The Real Reason to Wear a Mask

To better understand what level of mask-wearing we need in the population to get this pandemic under control, we assembled a transdisciplinary team of 19 experts and looked at a range of mathematical models and other research to learn what would happen if most people wore a mask in public. We wrote and submitted an academic paper as well as a layperson’s summary. Every infectious disease has a reproduction rate, called R. When it’s 1.0, that means the average infected person infects one other person. The 1918 pandemic flu had an R of 1.8—so one infected person infected, on average, almost two others. COVID-19’s rate, in the absence of measures such as social distancing and masks, is at least 2.4. A disease dies out if its R falls under 1.0. The lower the number, the faster it dies out.

Read: Why some people get sicker than others

The effectiveness of mask-wearing depends on three things: the basic reproduction number, R₀, of the virus in a community; masks’ efficacy at blocking transmission; and the percentage of people wearing masks. The blue area of the graph below indicates an R₀ below 1.0, the magic number needed to make the disease die out.

Models show that if 80 percent of people wear masks that are 60 percent effective, easily achievable with cloth, we can get to an effective R₀ of less than one. That’s enough to halt the spread of the disease. Many countries already have more than 80 percent of their population wearing masks in public, including countries such as Hong Kong, where most stores deny entry to unmasked customers, and the more than 30 countries that legally require masks in public spaces, such as Israel, Singapore, and the Czech Republic. Mask use in combination with physical distancing is even more powerful.

While cloth masks are sufficient for protecting others, people who are immunocompromised or those who have a few left over from fire season or hobbies may be considering wearing N95s, to better  protect themselves. One note of caution: Many nonmedical N95s have exhalation valves (to make them less stuffy to wear) that let out unfiltered air, and thus they won’t  stop the wearer from infecting others—so they shouldn’t be worn around other people unless the valve is covered over with tape or cloth.

The community use of masks for source control is a “public good”: something we all contribute to that eventually benefits everyone—but only if almost everyone contributes, which can be a challenge to persuade people to do. It’s like emission filters in our car exhausts and chimneys: They need to be installed in all cars, factories, and houses to guarantee clean air for everyone. Usually, laws, regulations, mandates, or strong cultural norms ensure maximal participation. And once that happens, the result can be amazing.

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