Now, a growing body of science suggests that by wearing a mask to prevent spreading the virus, you may be protecting yourself, too. It is further evidence that knowledge about masks, and their benefits, continues to evolve — much as does understanding of the pandemic more broadly.

The Centers for Disease Control and Prevention publicly acknowledged that for the first time, writing in a scientific bulletin posted to its website this week that “the benefit of masking is derived from the combination of source control and personal protection for the mask wearer.” Masks are neither completely selfless nor selfish — they help everyone.

John Brooks, chief medical officer for the CDC’s coronavirus response, told The Washington Post there was an urgency to explain this clearly, because the widespread wearing of face coverings can help prevent the need for national lockdowns.

The CDC published this scientific brief to fix what the agency saw as the lack of “a concise summary of the powerful scientific evidence demonstrating the benefit of masking,” he said. The bulletin marks the start of the agency’s renewed push to bolster public messaging as infections surge to their highest-ever levels in many U.S. regions. The agency is now updating all its communications about masks to include the new information.

Because the CDC cannot impose mandates, the agency wants the public to understand masks are “good for them,” Brooks said.

The new document attempts to dangle a persuasive carrot of information before the public: A “likely complementary and possibly synergistic” relationship exists between controlling the source of infection for others and being protected yourself, the agency said. Put another way, the more people wearing masks in the community, the greater the individual benefit.

Masks create a barrier that stops some of the droplets from flying outward when someone breathes, talks, sings or coughs. A study released last week showed that, in experimental conditions, simple fabric masks blocked about three-fourths of the particles expelled by coughing volunteers.

But it was a logical hypothesis for researchers to investigate, too, whether masks might also block incoming particles. And laboratory tests in recent months indicated that masks can filter out the types of incoming particles able to carry virus, Brooks said.

The CDC official added that personal protection for the mask-wearer is not absolute. “The real benefit is when all of us do it, that’s how we bring down the viral load of covid-19 in communities,” Brooks said.

In the nine months since the virus hit the United States, the CDC has come under increasing criticism that its guidance on mask protection has not been sufficiently clear because it conflated scientific data with a concern over mask availability.

During the first wave of the pandemic, surveys indicated a majority of both Democrats and Republicans wore masks, though Democrats were more likely to. But a few Americans were strongly skeptical. This opposition became so pronounced in the summer that online shops sold fake “mask exemption” cards, labeled with stolen Justice Department imagery, by the hundreds.

By midsummer, Republican members of Congress were encouraging face coverings. Senate Majority Leader Mitch McConnell (R-Ky.) said in late June putting on a mask was the “single most important thing you can do” until vaccines were available.

But President Trump rarely insisted Americans wear them. He often described masks as optional and once, misleadingly, as a “double-edged sword” because people might touch the fabric and then their faces.

Images of the president covering his face were scarce until he tweeted a photo of himself in a mask in July.

“Overall, this seems like a win in terms of messaging that would appeal to Republicans,” said Katherine White, an expert in consumer behavior at the University of British Columbia. She said this was because conservatives are strongly motivated by a personal responsibility to care for themselves.

Brooks, who oversaw the publication of the new bulletin, said the language went through lengthy vetting to make sure there was agreement on the best evidence. In this case, laboratory, epidemiological and population-based studies all showed “very substantial benefits” of mask-wearing, he said.

An evolution of scientific thinking about masks in the United States occurred amid the pandemic. Monica Gandhi, an infectious-diseases expert at the University of California at San Francisco, said science suggests face coverings “of higher thread count, more than one layer, and those that employ ‘electrostatic filtration’ like surgical masks” provide the best protection.

“Messaging that allows the public to know that masks protect you as well as others will be more powerful in convincing skeptics that masks are important in public spaces to slow down spread and disease from covid-19,” Gandhi said.

It is less clear whether masks — by potentially stanching some, but not all, incoming viral particles — might also lessen the severity of illness. On Sept. 8, Gandhi and her UCSF colleague George Rutherford suggested the dose of virus that people are exposed to may impact the seriousness of disease. The scientists said face coverings could filter enough droplets to lead to asymptomatic or mild illness in some cases — and lead to some immunity.

On Oct. 23, a group of other prominent experts — including Columbia University’s Angela Rasmussen and George Mason University’s Saskia Popescu — argued in a letter to the editor that the science is preliminary and that it is only a hypothesis that mask-wearers inhale small amounts of virus.

The CDC did not address this theory in the bulletin released this week. The agency emphasized this update did not reflect a change in the agency’s recommendation to wear masks.

“Our guidance is still the same,” Brooks said. “This is more data in support of that, and it highlights that there is personal protection that we want to make sure people know about.”

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