not use bleach. Bleach can damage the fabric of a face mask, reducing its ability to block small particles. Also, breathing bleach can damage the respiratory tract.

The effectiveness of single-use masks. Choose disposable masks that have at least three and preferably four layers…three pleats to enable a good fit around the nose and mouth…and a metal strip to secure the top of the mask snugly around the bridge of the nose.

The importance of eye covering. A meta-analysis of SARS-CoV-2 research published in The Lancet suggests that wearing some form of eye protection, such as a face shield, a visor or goggles, in addition to a face mask could reduce infection risk by more than half.

Effectiveness of N95 masks. N95s, currently reserved primarily for medical workers, seal very tightly to the face, which allows them to filter out at least 95% of particles 0.3 microns in size. I suspect N95s will be available to consumers within a year. The only problem is that it’s not always easy to breathe through them—some people feel like they’re suffocating. Practice with an N95 mask at home, and don’t wear one in public until you can do so without feeling an urge to pull it off. Choose a different mask for cardio workouts. 

Effectiveness of KN95 masks. The KN95 filtration standard is the ­Chinese equivalent of the US N95 standard. The two are very similar—both require the filtration of at least 95% of particles 0.3 microns or larger. The concern would be whether masks made by obscure companies and sold as KN95 achieve this standard. For a list of FDA-authorized brands of KN95 masks, go to bit.ly/2Z5bM0w.