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Still Confused About Masks? Here’s the Science Behind How Face Masks Prevent Coronavirus

As states reopen from stay-at-home orders, many, including California, are now requiring people to wear face coverings in most public spaces to reduce the spread of COVID-19.

Both the Centers for Disease Control and Prevention (CDC) and the World Health Organization now recommend cloth masks for the general public, but earlier in the pandemic, both organizations recommended just the opposite. These shifting guidelines may have sowed confusion among the public about the utility of masks.

But health experts say the evidence is clear that masks can help prevent the spread of COVID-19 and that the more people wearing masks, the better.

We talked to UC San Francisco epidemiologist George Rutherford, MD, and infectious disease specialist Peter Chin-Hong, MD, about the CDC’s reversal on mask-wearing, the current science on how masks work, and what to consider when choosing a mask.

Why did the CDC change its guidance on wearing masks?

The original CDC guidance partly was based on what was thought to be low disease prevalence earlier in the pandemic, said Chin-Hong.

“So, of course, you’re preaching that the juice isn’t really worth the squeeze to have the whole population wear masks in the beginning – but that was really a reflection of not having enough testing, anyway,” he said. “We were getting a false sense of security.”

Rutherford was more blunt. The legitimate concern that the limited supply of surgical masks and N95 respirators should be saved for health care workers should not have prevented more nuanced messaging about the benefits of masking. “We should have told people to wear cloth masks right off the bat,” he said.

Another factor “is that culturally, the U.S. wasn’t really prepared to wear masks,” unlike some countries in Asia where the practice is more common, said Chin-Hong. Even now, some Americans are choosing to ignore CDC guidance and local mandates on masks, a hesitation that Chin-Hong says is “foolhardy.”

What may have finally convinced the CDC to change its guidance in favor of masks were rising disease prevalence and a clearer understanding that both pre-symptomatic and asymptomatic transmission are possible – even common. Studies have found that viral load peaks in the days before symptoms begin and that speaking is enough to expel virus-carrying droplets.