Your questions, answered "I do not question that masks, such as surgical masks, can play a key role in preventing the spread of the coronavirus. But isn't how such masks play this role widely misunderstood? Isn't the job of a surgical mask to protect people nearby in case the person wearing it is infected but does not yet know it (where the mask prevents infected droplets and aerosol from traveling too far forward)? For a mask to protect the person wearing it, must it not be a well-fitted, high-filtering mask such as an N95?" – Joe in Virginia You're right on this: Well-fitted and filtered medical-grade masks offer the best protection. But research shows that when worn properly, many masks can help keep coronavirus droplets and aerosols from spreading — both by blocking particles from escaping a mask worn by a person infected with the virus and keeping them from penetrating a mask worn by a person trying to stay healthy. The confusion may come in part by how the guidance has evolved over the course of the pandemic. Early last year, mask recommendations from the Centers for Disease Control and Prevention were geared more toward protecting others to stop the spread from those with asymptomatic infections. However, as scientists learned more about SARS-CoV-2 — the technical name for this coronavirus — the guidelines changed, indicating that masks also protect wearers from possible infection. Monica Gandhi, an infectious-diseases physician at the University of California at San Francisco, said masks have different layers of effectiveness in terms of blocking particles from getting in and getting out, and that effectiveness depends largely on the type of material, whether the mask is the right fit and whether it is worn properly — covering the nose and mouth and fitting tight against the cheeks. "Essentially, masks do protect both you and the other wearer no matter what you wear, but the more fit and filtered masks protect you more," said Gandhi, who co-wrote a paper on mask efficacy. The paper, which was published earlier this year in the peer-reviewed journal Med, summarized that, in physical science experiments, surgical masks were 60 to 70 percent effective at protecting others from the coronavirus and 50 percent effective at protecting the wearer. Cloth masks, on the other hand, showed quite a lot of variability, from less than 10 percent to more than 60 percent depending on the material of the mask. Here's how Gandhi and a colleague explained it in the paper: Masks work by blocking or filtering out viruses that are carried in aerosols. Filtering is not sieving out things that are too large to pass through holes in the material. Rather, air must curve as it flows around individual, tightly packed fibers of the material, like a race car swerving around cones of an obstacle course. As the air curves, the aerosols it carries cannot make the sharp bends and therefore slam into the fibers, or they come too close to the fibers and stick to them. Very small aerosols acquire random motion from air molecules bouncing off them and end up crashing into the fibers. This process works in both directions as air flows through a mask. It is true that a recent randomized controlled trial involving 600 villages in Bangladesh, which has not yet gone through peer-review, showed that in villages where people were given surgical masks instead of cloth masks, they appeared to be better protected from symptomatic infection. Still, participants who were told to wear either kind of mask had fewer infections vs. control groups who weren't given instructions to wear masks. Gandhi's paper and one published by the CDC showed that combining cloth and surgical masks together provide even greater protection. The takeaway, she said, is that nearly every mask provides some protection for the wearer, but the level of protection will vary based on the material and fit. | 
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