No one loves masking. But... places require it. So, which mask?

Science is clear is on mask fit. Gaps at the nose or leaks at the sides allow perhaps half the air to go around the mask without filtration. In one study, a medical mask worn normally provided 40% filtration, but after sealing leaks filtration improved to 80%. Large droplets from a sneeze fly like projectiles and most any mask stops them, but smaller particles go around.

Science on mask construction is evolving. An N95 respirator filters >95% of 0.3um particles, the most difficult size to filter. A two layer woven cloth masks filters perhaps 50% of 0.3um particles but better for larger particles, perhaps 90% of 5um particles. What sizes drive Covid transmission is an open question. A 0.3um droplet likely holds too few 0.1um coronaviruses to make you ill. Probably 1um to 10um particles are the main concern... but ithe science is ongoing. Tiny droplets dry out fast, a 10um droplet in about 0.1 seconds. Droplets we emit are larger and thus easier to filter immediately than their dried up remnants floating in air. Cloth masks work well as source control, that is, when worn by an infected person, but seem to be only moderately effective for protecting the wearer.

If you're wearing a mask in kindness to others, then, any two layer cloth mask helps. If you're wearing a mask to protect yourself, choose mindfully.

• N95 is an American standard for filtering >95% of 0.3um particles. Elastics around the head provide a better fit than ear loops. This is the safest option.
• KN95 and KF94 are Chinese and Korean standards. Ear loop and around-the-head elastic versions exist. Unfortunately the market is flooded with counterfeits, so check the FDA (Food and Drug Administration) EUA (Emergency Use Authorization) for a KN95 on the "Approved" list.
• Double-masking: A medical mask under a well-fitting cloth mask can provide good protection at low cost. The medical mask filters well, while the outer cloth mask seals leaks.
• Medical or well-fitting two-layer cotton masks provide around 40% to 60% protection. That's significant for reducing disease transmission. If my mask filters 50%, and your mask filters 50%, the overall risk is reduced four-fold (1/2 * 1/2 = 1/4).
• Cloth mask with gaps or single layer may provide 10% or 50% protection (more likely on the low end), and with no labeling standards (yet) it's a gamble.

Out-valves allow unfiltered exhalation to escape. Avoid valved masks to protect your community.