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Do cloth masks actually work?

MIT Medical answers your COVID-19 questions. Got a question about COVID-19? Send it to us at CovidQ@mit.edu, and we’ll do our best to provide an answer.

I recently came across this study on cloth masks being ineffective in “high-risk situations.” The study concluded that moisture retention, reuse, and poor filtration could increase wearers’ risk of infection. We are being asked to use cloth masks on campus and in other places; should we be doing something different?

Illustration of a waiting room with a person wearing a cloth PPE masks visible at left and right sides of the image

The study you cite involved healthcare workers who experienced repeated exposures to the virus due to very close, often physical, contact with infected patients — the very definition of “high-risk situations.” Not surprisingly, the study showed that cloth masks did not protect against infection as well as medical-grade masks.

But unless you are a frontline healthcare worker, this study does not have much relevance to the precautions you should be taking on campus or elsewhere. Remember, since the virus is spread through respiratory droplets, and since people are most contagious in the 48 hours before symptom onset, our cloth face coverings are meant to protect other people, not ourselves. And there is steadily accumulating evidence that cloth masks perform that function well, by containing respiratory droplets before they can be expelled into the air.

Evidence for the efficacy of masks comes both from laboratory studies and real-world scenarios. For example, one recent laboratory experiment used a laser-light-scattering methodology to visualize respiratory droplets generated while subjects repeated the phrase “stay healthy.” While each utterance generated hundreds of droplets ranging in size from 20 to 500 micrometers, the researchers showed that covering the speaker’s mouth with a damp washcloth blocked nearly all of them. 

The evidence from epidemiologic data and case studies may be even more compelling. A recent study, for example, used publicly available data to calculate the COVID-19 growth rate before and after mask mandates in 15 states and the District of Columbia between the end of March and late May of 2020. Researchers found that mask mandates led to a marked slowdown in the daily growth rate, estimating that mask mandates may have prevented up to 450,000 cases of COVID-19.

Real-world “experiments” are also persuasive. In one early case, a man flew from Wuhan to Toronto with a dry cough and subsequently tested positive for COVID-19. He wore a mask during the flight, and no other passengers tested positive. In a more recent instance, two hair stylists employed in a newly reopened hair salon in Springfield, Missouri, worked on a combined 140 clients while sick with COVID-19. Everyone wore a mask per state order, and no clients tested positive.

Of course, no public venue is without risk at this point, and masks will never be 100 percent protective, even if used 100 percent of the time. However, in combination with social distancing, our cloth masks are very effective at preventing the spread of the virus. Remember, your mask protects other people; their masks protect you.

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