To see the true power of masks as a public health tool, we have to examine them in the context of everyone wearing them, where the power of each mask doubles. That’s because the particles have to pass through the material twice — once after being emitted and again before someone breathes them in. Take the example of two 70 percent efficient masks, which combine to reduce 91 percent of particles. Not bad. But two N95s result in greater than a 99 percent reduction in exposure.

Think about that for a minute. We could reduce exposure by 99 percent for what should be $1 a mask. (Prices are higher now because of the failure to produce an adequate supply.) Throw in better ventilation and some distance between people, and you have hospital-grade protections.

How well a mask works isn’t just about filtration; it must also fit well. A mask with a good set of filters doesn’t do much good if your breath can escape out the sides or top. The mask needs to go over the bridge of your nose, down around your chin and be flush against your cheeks.