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You may have already thought it, but it’s official now. The US Centers for Disease Control and Prevention says wearing cloth masks not only keeps you from unknowingly spewing out the virus, it keeps you from inhaling the virus from the air.

The agency reiterated that masks are primarily intended for “source control” — to reduce the emission of respiratory particles carrying the virus, particularly because many infected people might not have symptoms and might spread the virus without realizing it, if they don’t wear masks.

But the agency also said in a scientific brief posted Tuesday that masks “help reduce inhalation” of the particles by the wearer.

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“The prevention benefit of masking is derived from the combination of source control and personal protection for the mask wearer,” the brief said. “The relationship between source control and personal protection is likely complementary and possibly synergistic, so that individual benefit increases with increasing community mask use.”

The CDC had previously emphasized the use of masks for source control. A CDC web page on “Considerations for Wearing Masks” as of Wednesday morning still cited only source control as a reason for wearing masks.

The brief surveyed the research on how masks can block the virus from being emitted — and the research on the ability of masks to filter out the virus.

“Studies demonstrate that cloth mask materials can also reduce wearers' exposure to infectious droplets through filtration, including filtration of fine droplets and particles less than 10 microns,” it said, noting that studies have differed on how effective various masks are, in large part due to variation in experimental design and the particle sizes analyzed.

“Multiple layers of cloth with higher thread counts have demonstrated superior performance compared to single layers of cloth with lower thread counts, in some cases filtering nearly 50% of fine particles less than 1 micron,” the brief said.

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Some materials, such as polypropylene, “may enhance filtering effectiveness by generating triboelectric charge (a form of static electricity) that enhances capture of charged particles while others (e.g., silk) may help repel moist droplets and reduce fabric wetting and thus maintain breathability and comfort,” the brief said.

It called for more research in the area, saying it was “needed to expand the evidence base for the protective effect of cloth masks and in particular to identify the combinations of materials that maximize both their blocking and filtering effectiveness, as well as fit, comfort, durability, and consumer appeal.”

“Adopting universal masking policies can help avert future lockdowns, especially if combined with other non-pharmaceutical interventions such as social distancing, hand hygiene, and adequate ventilation,” the brief said.

Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, said Tuesday on MSNBC about wearing masks, “Recent data has now shown that, as a matter of fact, there’s also the added benefit to protect you from droplets and virus that’s coming your way. … So it’s a two-way street.”

“It’s very clear that [mask-wearing] is very helpful,” he said.

Experts welcomed the change in the message from the CDC.

“This is good. Formal acknowledgment by CDC on masks is important,” tweeted Joseph Allen, a professor at Harvard T.H. Chan School of Public Health and director of the school’s Healthy Buildings Program. He also wondered, “What took so long?”

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Dr. Monica Gandhi, an infectious disease expert at the University of California, San Francisco, told The New York Times that the change from the CDC “matters for public-health messaging, because we don’t have people yet who are completely convinced about the benefits of masking until they see the CDC say that it also protects you and your family.”

“I would encourage every American to adhere to masking guidelines now that we hear more clearly today that this will protect you and others,” Gandhi said. “We cannot afford more lockdowns, but we can do our part to stop COVID-19 transmission and disease.”

Saskia Popescu, an infectious disease epidemiologist and adjunct professor at the University of Arizona, tweeted, “We know that even fabric masks offer some variable protection for the wearer, but I think the bigger issue is that the CDC has to reiterate this to get people to wear a mask."

Angela Rasmussen, a virologist at the Center for Infection and Immunity at Columbia University’s Mailman School of Public Health. tweeted that she was “saddened and disappointed that the greater good isn’t sufficiently motivating to get everyone on board and we have to put critical guidance into a selfish context.”




Martin Finucane can be reached at martin.finucane@globe.com.